New Zealand Journal of Crop and Horticultural Science ISSN: 0114-0671 (Print) 1175-8783 (Online) Journal homepage: http://www.tandfonline.com/loi/tnzc20 Fusaria and Fusarium mycotoxins in leaves and ears of maize plants 2. A time course study made in the Waikato region, New Zealand, in 1997 D. R. Lauren & M. E. Di Menna To cite this article: D. R. Lauren & M. E. Di Menna (1999) Fusaria and Fusarium mycotoxins in leaves and ears of maize plants 2. A time course study made in the Waikato region, New Zealand, in 1997, New Zealand Journal of Crop and Horticultural Science, 27:3, 215-223, DOI: 10.1080/01140671.1999.9514099 To link to this article: https://doi.org/10.1080/01140671.1999.9514099 Published online: 22 Mar 2010. Submit your article to this journal Article views: 85 View related articles Citing articles: 13 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalinformation?journalcode=tnzc20 Download by: [46.3.203.125] Date: 30 November 2017, At: 14:22
New Zealand Journal of Crop and Horticultural Science, 1999, Vol. 27: 215-223 0014-0671/99/2703-0215 $7.00 The Royal Society of New Zealand 1999 215 Fusaria and Fusarium mycotoxins in leaves and ears of maize plants 2. A time course study made in the Waikato region, New Zealand, in 1997 D. R. LAUREN The Horticulture and Food Research Institute of New Zealand Ltd Ruakura Research Centre Private Bag 3123 Hamilton, New Zealand email: dlauren@hort.cri.nz M. E. DI MENNA New Zealand Pastoral Agriculture Research Institute Limited Ruakura Research Centre Private Bag 3123 Hamilton, New Zealand Abstract The patterns of fungal infection and mycotoxin contamination in leaf and ear sections of plants of two maize (Zea mays L.) hybrids, one resistant to mycotoxin accumulation under New Zealand conditions (Pioneer 3902 (P3902)) and one less so (Pioneer 3751 (P3751)), have been measured. Sampling commenced early in the season, well before ear and tassel formation, and continued until harvest. A number of fungi were isolated, the most common overall being Fusarium. Most common in leaf fractions were Epicoccum, Fusarium, and Alternaria, whereas in ear fractions the most common were Fusarium, Penicillium, Cladosporium, and Mucor. The most common fusaria isolated from leaf fractions were the toxigenic species F. crookwellense and F. graminearum. These species were evident from late February although other, non-toxigenic, species were present in leaf axils from early January. For ear fractions the most common species were F. graminearum, F. crookwellense, and F. subglutinans. Fusarium infection was evident in ears of P3902 from March to April, although heavy infection by the toxigenic species tended to occur later H98061 Received 10 December 1998; accepted 3 May 1999 towards May-June, especially for the basal ear fractions. For P3751 ear infection commenced in May, and then was predominantly by toxigenic species. Mycotoxins were found in most plant fractions measured, especially as the plants aged. The toxins found reflected the particular toxigenic Fusarium species present in the fraction. The highest mycotoxin concentration in a leaf fraction was 16.6 mg/ kg of zearalenone (ZEN) in an upper leaf axil sample. Nivalenol (NIV) was also found at up to 7.4 mg/ kg in leaf axils. The most contaminated ear fraction was the rachis, with over 40-95 mg/kg of ZEN, NIV, or deoxynivalenol (DON) at various times. The highest concentration found in kernels was 3.8 mg/kg of DON found in apical kernels of P3751 two weeks before harvest. The results suggest that the mechanisms of maize infection by Fusarium in New Zealand may not be controlled by factors at silk emergence but rather by later season events such as high rainfall and warmer temperatures. Keywords maize; mycotoxins; Fusarium; F. graminearum; F. crookwellense; infection; plant fractions INTRODUCTION In 1995 and 1996, time-course studies were made of natural infection by fusaria and the resulting contamination by Fusarium mycotoxins, nivalenol (NIV), deoxynivalenol (DON), and zearalenone (ZEN), in various fractions of whole maize (Zea mays L.) plants growing in New Zealand (di Menna et al. 1997). The 1995 study covered the period April-June, and examined three hybrids grown in the Manawatu region, the most southern and coolest maize-growing area of the country. The eight fractions examined were kernels and rachis from the apices and bases of ears, peduncles, leaf axils, and upper and lower parts of stalks. The 1996 study covered the period January-May, and examined two commercial hybrids grown near Hamilton airport in the Waikato region, a warmer region and the major
