AP95021 Disease monitoring & control of pear scab. W S Washington & O N Villalta Agriculture Victoria

Transcription

1 AP95021 Disease monitoring & control of pear scab W S Washington & O N Villalta Agriculture Victoria

2 AP95021 This report is published by the Horticultural Research and Development Corporation to pass on information concerning horticultural research and development undertaken for the apple and pear industry. The research contained in mis report was funded by the Horticultural Research and Development Corporation with the financial support of the apple and pear industry and Agriculture Victoria. All expressions of opinion are not to be regarded as expressing the opinion of the Horticultural Research and Development Corporation or any authority of the Australian Government The Corporation and the Australian Government accept no responsibility for any of the opinions or the accuracy of the information contained in this report and readers should rely upon their own enquiries in making decisions concerning their own interests. Cover price: $20.00 HRDC ISBN Published and distributed by: Horticultural Research & Development Corporation Level 6 7 Merriwa Street Gordon NSW 2072 Telephone: (02) Fax: (02) S2 hrdc@hrdc.gov.au Copyright 1999 HRD\C HORTICULTURAL RESEARCH & DEVELOPMENT CORPORATION Partnership in horticulture

3 M.imral Rcsoimvs and l.mironvnem Disease monitoring and control of pear scab *RO Final Report HRDC project number AP July 1995 to June 1998 W.S.Washington and O.N.Villalta Agriculture Victoria, Institute for Horticultural Development, Knoxfield a Canned Fruits Industry Council of Australia icrono ON THE MOVE

4 HRDC project number AP Principal investigator: W.S.Washington, Agriculture Victoria, Institute for Horticultural Development, Knoxfield, Private Bag 15, SouthEastern Mail Centre, Vic 3176 Phone , fax This report summarises work done to fulfil the requirements of the HRDC project number AP Acknowledgement: This work was supported by funding from the Horticultural Research and Development Corporation, the Australian Apple and Pear Growers Association and Agriculture Victoria. Any recommendations contained in this publication do not necessarily represent current HRDC policy. No person should act on the basis of the contents of this publication, whether as to matters of fact or opinion or other content, without first obtaining specific, independent professional advice in respect of the matters set out in this publication.

5 1. SUMMARIES Industry Summary Technical summary 3 2. INTRODUCTION 4 3. PAD MONITORING 6 Materials and methods 6 Orchards Sites 6 Scab Management 7 Results 8 4. ASCOSPORE MATURITY MODEL 15 Materials and Methods 15 Ascospore monitoring in the field 15 Microscopic examination of pseudothecia 16 Prediction model for ascospore maturity 16 Results REDUCTION OF OVERWINTERING INOCULUM 21 Materials and Methods 21 Ascospore productivity 21 Leaf decomposition study 22 Orchard sanitation study 23 Results GENERAL DISCUSSION TECHNOLOGY TRANSFER RECOMMENDATIONS ACKNOWLEDGEMENTS REFERENCES 31 1

6 1. Summaries 1.1 Industry Summary This work has studied three interrelated aspects of pear scab management which together, have the potential to improve scab control by better timing of sprays and by possible reduction in the number of sprays. These aspects are 1). the relation between autumn leaf scab and the numbers of ascospores produced in the following spring, 2). the relation between ascospore maturity in spring and the accumulated degree-days, and 3). the relation between treatments which reduce the amount of overwintering inoculum and the disease pressure in the following spring. Outcomes should be improved spray timing and, in some cases, the possibility of reduced spray programs in spring. 1. An autumn monitoring method to quantify the level of scab in an orchard after harvest has been developed and tested. This method may be used to predict the potential disease risk which could occur in the following spring (potential ascospore dose or PAD). It involves examining all leaves on 600 shoots per block of trees (10 shoots per tree on 60 trees per block), and scoring the incidence of scabbed leaves and the number of lesions per leaf. At present this method can be used to help decide whether autumn or winter treatments such as leaf mulching or spraying to reduce the level of overwintering disease are required. Monitoring must cover both sides of the leaf as most scab lesions are found on the underside of leaves. Monitoring must occur before leaves have started senescence, but not too early as late leaf infections may occur after an early sampling, resulting in an under-estimate of disease. Twigs should also be checked for scab pustules in autumn, to ensure that the disease is not overwintering as conidia on twigs. Results indicate that when autumn scab is below about 0.5% up to two infection periods after green tip (but before white-bud) may be left unprotected, or sprayed with reduced rates of fungicides or other "softer" sprays. More work is needed to refine this action threshold to allow a safe reduction in the spring spray program. 2. Two ascospore maturity models have been developed from spore trapping and weather data collected over five seasons. These models will provide a simple guide to the most critical time to apply fungicide sprays in the primary infection period in spring. They relate cumulative degree days after green tip to ascospore maturity. This relationship can be used to predict peak ascospore maturity and allow more precise timing of sprays. The models predict that 95% of ascospores would be mature after 400, and 650 degree days accumulation for Southern, and Northern Victoria, respectively. The models will be further developed and trialed during spring in coming seasons. 3. Several materials (fungicides and a nitrogenous fertilizer) have been shown to have potential to reduce the amount of overwintering disease when applied to scabbed leaves after harvest. One application of a curative fungicide (myclobutanil) to leaves after harvest but before leaf-fall substantially reduced the amount of overwintering inoculum produced in the following spring. Similarly, 2% urea (low 2

