Development and testing of a recommendation system to schedule copper sprays for citrus disease control

J. ASTM Intnl., Sept 200X Vol XX, No. X Paper ID:12904 Available online at: www.astm.org Published XXXX 25 TH SYMPOSIUM ON PESTICIDE FORMULATIONS AND DELIVERY SYSTEMS: ADVANCES IN CROP PROTECTION TECHNOLOGIES 1 L. G. Albrigo, 2 H. W. Beck, 3 L. W. Timmer 2, E. Stover, 4 Development and testing of a recommendation system to schedule copper sprays for citrus disease control ABSTRACT: Copper (Cu) fungicides are common contact products for control of citrus fungal diseases. Cu residues are subject to weathering (rain and wind) and dilution from fruit expansion during early growth. Growers often over- or under-spray, which leads to environmental concerns, Cu phytotoxicity to fruit or poor disease control. After spraying trees with different amounts of copper fungicides using different water volumes per tree, Cu on fruit surfaces was analyzed over time. These data were used to build a Cu spray scheduling recommendation system (CuSSRS) that predicts initial residue after application depending on spray volume rate and Cu content. In addition, the CuSSRS predicts dilution and loss of the Cu residue as fruit grow and rainfall weathering occurs. The model indicates a warning and a danger level of low residual Cu based on necessary levels of Cu for disease control, thus providing the grower with advanced notice of the need to re-spray. The system was grower tested for six years. Verification data on efficacy and actual versus predicted residues were collected over a three-year period. These tests indicated the CuSSRS works reasonably well and can reduce the need for copper fungicide sprays in some cases. KEYWORDS: melanose, greasy spot, fungal control, fruit blemishes, spray burn, Cu residues, 1 This research was supported by the Florida Agricultural Experiment Station and the Florida Production Research Marketing Order. It was approved for publication as Fla. Agric. Expt. Station Journal Series No.. 2 Professors, Citrus Research and Education Center, University of Florida, 700 Experiment Station Road, Lake Alfred, Florida, USA. 3 Professor, Agricultural and Biological Engineering, University of Florida, Gainesville, Florida, USA. 4 Indian River Research and Education Center, University of Florida, Ft. Pierce, FL USA, Currently Curator & Res. Leader, USDA, ARS, PWA National Clonal Germplasm Repository, One Shields Av., Davis, CA 95616

2 Citrus growers in Florida use copper fungicides for control of several fungal diseases on fruit from spring flush through summer. Copper fungicides function as protectants on the plant surface to prevent fungal growth. There must be sufficient Cu residue to provide soluble copper ions whenever moisture is on the fruit or leaf surface long enough for target fungal spores to germinate. The most common form of copper used in Florida is copper hydroxide, which provides soluble Cu ++ ions in water at six mg/l, while other products may provide higher Cu ++ levels. Frequent problems with copper use as a fungicide have been inadequate control, copper stippling (phytotoxic burn) from excessive uptake of Cu ++ by the fruit and build-up of toxic levels of Cu in the soil because of multiple applications of high amounts of Cu over several years (Alva and Graham, 1991, Albrigo and Grosser, 1996). These problems led to a number of studies to better understand the behavior of Cu as a fungicide and its residual behavior on citrus (Albrigo et al. 1997). The data from these studies were used to create a copper spray scheduling recommendation system(cussrs) to aid growers in scheduling copper fungicide sprays for early season disease control. The CuSSRS is based on evaluation of initial deposits of Cu residues from spray conditions and the decrease in residue (per unit of surface area) as the fruit surface expands and as weathering events (mostly rainfall) remove Cu from the surface. Grapefruit is a major fresh fruit in Florida which receives Cu sprays for fungal diseases, particularly melanose. All other citrus cultivars are susceptible to melanose, but this blemish on grapefruit is more noticeable. Sufficient Cu residue must be present to protect the fruit from petal fall until late June or early July, when fruit are no longer susceptible to penetration by the germinating fungal spores of the melanose causal agent Diaporthe citri, F.A. Wolf., anamorph Phomopsis citri H. Fawc. (Timmer and Zitko 1996). Copper fungicides are also options for control of Phytophthora

brown rot, alternaria, scab and greasy spot rind blotch fungus, Mycosphaerella citri Whiteside on grapefruit. 3 This paper reports development details of this copper fungicide expert system and verification testing. CuSSRS was verified by 1) monitoring the actual Cu residue levels compared to predicted Cu values and 2) comparing disease control efficacy for plots sprayed using CuSSRS to a standard 21-day calendar spray schedule. Materials and Methods Data for the model The basic information for development of the model included initial fruit Cu residues after sprays using standard low volume or speed sprayers calibrated at different volume delivery rates of a given metallic Cu amount per hectare and the changes in those residues as the fruit grew and weather events occurred. Both fruit growth and rainfall were measured to calculate dilution by fruit surface expansion and weathering reductions in Cu residues. Most of this information was developed and reported earlier (Albrigo et al. 1997). Effect of copper concentration on germination of conidia of D. citri Cultures of D. citri were grown on potato dextrose agar (PDA) containing embedded, sterilized citrus twigs. Pycnidia were formed on the twigs in the agar after three to four weeks. Spore tendrils were collected, suspended in sterile, distilled water and washed three times by centrifugation to remove the gelatinous matrix that inhibits spore germination. One-hundred twenty-five ml flasks were prepared with 20 ml solutions of CuSO 4 5H 2 O at 0, 0.1, 1.0, 10, and