216 New Zealand Journal of Crop and Horticultural Science, 1999, Vol. 27 maize-growing region of New Zealand. The 11 fractions examined were tassels, husks, kernels, and rachis from the ears, peduncles, lower and upper leaf axils, lower and upper leaf blades, and lower and upper stalks. Findings from the two studies were similar for equivalent sampling times and plant fractions, and were also consistent with earlier work on Fusarium infection of Waikato maize by Sayer (1991). It was found that Fusarium infection occurred in leaf axil fractions by early January, was detectable in leaf blades and stalks by late January, in rachis and peduncle during February, and in kernels by early April. Fusarium spp. isolated from apical kernels and rachis sometimes differed from those isolated from basal kernels, rachis and peduncle from the same ears, suggesting that infection could occur both at the exposed apices and systematically from the infected stem. There was no evidence to support the theory that infection by the silk channel was the primary source of infection (e.g., Reid & Sinha 1998 and references therein). It was also observed that the Fusarium mycotoxins NIV, DON, and ZEN were detectable in the various fractions only some time after infection by fusaria was demonstrable. At the Manawatu site, where F. crookwellense and F. culmorum were the predominant toxigenic species isolated, NIV was found in high concentrations relative to DON, and was the most frequent contaminant in the kernels. At the Waikato site where F. graminearum and F. subglutinans predominated, NIV and DON occurred at similar levels. The relative incidence of these trichothecenes is consistent with the known toxigenic potential of New Zealand isolates of the dominant species recovered (Lauren et al. 1991). In both trials, the highest levels of NIV and DON were in rachis and peduncle, which had up to 100 times more than in most other fractions. ZEN was found most consistently in leaf axils and blades at both sites, and highest levels occurred in rachis followed by lower leaf axils and blades. The results of these earlier studies by di Menna et al. (1997) showed a similar trend to the observations of Chulze et al. (1996) with Fusarium and fumonisins in naturally-contaminated Argentinian corn. They found that fumonisin contamination in kernels at times throughout the season reflected the relative infection by fumonisin-producing species, F. moniliforme and F. proliferatum, compared with the non-producing F. subglutinans. Most other studies of maize infection mechanisms have used artificial inoculation as a primary means of infection. One such recent intensive investigation by Munkvold et al. (1997) with F. moniliforme concluded that kernel infection could occur via systemic development from the stalk but that silk infection was more important. However, inoculation studies may not truely reflect natural events. Indeed, maize hybrid susceptibilities to natural infection by F. graminearum are reported to be poorly matched by ratings from inoculation trials (Miller 1994). Also, hybrid P3902 which is described as "susceptible" (to Gibberella ear rot) based on inoculation tests (Reid et al. 1996; Reid & Sinha 1998) has been found in New Zealand trials to be among the least susceptible to Fusarium mycotoxin accumulation from natural infection (Genetic Technologies Ltd 1997). The current trial was set up to obtain a third season's data and further insights on the development of natural infection of maize plants by Fusarium and contamination by Fusarium mycotoxins through the growing season. Apical and basal kernels and rachis were examined separately to gain further information on the mechanisms of kernel infection, and of the timing of invasion by different Fusarium species, whether toxigenic or non-toxigenic. Two hybrids were examined, one considered resistant, and one less resistant, to natural accumulation of Fusarium mycotoxins under New Zealand conditions. These two hybrids were chosen with the aim of highlighting differences in infection patterns that might result in subsequent differences in mycotoxin concentrations. MATERIALS AND METHODS The hybrids for the trial were Pioneer 3902 (P3902), a short season hybrid rated resistant to mycotoxin accumulation under New Zealand conditions, and Pioneer 3751 (P3751), a mid season hybrid rated less resistant (Genetic Technologies Ltd 1997). These were planted on 4 November 1996 in blocks of six rows by 100 m long at a site 1 km from that used for the 1996 study. Maize had been grown on the site in the previous four seasons. Samples were collected from the centre four rows, by randomly selecting plants at least 5 m apart. Samples for whole plant study Collection commenced on 9 January 1997 when the plants were typically 1.2 m (P3902) to 1.5 m (P3751) high and continued each 2 weeks until 3 July, except for a 3-week gap from 15 May to 5 June. On each collection day, three randomly selected plants of each hybrid were cut at ground level and