7 biuret) applied in the same way also reduced the amount of overwintering inoculum. Urea sprays applied to leaves before leaf-fall, at leaf-fall, or to leaves on the ground reduced the amount of leaf litter which survived into spring. Shredding the leaves by slashing and then spraying with urea gave the best effect. These materials have the potential to reduce the level of overwintering inoculum to allow reduced spring spraying. Note: these materials are not registered for this use. Recommendations: 1.Growers consider testing the autumn leaf monitoring method to identify blocks with low disease potential. 2. Output from the ascospore maturity model be provided to some growers and consultants in spring 1998, for feedback and further development. 3. Growers consider management practices to reduce leaf litter which remains on the orchard floor after winter. 4. Further work is needed to develop these three methods. This is planned as part of the new project proposal "Improving scab management in pome fruit using disease and weather monitoring systems" 1.2 Technical summary This work has studied three interrelated aspects of pear scab management which together, have the potential to improve scab control by better timing of sprays and by possible reduction in the number of sprays. These aspects are 1). the relation between autumn leaf scab and the numbers of ascospores produced in the following spring, 2). the relation between ascospore maturity in spring and the accumulated degree-days, and 3). the relation between treatments which reduce the amount of overwintering inoculum and the disease pressure in the following spring. 1). A monitoring method which quantifies the autumn leaf scab in an orchard and which can estimate the scab pressure which may occur in the following spring (potential ascospore dose or PAD) has been developed and tested in 15 replicated spray timing trials on growers properties. The method, based on a similar procedure developed in USA for apple scab, involves examining all leaves on 600 shoots per block of trees (10 shoots on each of 60 trees). The incidence of scabbed leaves and the number of scab lesions per leaf are recorded after harvest but before leaves have started senescence in late autumn. Results support the idea that when leaf scab levels are very low in autumn (less than 0.5%), the amount of inoculum (ascospores) which develops in the following spring is low. This can allow a reduction in the spring spray program (leaving trees unprotected for the first 1-2 infection periods after bud burst, but no later than white bud by delaying the early sprays or reducing fungicide rates or applying "softer" treatments during the lower risk period). Twigs should also be checked for scab pustules in autumn, to ensure that the disease is not overwintering as conidia on twigs. 3

8 Although the work provides evidence to support this strategy, more research is needed to refine thresholds which will allow the application of these reduced spray programs. The monitoring method will allow rational decisions to be made about the need for management practices such as post-harvest pre-leaf fall sprays, or leaf sweeping and mulching to reduce overwintering inoculum. 2). Two ascospore maturity models, one for Northern Victoria and one for Southern Victoria, have been developed from spore trapping and weather recording in pear orchards over five seasons. These models will provide a simple guide to the most critical time to apply fungicide sprays in the primary infection period in spring. They indicate a relation between cumulative degree days after green tip and ascospore maturity. This can be used to predict peak ascospore maturity and allow more precise timing of sprays. The models predict that 95% of ascospores reach maturity after 400, and 650 degree days accumulation for Southern, and Northern Victoria, respectively. The models will be developed further and trialed during spring in coming seasons. 3). Urea (low biuret), hydrated lime, and the fungicides myclobutanil, fenarimol, flusilazole, hexaconazole, fluquinconazole, benomyl and pyrimethanil were tested for their effect on reducing ascospore production from scabbed leaves. All treatments when applied as a leaf dip in autumn reduced ascospore production in at least one test, but benomyl, myclobutanil and to a lesser extent urea gave the most consistent effect. Myclobutanil (12 g/lool) reduced ascospore production from leaves more than urea (2%) when sprayed onto trees once before leaf fall in autumn. A single autumn spray of urea applied to leaves on trees or on the ground reduced the amount of leaf litter which survived into the spring. A reduction in leaf litter correlates with a reduction in overwintering ascospores or PAD. The most effective treatment was a combination of ground leaf spray of urea following mulching of leaves by a slasher. These treatments have the potential to reduce the level of overwintering inoculum of scab. They could be developed for use with the monitoring method to allow the reduction of PAD in orchards which otherwise would have too much inoculum carryover to allow a reduced spring spray program. Note: these treatments are not registered for this use. Further work is needed on the autumn monitoring approach to establish thresholds for reduced spring spray programs. The ascospore maturity model also needs more data which will improve its accuracy. Further development of methods to reduce overwintering inoculum is required to optimise promising treatments. These aspects will be addressed in the proposed new project "Improving scab management in pome fruit using disease and weather monitoring systems" 2. Introduction Black spot or scab of pear (Pyrus communis L.) caused by the fungus Venturia pirina Aderhold, occurs wherever pears are grown. It can be a major problem in Victoria, where around 85% of the Australian crop of 155, 000 tonnes is grown 4