Lauren & di Menna Fusaria and Fusarium mycotoxins in maize 217 transported (within 1 h) to the laboratory, hi the laboratory on the day of collection the three plants of each hybrid were dissected into eight fractions. Fractions collected were: lower leaf axils (two from the lowest leaves of each plant cut 4 cm above and below the ligule), upper leaf axils (from the two leaves subtending the two ears, developing and abortive, cut as for the lower axils), lower leaf blades (segments 20 cm long above the cut edges of the axils), upper leaf blades (cut as for the lower blades), basal kernels from the lower third of the developing ear of each plant, apical kernels from the upper third of ears, basal rachis from which the basal kernels had been removed, and apical rachis from which the apical kernels had been removed. For the fractions other than kernels, the plant segments were cut into pieces of 5 mm side before mixing. Equivalent fractions of each hybrid were bulked for each collection day. Each bulk fraction, including kernels, was mixed thoroughly then split into two equal parts. One part was used for culturing and the other frozen (-20 C) for subsequent mycotoxin analysis. Although bulking of samples precluded any statistical evaluation of the results, this strategy was necessary to allow the processing to proceed without delays which could result in concerns about sample integrity. Not all plant fractions were available on all sampling days. Tasseling and silking was first evident for both hybrids on 6 February, and ears were too small to sample until 20 February. Kernels had to be cut from the rachis on 20 February and 6 March, but after this they could be removed by hand. Ears drooped as they matured and wrappings fell away from the apices. Lower leaves were senescent by 20 March and had disintegrated by 15 May, therefore no samples of lower leaf axils or blades were obtained from that date. Upper leaves were senescent by 17 April. Culture methods These were as described by di Menna et al. (1997). Half of each fraction was surface sterilised by immersion in 10% commercial bleach (31.3 g sodium hypochlorite/litre) for 5 min followed by a sterile distilled water rinse. One hundred pieces of each fraction were inoculated on plates of potatoglucose-chlortetracycline agar at 5 pieces/plate, and these incubated at ambient temperature in daylight for 3-5 days. The shorter incubation period was used when Mucor or Nigrospora was likely to overgrow the plates. Developed fungal colonies were identified to genus and recorded, and Fusarium colonies not overgrown were subcultured for species identification, up to a maximum of 30 per fraction. Where there were more than this, only those on the required number of plates to yield 30 colonies per fraction were subcultured. All non-sporing colonies were subcultured as some were fusaria which were not sporing on the isolation plates. Fusarium identifications were made by the criteria of Burgess et al. (1994). The degree of infection of each plant fraction by the different fungal genera was expressed as a percentage for the proportion of the 100 pieces plated out which showed that genus. The percentage Fusarium infection was further split into the relative percentage infection by each species isolated. Analysis for mycotoxins Analysis for NIV and DON used a method adapted from that described by Lauren & Ringrose (1997). Briefly, the samples were extracted with acetonitrilemethanol-water (85:5:15), and aliquots subjected to cleanup through an alumina-carbon/cation exchange column, followed by hydrolysis to convert all trichothecenes to parent alcohols. After neutralisation, the extracts were passed through a 216 Trichothecene Charcoal-Column (Romer Labs, MO, United States). These cleaned-up extracts were then used for analysis by HPLC with UV detection at 245 run. Analysis for NIV and DON used a Zorbax SB-C8 column held at 35 C with a mobile phase of methanol-water (12:88). Under these conditions NTV in some samples was subject to interference, and in these instances NIV was analysed using a Zorbax SB-Phenyl column held at 35 C with a mobile phase of methanol-water (1:99). For zearalenone analysis, aliquot extract solutions were analysed directly by HPLC without any cleanup treatment using a Zorbax ODS column held at 35 C with a mobile phase of methanol-water (60:40) and fluorescence detection (excitation 280 nm, emission 460 nm). Analysis was commenced on samples from the latest collection days and continued back to previous collections until at least two consecutive samples showed no contamination. Concentrations are expressed on a dry weight basis. RESULTS Over 15 genera of fungi were isolated from the various plant fractions. The most common overall was Fusarium which was either the main or a major genus from all fractions except the upper blades (of both hybrids) in which Epicoccum and Alternaria