9 (Anon. 1996). The disease is a result of variable spring rainfall which can in some years provide suitable conditions for the development of severe epidemics of scab. Pear scab affects leaves, shoots, flowers and fruit, and can cause serious crop loss, especially in wet seasons when control measures are inadequate. The disease survives from one season to the next in infected leaves on the orchard floor. In spring these leaves are the source of ascospores which are considered to be the most important source of primary infection. During wet periods in spring, these ascospores are released and infect susceptible new pear tissue (Shabi 1990). Control measures are based largely on protectant fungicide sprays timed according to the developmental stage of the trees and weather conditions favourable for infection. These are supplemented by curative sprays applied after conditions suitable for infection have occurred (Williams et al, 1996), in a similar manner to sprays for apple scab (Penrose 1989). Up to fifteen sprays may be applied in some orchards during seasons with very wet spring and summer weather. In drier seasons fewer sprays are applied but the uncertainty about the likelihood of suitable infection weather leads to significant spraying in almost every season. Earlier work (Washington and Villalta, 1996) has shown that ascospores of the scab fungus appear to be the major source of overwintering infection in Victoria, though wood lesions which may allow carryover of the conidia can occur under heavy infection conditions. This earlier work also showed that ascospores are released from infected, fallen leaves from the previous year, usually from budburst until calyx stage of tree growth. Information on the critical times for ascospore release and infection are therefore vital to growers seeking to optimise timing of fungicides during spring. Methods to quantify ascospore maturation include direct microscopic examination of pseudothecia in leaves (Gadoury and MacHardy 1982, Spotts and Cervantes 1994) and spore trapping using slides and/or spore traps and wind tunnels (Brook 1966, Gilpatrick and Smith 1972). Indirect methods involving the development of mathematical models which relate ascospore maturity to accumulated degree days (MacHardy and Gadoury 1985, Spotts and Cervantes 1994). These systems have the advantage that, once developed and validated, they are less labour intensive than direct examination or spore trapping. A number of treatments have been shown to reduce the overwintering of ascospores of scab in apples, including burying leaves by ploughing (Curtis 1924), inorganic chemicals (Hutton 1954), organic fungicides (Gadoury and MacHardy 1984) and nitrogenous compounds (Burchill et al. 1965). Similarly, several reports have shown the effects of chemicals on ascospore production of pear scab (Williamson and Burchill 1970, Latorre and Marin 1982). Such methods have potential to reduce scab pressure in the spring following treatment. Orchard urea treatments after harvest are used by some pome fruit growers in Victoria following a season of high disease pressureto minimise the risk of infection in the next spring. In some well managed orchards where scab control is consistently good from year to year, the overwintering supply of ascospores is likely to be very low. There may be scope to reduce sprays under these conditions, but uncertainty about the risk leads to 5

10 application of more sprays than may always be necessary. Monitoring of potential ascospore dose (PAD) has been developed by Gadoury and MacHardy (1986) for apple scab in USA. Such a monitoring method may enable growers with consistently low disease levels to adjust (reduce) the level of control measures applied in the following spring. This has been shown to be possible for apple scab in USA (Wilcox et al 1992, MacHardy et al. 1993). This project aimed to develop a method to monitor the level of scab infection in autumn which could relate seasonal disease carryover to the intensity of control measures required in the following season. A second aim was to develop a degree day model to predict the maturity of ascospores during the spring, which would allow growers to target sprays to times of highest risk of ascospore infection. A third aim was to evaluate methods for reducing the amount of overwintering scab, which could then allow a reduction in scab control measures during the following spring. 3. PAD monitoring Introduction Gadoury and MacHardy (1986) recently developed a method to quantify the amount of overwintering ascospore inoculum, or potential ascospore dose (PAD), for apple scab {Venturia inaequalis). In a six year study, MacHardy et al. (1993) used this method to identify low inoculum apple orchards that could sustain early season fungicide sprays savings without significant risk of disease increase. This approach is based on the hypothesis that in low inoculum orchards risk of infection at the beginning of the growing season is minimal and in high inoculum orchards the risk is high. Our research aimed to test whether this method (PAD) could be used to quantify the dose of ascospores of V. pirina in pears. One outcome would be that in low inoculum pear blocks early fungicide sprays could be delayed or reduced in rate, when compared with those needed in high inoculum pear blocks. Materials and methods Orchards Sites From 1995 to 1998, in each of six commercial pear orchards in the Goulburn Valley, Victoria, a research block was established (Table 1). Three blocks consisted of Williams Bon Chretien (WBC) pear trees, two of Packhams Triumph (PK) and one of both cultivars. All blocks varied between ha in size, and were bordered by pear or apple blocks under conventional spray programs. The research block in each orchard consisted of 2-3 sub-plots in which a conventional spray program was applied and 2-3 sub-plots in which one to three early season sprays were omitted. 6

11 Table 1. Characteristics of the pear orchards used in the commercial orchards in the Goulburn Valley, Victoria, to evaluate the potential ascospore dose measurements in 1995/97. Location Orchard Cullivar' Age Number of Tree/ha Irrigation trees in research block Goulburn A WBC sprinkler Valley B WBC sprinkler C WBC/PK drippers D PK sprinkler E WBC sprinkler F PK sprinkler Cultivars WBC = Williams Bon Chretien and PK = Packhams Triumph Scab Management In conventional (early sprayed) plots, growers began the fungicide applications and selected fungicides at their own discretion, largely based on a calendar spray program and Mills infection periods. In reduced spray (delayed spray) plots, growers were asked to delay 1-3 spray applications after green tip. Fungicide applications made 2 weeks or more after the end of the ascospore season were classified as part of the secondary scab season and earlier applications were classified as primary-season applications. Infection periods were determined using thermohygrographs and mechanical leaf wetness recorders (De Wit and Belfort). Airborne ascospores were monitored using a 7-day recording Burkard spore trap in a WBC orchard at the Institute for Sustainable and Irrigated Agriculture, at Tatura (1995), or in an unsprayed WBC orchard at Merrigum (1996 and 1997). In all years, green tip was used as a date to determine how long the first spray was delayed. Disease incidence and severity were recorded 1, 5 and 8 weeks before the start of leaf fall in 1995, 1996 and 1997, respectively, by recording the number of infected leaves and lesions per scabbed leaf on 10 terminal shoots from 60 trees in each site (sites A, C, E and F) or the number of infected leaves and lesions per scabbed leaf were recorded on 10 terminal shoots from 5 and 20 trees (sites B and D). At the same time, the total leaf area of each shoot was measured at several sites with an area leaf meter (Li Cor ). At sites E (1996), A and C ( ) and D and F (1997), the leaf litter density (LLD) was assessed at leaf fall, bud break and petal fall, using the point-intercept method (Gadoury and MacHardy, 1986). Disease assessment and statistical analysis Scab development was determined each season after the end of the primary scab season (late November) and before harvest (late January). The scab incidence on leaves and fruit was determined by assessing 10 terminal shoots per tree on each of 3-4 interior trees per sub-plot and on 100 fruit per tree on each of 3-4 interior trees 7