218 New Zealand Journal of Crop and Horticultural Science, 1999, Vol. 27 were the most common. Fungi isolated from the upper axils were predominantly Epicoccum and Fusarium followed by Alternaria whereas from the lower blades were mostly Epicoccum and Fusarium. Fusarium predominated in the lower axils. The general pattern for leaf fractions was similar for both hybrids as shown in Table 1. In the ear fractions (kernels and rachis) the two hybrids differed. For P3751 Fusarium was dominant in all fractions (38-63% of all fungi isolated from individual ear fractions) with contributions also from other fungi as indicated in Table 1. Penicillium, when it occurred was primarily in apical ear fractions, and then mostly in samples from the last sampling date (3 July) when infection by Fusarium was very low. In all P3902 ear fractions, Fusarium (27-39% of fungi isolated from individual ear fractions) and Penicillium (27-51%) were most common. Generally the two fungi occurred together, but there were some samples in which one or other dominated. For example, in apical fractions of 15 May Fusarium were 84-92% of all fungi isolated whereas on 5 June Penicillium made up 93-100% of fungi. The time course results for percentage Fusarium infection and the Fusarium mycotoxin content for the eight plant fractions of each hybrid are shown in Fig. 1 (for hybrid P3902) and in Fig. 2 (for hybrid P3751). The degree of Fusarium infection is shown both as total Fusarium infection and as percent infection by the three most common toxigenic species present (F. crookwellense, F. culmorum, and F. graminearum). The mycotoxin content is expressed as the sum of trichothecenes NIV plus DON, and as zearalenone. There was little Fusarium infection of leaf axils and blades at the first sampling on 9 January but this increased during February so that by March the Table 1 Mean percentages of isolates of predominant mould genera recovered over the entire sampling period from all ear (kernels and rachis) and leaf (axils and blades) fractions of maize (Zea mays) hybrids P3902 and P3751. Alternaria Cladosporium Epicoccum Fusarium Mucor Nigrospora Penicillium Ear fractions 3902 3751 1 10 4 36 6 4 37 1 15 6 45 14 9 7 Leaf fractions 3902 3751 16 8 29 18 6 5 <1 14 9 29 23 8 6 <1 lower axil pieces in particular were highly infected (see Fig. 1 and 2). The incidence of toxigenic fusaria was generally below the total Fusarium, although by March or April (depending on the plant fraction) a significant proportion of the fusaria isolated from the leaf fractions were toxigenic. As the sampling method was destructive, a steady rate of increase in infection was not to be expected, and was not seen. Generally, infection rates were higher in axils than in blades and in lower fractions than in upper. There was little difference in infection rates between the two hybrids. In kernels and rachis total Fusarium infection rates were low in February and March but increased in April in hybrid P3902 and in May in P3751 (see Fig. 1 and 2 respectively). These events occurred when the moisture contents of the respective hybrids had dropped to 30-40% in the whole ears and 25-35% in the kernels; P3902 reached these levels c. 4 weeks before P3751. Infection rates of apical and basal portions of ears were generally similar, especially for P3751, but there were some samples with noticeable differences. For example, in hybrid P3902 there were marked differences in infection levels between apical and basal ear sections on 17 April (15-39% in apical and 1-6% in basal) and 5 June (0% in apical and 10-27% in basal), whereas in hybrid 3751 there was a large difference between apical sections (2-10% infection) and basal sections (71-77% infection) on 3 July. Because the sampling method was destructive, these differences were representative only of the plants collected on the particular sampling day, and because each fraction was a mixture from three ears, similar differences in individual ears on other days may have been obscured. The infection patterns for the apical and basal kernel fractions reflected the infection patterns of their respective rachis sections. The relative proportions of toxigenic fusaria compared with total fusaria showed different results for the two hybrids. For P3902, although the total Fusarium infection level was generally appreciable from early April, on many dates the proportion of toxigenic Fusarium was much less or zero. This was especially so for the basal fractions where infection by toxigenic species was very low until early June. For hybrid P3751, infection by the toxigenic fusaria was generally a major portion of the total fusaria. This was especially so for the basal fractions where, with this hybrid, infection by mostly toxigenic species increased from early May. A total of 14 Fusarium species was recovered from the various plant fractions. Of the 2367 isolates
Lauren & di Menna Fusaria and Fusarium mycotoxins in maize 219 Fig. 1 Incidence of infection by total Fusarium (% fusaria) and common toxigenic Fusarium (% toxigenic) and contamination by Fusarium mycotoxins nivalenol plus deoxynivalenol (NIV + DON) and zearalenone (ZEN) in fractions of plants of maize {Zea mays) hybrid P3902 sampled between January and July 1997. (Note that the mycotoxin concentration scales differ for the eight subtractions.)
220 New Zealand Journal of Crop and Horticultural Science, 1999, Vol. 27 ^^NIV+DON - -x - - % fusaria ESS3ZEN A % toxigenic Fig. 2 Incidence of infection by total Fusarium (% fusaria) and common toxigenic Fusarium (% toxigenic) and contamination by Fusarium mycotoxins nivalenol plus deoxynivalenol (NIV + DON) and zearalenone (ZEN) in fractions of plants of maize (Zea mays) hybrid P3 751 sampled between January and July 1997. (Note that the mycotoxin concentration scales differ for the eight subfractions.)
Lauren & di Menna Fusaria and Fusarium mycotoxins in maize 221 identified, F. crookwellense made up 33%, F. graminearum 27%, F. subglutinans 12%, F. culmorum 7%, and F. poae 7%. The proportions of these species in the leaf fractions of the two hybrids were similar, with F. crookwellense the most often recovered (Table 2). After early March this was generally the most abundant species in leaf fractions followed by F. gramineanim. With ear fractions the predominant species showed more variation from collection date to collection date and also sometimes between basal and apical fractions, although the same species would predominate in corresponding kernel and rachis fractions on any particular collection day. For P3 751, F. gramineanim was generally the predominant species in apical ear fractions with F. subglutinans the next most common. F. graminearum followed by F. crookwellense dominated in the basal fractions. For P3902, F. crookwellense, F. graminearum, F. poae, and.f. subglutinans were all dominant in apical ear fractions on different sampling days. This is reflected by the differing proportions of total and toxigenic fusaria in Fig. 1. This changing pattern of infection was also evident in the basal fractions. F. subglutinans and F. poae were dominant up to the 15 May sampling; after that F. graminearum and F. crookwellense were dominant. Other species recovered in relatively small proportions were F. anthophilum, F. avenaceum, F. equiseti, F. graminum, F. moniliforme, F. oxysporum, F. sambucinum, F. sporotrichioides, and F. tricinctum. From Fig. 1 and 2 it can be seen that mycotoxins were found in almost all fractions analysed, especially as the plant aged, but that the concentrations found were quite different. (Note that the concentration scales differ for the eight sub-figures of each hybrid.) The most contaminated leaf fractions were the axils. ZEN was found in both upper and lower axils of P3902 at from 0.6 to 16.6 mg/kg, but did not occur in the associated blades. The highest level of NIV or DON was c. 0.6 mg/kg NTV in the upper axils on 3 July. For hybrid P3751 the most contaminated leaf fractions were again the axils; in this instance contaminated by ZEN at from 0.3 to 11.5 mg/kg, by NIV at from 0.1 to 7.4 mg/kg, and by DON at from 0.1 to 0.8 mg/kg. The occurrence of these three toxins is consistent with the dominance of F. crookwellense and F. graminearum. The most contaminated ear fractions were the rachis fractions which, for the plant fractions tested, had the highest overall concentrations of mycotoxins: 40 mg/kg of ZEN in the apical rachis of P3902 on 15 May, 41 mg/kg of NIV in the same sample, and 95 mg/kg of DON in the apical rachis of P3751 on 19 June. It was most noticeable for the ear fractions that the presence of mycotoxins coincides with infection by toxigenic fusaria. The highest concentrations found in kernels were in apical kernels (3.8 mg/kg of DON for P3751 on 19 June). The basal kernels of P3902 showed only a low concentration of mycotoxin (0.17 mg/kg of NIV) even late in the season. When contaminated, ear fractions of P3902 tended to contain NIV or ZEN. This is consistent with the presence of F. crookwellense, F. culmorum, or a NlV-producing strain of F. graminearum. The contaminated ear fractions of P3751 tended to contain either or both of NIV and DON with ZEN. This is consistent with the presence of F. crookwellense and with DON- and NlV-producing strains of F. graminearum. DISCUSSION When the sampling month and the fraction sampled were the same, the overall results from this study were usually similar to those from the 1995 and 1996 study (di