12 per sub-plot. An analysis of variance was conducted on the data using Genstat software. Results PAD components In general, disease incidence in autumn before leaf fall was low in the commercial orchards examined, as would be expected as this was the main criterion for selecting blocks of trees for these experiments (Table 2). In 6 out of 15 blocks, scab was not observed on the leaves monitored in autumn. In 3 out of 15 blocks, more than 1% of the leaves were infected. In only one block of the 15 trials discussed here, was the number of lesions per 600 shoots was higher than 100. Scab lesions were found mostly on the lower side of the leaf, and on average about 1.1 lesions were found per infected leaf. Table 2. Incidence and severity of pear scab on terminal shoots assessed from research sites in commercial orchards in the Goulburn Valley, Victoria, in autumn 1995,1996 and Orchard/cv' v Year No. shoots No. % leaves Average assessed lesions/total shoots infected lesions/infected leaf assessed A-WBC A-WBC A-WBC B-WBC B-WBC C-PK C-WBC C-PK C-WBC C-PK D-PK D-PK D-PK E-WBC F-PK Cultivars WBC = Williams Bon Chretien and PK = Packhams Triumph Scab Management 1995/96 season. In this year scab was observed in autumn only in orchard B, with scab incidence of 1.4 % (Table 2). In the following spring, ascospores were discharged on five occasions from the 5-22 of October, (between petal fall and calyx stages of WBC trees (Table 3). Between 8-9 infection periods occurred during the primary infection 8

13 period in the four orchards studied (Table 4). Six of these infection periods occurred between green tip and petal fall stages of tree development. Three growers applied their first fungicide spray (copper) at green tip and one grower delayed his first fungicide spray (protectant plus curative) until about late finger, 16 days after green tip. In all orchards, most sprays were applied between green tip and calyx stages of tree development. No infection periods were left unprotected in the delayed spray plots, thus no conclusions can be reached regarding the relation between autumn leaf infection and need for early spring protection of infection periods. Control of fruit scab was complete in orchards A B and D (Table 5). Control of leaf scab was also excellent in orchards A and D (0.0%) but poor in orchard B, with an incidence of 1.6 and 5.6 % at the end of the primary season and at harvest respectively. Control of leaf and fruit scab was good in orchard C in which the grower applied only 2 fungicide applications around full bloom. 1996/97 season. In autumn, foliar scab was observed in four of the five orchards studied, and ranged from 0.4 to 1.9%. The first ascospores were discharged on the 5th of September and the last ascospore discharged on the 17th of November (Table 3). The majority of ascospores were discharged between green tip (5 and 12 of September) and calyx (13th and 21st of October) stages of Packhams Triumph and WBC tree development, respectively. Between 7-10 infection periods were recorded during the primary infection season in the five orchards studied (Table 4). Between 6-8 of these infection periods occurred between green tip and petal fall stages of tree development. In conventional plots during the primary infection period, growers in orchards A, B, C, D and E applied 9,1,5,1 and 12 fungicide applications respectively for the control of primary scab. All delayed sprays were applied between finger and full bloom stages of tree development. Between 1 and 4 infection periods were left unprotected in delayed spray plots. At 2 sites (A and B), this resulted in significant increase in fruit or fruit and leaf scab (Table 5). In the two blocks at site C, scab infection was very high in both early and delayed plots significant. Possible reasons for this unexpected amount of disease in both conventional and delayed plots include a high carryover of intact leaves at this site, and the possible influx of ascospores from a nearby pear orchard. 1997/98 season. In autumn, foliar scab was observed in 2 of the 4 orchards studied, and ranged from 0.1 to 0.7% (Table 2). The first ascospores were discharged on the 4th of September and the last ascospores discharged on the 29th of November (Table 3). The majority of ascospores were discharged between green tip (12th and 15th of September) and calyx (22nd and 25th October) stages of Packhams Triumph and WBC tree development, respectively. Between 8-10 infection periods were recorded during the primary infection season in the four orchards studied (Table 4). Between 5-7 of these infection periods occurred between green tip and petal fall stages of tree development. In orchard C, 9

14 the grower did not delay any spray application. The weather was very dry for the remainder of the season, limiting the potential for secondary scab development. Infection periods were not recorded during this period and growers did not apply any fungicide sprays. In orchards C, D and F, pear scab control was complete in both conventional and delayed plots. In orchard A, scab control was good, with an incidence of 0.0 and 0.3 % on terminal leaves and fruit at harvest time (Table 5). 10