Menna et al. 1997). Fusarium infection was first apparent in leaf axils as early as January, and this tended to increase with plant age, and to be heavier than in leaf blades. Kernels and rachis did not become appreciably infected until later, in April or May depending on the hybrid. A novel finding that has been identified in the present study is the different infection pattern for the two hybrids in terms of toxigenic fusaria and thus in mycotoxin contamination. The hybrid least prone to mycotoxin contamination under natural conditions in New Zealand (P3902) showed a relatively delayed infection by toxigenic species when compared with total fusaria. This was especially so for the basal fractions which only showed infection by toxigenic species, and contamination, in June and July. This Table 2 Mean percentages of isolates of predominant Fusarium spp. recovered over the entire sampling period from all ear (kernels and rachis) and leaf (axils and blades) fractions of maize (Zea mays) hybrids P3902 and P3751. F. crookwellense F. culmorum F. graminearum F. poae F. subglutinans Ear fractions 3902 3751 25 8 23 23 16 26 3 50 <1 15 Leaf fractions 3902 3751 41 7 18 0 9 43 8 19 0 7
222 New Zealand Journal of Crop and Horticultural Science, 1999, Vol. 27 was 4 weeks after the kernel moisture first dropped to 20%. An interesting possibility raised by this finding is that the prior infection by non-toxigenic species could act as a form of biological control against invasion by toxigenic species and subsequent mycotoxin contamination. Unnecessarily delayed harvest has been cited by Lauren et al. (1996) as a factor contributing to elevated mycotoxin levels in harvested maize, and in the present study, toxins were not detectable in the various fractions until, or some time after, infection by toxigenic fusaria was demonstrable. This is as described by Chulze et al. (1996) with kernels of Argentinian corn. Toxin levels in 1997 (and in 1995) were higher than in 1996 but this could be attributed to the fact that the 1996 trial was harvested in early May. Highest toxin levels in the 1995 and 1997 trials were in plants collected in late May, June, or July. In all 3 years toxin concentrations in kernels were low in relation to those in the rachis fractions, and sometimes also in relation to those in the leaf axils. These findings have implications for the use of maize stubble in particular as an animal feed. Although ruminants are relatively resistant to the toxic affects of NIV and DON, consumption of the known oestrogen, ZEN could lead to reproductive problems. The presence of NIV and/or DON in particular plant fractions was related to the dominant toxigenic species present. In culture, individual New Zealand isolates of F. graminearum produce either NIV or DON, whereas those of F. crookwellense and F. culmorum tend to produce NIV (Lauren et al. 1991). F. crookwellense and F. culmorum predominated in the 1995 Manawatu material in which NIV concentrations were high in relation to those of DON. In the 1996 study, where F. graminearum was dominant, NIV and DON concentrations were similar. In the present study, F. graminearum predominated in ear fractions of hybrid 3751 and DON concentrations in them were as high as or higher than those of NIV. In the ear fractions of 3902 and the leaf fractions of both hybrids, where F. crookwellense was more common than - F. graminearum, NIV concentrations were high and those of DON were low. Factors determining the predominant toxigenic species and their relationship to total fusaria, are at present obscure but they do not appear to be directly related to hybrid or to site. However, there could well be a complex interaction between a particular hybrid's genetics and maturity, weather or climate at the time of infection, and the particular inoculum present at that time. Late in the season the toxigenic species F. crookwellense and F. graminearum tend to become dominant, and this no doubt accounts for the increased mycotoxin content in delayed-harvest maize (Lauren et al. 1996). The mechanism of maize infection by Fusarium is commonly considered to be primarily through the