15 Table 3. Relationship between tree phenology and cumulative percentage of ascospores of V.pirina discharged from pear leaves from an unsprayed orchard in the Goulburn Valley, Victoria, in spring 199S Tree Phenology^ Cumulative % Year/Date of ascospore Packham WBC discharged September Green Tip 9 September Green Tip 20 September Full Bloom 24 September Full Bloom 30 September Petal Fall 5 October Petal Fall October Calyx 11 October October October Calyx October September Green tip September Green Tip September Full Bloom October Full Bloom October Petal Fall October Petal Fall October Calyx October Calyx November September September Green Tip September Green Tip September Full Bloom October Full Bloom October Petal Fall October Petal Fall October Calyx October Calyx November 100 Cultivars WBC = Williams Bon Chretien and PK = Packhams Triumph Ascospore trapping was conducted with a 7-day recording Burkard spore trap in a WBC research block at ISIA (1995) and in an unsprayed WBC orchard at Merrigum ( ). Cumulative percentage are shown from the first ascospore discharge and the last discharge for the season. 11

16 Table 4. Number of fungicide applications and infection periods recorded in the primary infe season at research sites in commercial orchards in the Goulburn Valley, Victoria, 1995,1996 a Year Primary infection period Fungicide sprays Orchard Cultivar Infection periods liarly 1995/96 A WBC 9 (M,H,M,H,H,L,H,Ma,L) B WBC 8 (M,H,M,H,H,L,H,M) C PK 9 (M,H,M,H,H,L,H,Ma,L) D PK 9 (H,M,H,M,H,H,L,Ma,M) Delayed /97 A WBC 7 (M,L,M,L,L,H,M) B WBC 8 (M,L,Ma,M,L,L,H,M) C WBC/PK 7 (M,L,M,L,L,H,M) D PK 10(M,Ma,Ma,Ma,M,L,M,H,L,Ma) E WBC 8 (M,L,Ma,M,L,L,H,M) 9 8(c) ,5, (c) 1997/98 A WBC 8 (L,L,Ma,H,M,H,L,M) C WBC/PK 10(L,H,L,M,H,L,M,H,M,M) D PK 10 (L,LMa,L,M,H,M,H,M,L) F PK 10 (L,L,Ma,L,M,H,M,H,M,L) 8 7(c) ,7,8 (c 8 7(c) Cultivars WBC = Williams Bon Chretien and Pk = Packhams Triumph. B In sites B and E, spraying early in the season was conducted on an alternate row system. The severity of the infection periods as per Mills & Laplante (1951), L = light; M = moderate and H = heavy. Ma = the marginal perio for ascospores as per MacHardy & Gadoury(1989). In 1996/97, site B records used for site E (7 km); in 1997/98, site D for orchard F (5 D Number of fungicide sprays applied during the ascospore release period in early and delayed plots; (c) = the first spray delayed was co E Unprotected infection periods (UIP) in the remainder of the season and outside the period of peak ascospore discharge other than earl 12

17 Table 5. Incidence of pear scab on leaves and fruit, at the end of the ascospore season and at harvest, previous season and unprotected infection periods due to the delay of the first fungicide sprays in com Victoria, 1995,1996 and Year Leaf Infection Days between green tip Tree phenology scab Periods and appl cation at application previous between early Early year and delayed spray Site Cultivar (%) Early Delayed Earh Delayed Nov Jan 1995 A WBC 0.0 (0) GT 0.0 B WBC 1.4(22) 0 0. GT C PK 0.0 (0) LF D PK 0.0 (0) GT A WBC 0.4(16) GT F B WBC 1.4(23) GT EFB C WBC 0.7 (47) 1,2-9,20 F,FB 1.1 C PK 0.7 1,3-16,27 LF,FB 0.3 D PK 0.0 (0) 1,2,4 0 16,21,25 GT F,EFB,FB E WBC 1.9(150) GT LGT A WBC 0.7 (34) GT EF C WBC 0.1(31) 1. 8 GT - C PK GT D PK 0.0 (0) 2,3 0 3,9 GT GT,EF,F F PK 0.0 (0) GT EF Percent leaves with scab lesions measured on terminal shoots after harvest. Values in parenthesis indicate number of lesions / number of shoots The severity of the infection periods as per Mills & Laplante (1951)and MacHardy & Gadoury(1989), Ma = the marginal, L = light; M = modera number of infection periods unprotected in the different sub-plots. Early = first fungicide application in grower's program. Delayed = fungicide application delayed days after green tip as per treatment/sub-plot. D Tree phenology: GT = green tip, LGT = late green tip, F = finger, EF = early finger, LF = late finger, EFB = full bloom, FB = full bloom, LFB = E Mean percent of infected leaves in terminal shoots and mean percent of fruit infected. Two assessments conducted, the first at the end of the asco * = significant increase with delayed spraying (P<0.05). 13

18 Between 0 and 3 infection periods were left unprotected in delayed spray plots. There was no disease observed on leaves in either early or delayed plots at any site, and only a low level on fruit at site A. At site A there was no significant difference between disease in early or delayed plots. Discussion A method has been developed to monitor commercial pear orchards for autumn scab. This method will give information about the level of disease in the autumn and the potential for high (or low) disease in the following spring. Such a method will allow a rational decision about applying measures such as post-harvest pre-leaf fall sprays, or other management practices such as leaf mulching and leaf sweeping which can reduce overwintering inoculum. Results emphasise the need to examine the underside of leaves, as this is where most scab lesions were found. The time of sampling is also critical; if sampling is too close to leaf fall then senescence of leaves will make detection of scab very difficult; on the other hand sampling too early may allow late leaf infections to occur after monitoring if shoots continue growing in late autumn, resulting in an under-estimate of disease. These results also show that of the nine trials where a delayed spray was applied leaving an unprotected infection period and where disease could be compared with disease resulting from a conventional spray program, only 2 of the 9 resulted in a significant increase in scab with an unprotected infection period. Two others had very high disease in both early and delayed plots. The remaining 5 had disease levels no different from the conventional (early) spray program. The results also indicate that leaf litter is reduced from leaf fall to bud-burst, but in some orchards and some years this reduction is very small. Where little reduction of leaf litter occurs, their is greater potential for more ascospores to survive and be available for infection in the spring. This occurred at two sites in the second season, and may have been a factor in the high disease which developed in all plots at this site. Although this data supports the hypothesis that in orchards with a low PAD the spring fungicide program can be delayed to leave one or two early infection periods unprotected, further information is required to explain the apparent failure of the monitoring threshold to allow this in some instances. A major difficulty with this work is to get suitable co-operators who have consistently low disease and who are prepared to take the risk and inconvenience of setting up and treating a replicated trial with delayed or conventional sprays on part of their property. Each autumn requires more sites to be monitored than are trialed in spring because some co-operators drop out by the time spring arrives, or are unable to treat plots in the required manner. This unfortunately means that a lot of time is spent without obtaining any useful data to test the relationship between autumn leaf scab and potential to reduce early spring sprays. 14

19 4. Ascospore maturity model Introduction Ascospores are the primary source of inoculum for pear scab. Spore trapping (Washington and Villalta 1996) has shown that ascospores are available between green tip and calyx or even later. As seasonal conditions can affect their rate of maturity, it is useful to have more precise information about their maturity to help time sprays. Methods to quantify the maturation of ascospores include microscopic examination of leaves during spring and spore trapping. These methods are labour intensive and often cannot provide timely information which can benefit grower decision making in the current season. The development of an ascospore maturity model based on accumulated degree days has the advantage that, once validated, it is less labour intensive than direct examination or spore trapping, and can provide a quick prediction of ascospore maturity which can be used directly by growers to assist the optimal timing of fungicide sprays. Materials and Methods Ascospore monitoring in the field Airborne ascospores were monitored during spring to early summer in two different pear growing regions in Victoria, Australia, with a 7-day Burkard volumetric spore trap. In southern Victoria, airborne ascospores were monitored during 1992, 1993 and 1997 in a 55-yr-old Packhams Triumph pear orchard at Strathewen and during 1994 in the Institute for Horticultural Development, at Knoxfield (approximately 60 km from Strathewen). In northern Victoria, ascospores were monitored during 1994 in the centre of a 47- yr-old 0.5-ha research orchard at the Institute for Sustainable and Irrigated Agriculture, Tatura and during 1996 and 1997 in a 65-yr-old WBC unsprayed pear orchard at Merrigum (approximately 15 km from Tatura). Three to four m of the orchard floor under the traps was covered with infected pear leaves held by wire-meshes or bird netting to hold leaf litter in place near the spore traps. Orchard air was sampled continuously from late August-early September to early December. The spore trap tape was changed weekly and the number of ascospores deposited on the surfaces were counted at 240X with a microscope. Counts were made during each wet period initiated by rain or dew, and several hours before and after each wet period. Air temperature, rainfall, leaf wetness periods and relative humidity were monitored daily at each site. The dates of pear growth stages were recorded each season for cultivars Packhams Triumph and WBC at all sites. 15

20 Microscopic examination of pseudothecia Scab infected pear leaves (WBC) were collected and overwintered in an unsprayed orchard at Merrigum, northern Victoria, in 1996 and Five to ten leaves were removed every 7-10 days from each site prior to green tip until early December. Ascospore development was assessed by examination of crushed pseudothecia (Gadoury and MacHardy 1982). Fifteen pseudothecia were removed from the sample of leaves, crushed and examined microscopically and the asci classified as immature, mature and empty. The average number of asci per pseudothecium was determined for each sampling date and the percentage of asci in each maturity class was corrected for undeveloped asci in early spring and exclusion of disintegrated asci in late spring. The cumulative proportion of matured ascospores was calculated. Prediction model for ascospore maturity Ascospore discharge data from the field were transformed using logit transformation. Cumulative degree-days were calculated from daily minimum and maximum temperature data (Baskerville and Emin 1968). A base of 0 C was selected as the temperature most closely related to ascospore maturity. Degree-day and ascospore accumulations were then related using standard linear regression analysis. Degree-day accumulations were started at the date when the first ascospores were discharged. Preliminary validation of the southern Victoria model relating ascospore maturity to degree-days was conducted in 1997 at Strathewen with field data and at Knoxfield with laboratory data. Validation of the northern Victoria model was also conducted in 1996 and 1997 at Merrigum and in 1997 at Cobram with laboratory data. Ascospore maturity at these sites was compared with ascospore maturity as estimated by the model. Results Ascospore maturity model Spore trapping. In southern Victoria, the first ascospores discharged from Packhams Triumph leaves in the field were observed before or at green tip in early September, with the exception of spring 1992 when trapping began 2 weeks later. Ascospores continue to be discharged over a period of 2-3 months from green tip to mid November. Most ascospores were trapped before the calyx stage of Packhams Triumph. The percentages of the seasonal total of ascospores trapped before calyx were 87.8, 98.2, 95.0 and 93.8 % in 1992, 1993, 1994 and 1997 respectively. In northern Victoria, the first ascospores discharged from WBC infected leaves in the field were observed 8-10 days before green tip in early September, but at or before green tip of Packhams Triumph trees growing in the region. Ascospores continue to be discharged over a period of 3 months from green tip to late November. The percentages of the seasonal total of ascospores trapped before calyx of WBC were lower than the corresponding percentages for Packhams 16