silk at emergence or by insect or bird damage (Miller 1994; Reid et al. 1996). However, as inoculation experiments with F. graminearum designed to use these pathways give infection patterns with a poor relationship to natural epidemics, Miller (1994) suggested that secondary factors are important. These include a particular hybrid's resistance to fungal growth, and whether or not weather conditions are such as to promote fungal growth. He (Miller 1994) stated that although weather conditions early in the season have a role in determining inoculum pressure (for infection near silk emergence), late season events such as high rainfall and above normal temperatures (which could be considered typical of the common New Zealand temperate growth climate) which allow infection from dormant, rain-splashed, airborne, or insect/bird-vectored propagules, are necessary for epidemic events (in North America). It is most probable that in the main maize-growing regions of New Zealand, where a relatively high incidence of Fusarium infection and mycotoxin contamination by NIV and DON is common each year, that these late season events dominate. Infection patterns for this trial, and those in 1995 and 1996, do not support infection at silking as a major factor, since silking occurs in late January to early February whereas infection in the kernels is not evident until April-May. The infection present in leaf axils from early in the season, and the debris typically residing there (Sayer 1991), would be a suitable ready source of inoculum for either aerial or vertebrate-vectored infection into the ear, or indeed as a source for systemic infection. ACKNOWLEDGMENTS We thank Wendy Smith and Jane Veitch for assistance with mycotoxin analysis and Genetic Technologies Ltd for access to field trial plots. We also acknowledge funding support from the Foundation for Research, Science and Technology. REFERENCES Burgess, L. W.; Summerell, B. A.; Bullock, S.; Gott, K. P.; Backhouse, D. 1994: Laboratory manual for Fusarium research. 3rd edition. The University of Sydney. 133 p.
Lauren & di Menna Fusaria and Fusarium mycotoxins in maize 223 Chulze, S. N.; Ramirez, M. L.; Farnochi, M. C.; Pascale, M.; Visconti, A.; March, G. 1996: Fusarium and fumonisin occurrence in Argentinian corn at different ear maturity stages. Journal of Agricultural and Food Chemistry 44: 2797-2801. di Menna, M. E.; Lauren, D. R.; Hardacre, A. 1997: Fusaria and Fusarium toxins in New Zealand maize plants. Mycopathologia 139: 165-173. Genetic Technologies Ltd 1997: 1996-97 maize for grain hybrid performance information from Pioneer. Auckland, Genetic Technologies Ltd. P. 5. Lauren, D. R.; Jensen, D. J.; Smith, W. A.; Dow, B. W.; Sayer, S. T. 1996: Mycotoxins in New Zealand maize: a study of some factors influencing contamination levels in grain. New Zealand Journal of Crop and Horticultural Science 24: 13-20. Lauren, D. R.; Ringrose, M. A. 1997: Determination of the fate of three Fusarium mycotoxins through wet-milling of maize using an improved HPLC analytical technique. Food Additives and Contaminants 14: 435-443. Lauren, D. R.; Sayer, S. T.; di Menna, M. E. 1991: Trichothecene production by Fusarium species isolated from grain and pasture throughout New Zealand. Mycopathologia 120: 167-176. Miller, J. D. 1994: Epidemiology of Fusarium ear diseases of cereals. In: Miller, J. D.; Trenholm, H. L. ed. Mycotoxins in grain compounds other than aflatoxin. St Paul, MN, Eagan Press. Pp. 19-36. Munkvold, G. P.; McGee, D. C.; Carlton, W. M. 1997: Importance of different pathways for maize kernel infection by Fusarium moniliforme. Phytopathology 87: 209-217. Reid, L. M.; Sinha, R. C. 1998: Maize maturity and the development of Gibberella ear rot symptoms and deoxynivalenol after inoculation. European Journal of Plant Pathology 104: 147-154. Reid, L. M.; Stewart, D. W.; Hamilton, R. 1. 1996: A 4- year study of the association between Gibberella ear rot severity and deoxynivalenol concentration. Journal of Phytopathology 144: 431-436. Sayer, S. T. 1991: Fusarium infection in some Waikato maize. New Zealand Journal of Crop and Horticultural Research 19: 149-155.