21 Triumph in southern Victoria. The percentages were 59.2, 78.5 and 88.7 % in 1994, 1996 and 1997 respectively. Microscopic examination. The first mature ascospores detected in pseudothecia were observed before or at the green tip stage of WBC trees in samples overwintered at Merrigum in northern Victoria in 1996 and The percentage of asci with mature ascospores reached a maximum during the full bloom period in both years. Mature ascospores in about 2-3 % of the asci were shrunken and stained brown in examinations in November. In 1997, a late examination in early December showed a few olive coloured ascospores and shrunken brown ascospores. The first mature ascospores from leaves overwintered at Knoxfield and Cobram in 1997 were also observed before the green tip stage of Packhams Triumph and WBC trees respectively (Figure 1). The percentage of asci with mature ascospores also reached a maximum during the full bloom period in both sites (Figure 1). However, the proportion of empty asci increased more rapidly in samples from Knoxfield than in samples from Merrigum and Cobram. Ascospore maturity values were corrected on the basis of the maximum number of asci per pseudothecium. The two year average for V. pirina in all sites was 121, compared with 143 reported for V. pirina in Oregon (Sports and Cervantes 1984). Logit transformations performed on each year and the combined cumulative ascospore counts data for the two locations showed that ascospore maturity was closely related to accumulated degree-days with base temperature of 0 C. Only 7 of 103 and 6 of 83 data observations fell outside the 90 % confidence intervals for southern Victoria and northern Victoria respectively (Figure 2). The mean number of cumulative degree-days of combined years for 50 % and 95 % ascospore discharge were approximately 200 and 400 for southern Victoria and 350 and 650 for northern Victoria. 17

22 VI VI < J U U VJ 3-Sep 10-Sep 16-Sep 24-Sep 7-Oct 21-Oct DATE 4-Nov 11-Nov vi vi < U z u VI < B SV 0GV NV 0 3-Sep 10-Sep 16-Sep 24-Sep 7-Oct 21-Oct 4-Nov 11-Nov DATE Figure 1. Corrected assessments of pear scab ascospore maturity and discharge for southern Victoria (SV), Goulburn Valley (GV) and Cobram (NV) sites, during spring Fifteen pseudothecia of V.pirina were crushed and examined at approximately 7-15 days intervals and the number of mature asci (A) and empty asci estimated (B). 18

23 10 c 3 c3 1) 8. > o o '5b o I, I I I. I. I. I Degree-days (0 C base) II 1 a o D. > E 3 O o 3 9 B cr u 0 CD 0 osaea A 0, ' O A / coo o ocnq^ O 0 o >^co B 0 - O m j r 0 00 CO O o A 1997 GV 1997 NV 1996 GV I I I. I. I Degree-days(0 C base) Figure 2 Relation between cumulative degree-days and maturity of ascospores of pear scab beginning at detection of the first mature ascospores in southern Victoria (A) and northern Victoria (B). Cumulative percentage of ascospores discharged were transformed using logit transformation and analysed by linear regression analysis. In southern Victoria regression is significant at P = 0.01, R 2 = In northern Victoria regression is significant at P = 0.01, R 2 =

24 Discussion Ascospore maturity model In both southern and northern Victoria, ascospores were released in the field before or at green tip in early September and were released until mid November. Most ascospores were discharged between green tip and calyx stages of tree development. Our results are in agreement with those of Spotts and Cervantes (1994), who found (over a period of three years) that the first mature ascospore of WBC (Bartlett) pears were observed at or before bud swell. This pattern of discharge over three months is similar to the one observed in New Zealand (Curtis, 1921) and Australia (Washington et al. 1998). Ascospore maturity was described as a function of cumulative degree-days with a base temperature of 0 C. The description of the relationship between V.pirina ascospore maturity and degree-day accumulation for southern Victoria lacked the initial lag observed for V inaequalis (Gadoury and MacHardy 1982, MacHardy and Gadoury 1985,) but was similar to the one observed for V. pirina in Oregon (Spotts and Cervantes 1994). The description of the same relationship for northern Victoria was similar to the one observed for V. inaequalis. In our study, a model developed for southern Victoria estimated that 50 and 95 % of the ascospores were matured after 200 and 400 degree-days respectively. The model developed for northern Victoria estimated that 50 and 95 % of the ascospores were mature after 350 and 650 degree-days. Our model for southern Victoria predicted that ascospores would be matured and released much faster than in northern Victoria. Spotts and Cervantes reported that approximately 50 and 95 % of the V. pirina ascospores had matured in asci after an accumulation of 200 and 700 degree-days respectively in Oregon. A slower rate of ascospore maturity and release in northern Victoria compared to maturity and release in southern Victoria resulted in a shift of the entire curve. This large temporal shift may have been related to prolonged periods of dryness and lower moisture conditions in spring which interrupt maturation. In northern Victoria, periods of dry weather that last for some days are not uncommon during spring months. In contrast, southern Victoria has a relative mild and rainy spring which might be conducive to early peak in ascospore release. In the laboratory study, mature ascospores were observed before or at green tip in late August to early September and were present until early November in three locations in southern and northern Victoria and until early December in one location in northern Victoria. However, airborne ascospores were not detected in early December at the same site where samples for laboratory examination were overwintered. Peak ascospore maturation occurred during the bloom period. This pattern of maturity over four months is similar to that described in Oregon (Spotts and Cervantes 1994) and Chile (Latorre et al. 1982). Brown, shrunken ascospores were observed in about 2-3% of the asci in late November to early December. Spotts and Cervantes (1984) reported that mature ascospores of V.pirina remained in 5% of 20

25 the asci in early June (Oregon) and were not discharged. These methods to forecast ascospore maturity can help growers to schedule fungicide sprays to critical times when most ascospores are ready for discharge. They can also assist growers by determining when most ascospores are discharged and therefore when spray rates and timing can be relaxed, provided that no leaf or fruit infection has already occurred which will provide conidia for secondary infection. Assessment of crushed pseudothecia, and observations from Burkard traps is retrospective and labour intensive, thus a method such as the degree day model is more suitable for a predictive warning of ascospore maturity. 5. Reduction of overwintering inoculum The objective of this study was to assess the potential of urea, hydrated lime and curative fungicides, used as post-harvest applications, to reduce ascospore inoculum. The study also assessed the effect of urea on leaf decomposition. Materials and Methods Chemicals used. Ascospore production after autumn post-leaf fall dip treatment was investigated over three years 1995, 1996 and The chemicals used in 1995 were urea (Liqui Fert LB, 46% NPK w/v), hydrated lime ( Limil, approximately 82% calcium hydroxide and 8.5% magnesium oxide, David Mitchell Limited, Lilydale, Victoria), myclobutanil (Systhane, Agrevo Pty Ltd), flusilazole (Nustar, Du Pont Ltd.), fenarimol (Rubigan, Dow Elanco), hexaconazole (Anvil, ICI Australia Operations Ltd), benomyl (Benlate, Du Pont Australia Ltd.), fluquinconazole (Castellan, Agrevo Pty Ltd.) and pyrimethanil (Scala, Agrevo Pty Ltd.). In 1996 and 1997 the chemicals used were urea, hydrated lime, myclobutanil and benomyl. In 1996 the effect of urea on leaf decomposition was investigated following autumn post-leaf fall dip treatment and autumn pre-leaf-fall spray. The effect of urea on leaf litter density was also investigated in orchard trials in Ascospore productivity Leaf dipping experiments. Three experiments were conducted from 1995 to The 1995 experiment was conducted at the Institute for Horticultural Development (IHD), Knoxfield Victoria. In May 1995, heavily infected leaves were collected from beneath pear trees (Pyrus sp.) at early leaf fall. The 1996 and 1997 experiments were conducted in an organic orchard in the Goulburn Valley, Victoria. In June 1996 and 1997, heavily infected leaves were collected from beneath Packham trees at late leaf fall. On the day of collection, one hundred leaves were placed in onion-mesh bags (100 x 40 cm) and dipped in various chemical suspensions. All treatments were replicated 21

26 three times in a randomized complete block design with single onion-mesh bags as replicates. Each chemical suspension was prepared in 5 litres of water and the treatments were immersed for about 30 seconds. Control leaves were dipped in distilled water. The bags containing the treated leaves were overwintered on a grass lawn at IHD in 1995 and on the orchard floor between trees in 1996 and Tree-spray experiments. The field experiments were conducted in a heavily infected organic orchard on five year old Packham trees in the Goulburn Valley, Victoria. The trees had received no previous fungicide sprays before the experiment. More than 90% of the leaves were infected with scab prior to the application of treatments. Three treatments were replicated three times in a randomized complete block design with single-tree plots. In April 1996, the treatments were applied until runoff with a 40 litre backpack sprayer with approximately 3 to 4 litres used per tree before the start of leaf drop. Control trees were sprayed with water. After leaf drop, approximately one hundred infected leaves were collected from each tree, bagged and overwintered on the orchard floor between trees as described previously. This experiment was repeated in May Leaf sampling and ascospore determinations. In all years, leaves representing all treatments were sub-sampled in early spring to determine the production of ascospores. Fifteen (1995, 1996) or twenty (1997) leaf discs were cut from samples and the leaf area of the leaf discs was determined with a leaf area meter (model LI-3000, LiCor, Inc., Lincold, NE). The leaf discs were placed between wire-mesh in standard weather boxes to prevent wetting by rainfall and release of ascospores. The number of ascospores produced per square centimetre of leaf disc was determined by counting the number of ascospores liberated in water (Hutton and Burchill, 1965). Assessments were conducted three times in spring 1995 and twice in 1996 and The results are averaged over all assessment times for each experiment. Additional leaf discs from each treatment were tested for ascospore viability by discharging ascospores onto water agar in Petri dishes. An ascospore was considered germinated if the germ tube was greater that one half of the length of the spore. Leaf decomposition study Dipping experiment. In May 1996, pear leaves were collected from the orchard floor of WBC pear trees at the late leaf fall stage in two orchards in the Goulburn Valley, Victoria. On the day of collection, one hundred leaves were placed in onionmesh bags and then immersed in solutions of 2% or 5% urea for approximately 30 seconds. In another treatment, leaves were cut into 4-5 pieces before immersed in 2% urea. Control leaves were dipped in distilled water. All treatments were replicate three times in a randomized complete block design with single bags as replicates. After dipping, the leaves were allowed to drain for about 1 hour before bags containing the leaves were overwintered on the orchard floor. 22