4 100 mg/l as metallic copper. Conidia were added to all flasks to provide spores at about 10 3 /ml. Three replicate flasks were used for each concentration. Flasks were placed on a shaker and maintained at ambient temperature (23-25 o C). After 8, 24, 48, 72, and 96 h, 0.1 ml aliquots were removed from each flask and spread on three plates of PDA. Plates were incubated for one week at 27 C and the number of colonies per plate counted. Software CuSSRS was developed using the Java programming language. The model is designed to work as a stand-alone application or it can be integrated within a larger DISC Planning and Scheduling module (Beck et al. 2004). In the latter case it is easier to keep track of the information on spray application and grove/block configurations needed to run the model at many different sites. The object-oriented nature of the Java programming language facilitates creation of reusable components that can be easily combined for integration within larger decision support systems. Figure 1 shows the interface for CuSSRS. At the top are descriptions of the block for running the model. A local database used with the stand-alone application can store information for many different blocks (a more complete record keeping system is included in the DISC Planning & Scheduling module (Beck et al. 2004)). Required information for input includes bloom date, scion, and warning and danger threshold levels of Cu. Details on copper spray applications and rainfall data are included in the Data Entry tab (Fig. 2). The graph shows Cu concentration (green) over time (y-axis is µg/cm 2 ). The x-axis shows time on three scales, days since 1 st spray (x1), days since bloom (X2), and Julian date (X3). Peaks in the graph occur

whenever copper spray applications are made. Sudden drops in Cu concentration correspond to rainfall events. Gradual decay corresponds to increasing fruit surface area as the fruit expands through growth. When the Cu concentration decreases into the warning (yellow) or danger zone (red), a copper spray application is advised. Warning and danger threshold levels can be adjusted by the grower, but are set at recommended default values that provide a 6-fold safety margin. The model is run for one block at a time. However, multiple blocks can be grouped into Block Groups, indicating that the same spray or rainfall information applies to all the blocks within that group. The details required for copper spray application (Fig. 2) include the spray date, spray volume, and metallic Cu concentration. Rainfall events are defined by date and inches of rain. The configuration table allows for running the model for fruit located in the inside or outside of the tree (copper deposition and rainfall events impact the outer fruit more heavily than the inner fruit, but inside fruit grow faster). The configuration table can also allow for adjusting the resolution of the graph. Evaluation After the program was developed, three phases of testing were carried out. For the first year, internal tests were run to correct run problems in the program. For the second year, several growers were provided the program for testing in their grapefruit melanose spray programs. Feedback on problems encountered, desired features to add and general experience were obtained. By the third year, growth curves for other citrus were added so that the program would work on other fresh fruit cultivars, primarily mandarins and navel oranges. In the spring from 2000 to 2002, several tests were established in the Central Ridge and the Indian River Citrus Districts to compare results from using the program with different settings of the danger 5

6 threshold or different Cu levels in the sprays against a conventional 21-day interval spray program. The specifics of the comparisons are given with each data set. Cu determinations were made on three to five separate 10-fruit samples of inside or outside fruit from different trees in a treatment on the specified dates that were at different intervals after the date of spraying. The fruit samples were thoroughly washed in an appropriate volume of acidified water (0.0001% HCl) to dissolve any residual Cu on the fruit surface. An aliquot of the solution was mixed with a color reaction reagent for Cu ions and the developed color was read in a portable colorimeter (YSI 9000 Photometer, Fisher Scientific) set for the appropriate wavelength. Concentration of Cu was based on a standard curve. The residual concentrations were compared to the estimated values generated by the CuSSRS for the appropriate conditions. At harvest, 30 to 60-fruit samples of grapefruit from each plot were graded and reported for severity of melanose on a zero to seven scale with a rating of three or greater being unacceptable for fresh fruit for the domestic market and two or less required for export marketing. Results and Discussion Data for the model Data from a spray rate experiment show that initial residues are higher in several canopy locations when the middle range of volume rates were used (Figure 3). Generally, higher spray volume rates decrease Cu residue and the amount lost to run-off, but provide more uniform coverage. Very low spray volume rates can lead to excessive deposits on exterior surfaces of outer fruit and lead to copper stippling (Albrigo et al., 1997). The initial residue is determined by the spray volume of water per acre and the amount of Cu applied per acre. Higher metallic

Cu rates per area increase deposits but also increase the chance of spray burn. Optimization between these factors may be in the range of 935 to 1403 L/ha volume rate and 2.25 or 3.36 kg of metallic Cu/ha. The Florida Citrus Pest Management Guide makes a general recommendation of 2.25 kg metallic Cu/ha every three weeks, but this should be varied according to the product used, fruit growth, and weather conditions. The Expert System is meant to account for these variables. The major factor in dilution of the Cu residue after spraying is expansion of the fruit surface. Weathering events of rain and wind also play a major role in residue decline. Rain events may cause more copper loss from the surface by dislodgement of Cu particles than from dissolving and washing away soluble Cu (Albrigo et al, 1997). These two factors are the major principles for the decline in Cu residues programmed into the ES graphics program. Some additional considerations in the relationship of Cu, disease control and side effects are products used, variability in coverage, fruit growth, rain by in-tree location, and potential of Cu products plus spray additives to be phytotoxic. Equal rates of metallic Cu as a flowable or non-flowable product will probably give similar initial residues if the spray volume rate is moderate. The emulsifiers in a flowable product may increase the likelihood of Cu removal by rain events that occur soon after spraying (Timmer et al. 1998) as the Cu might be re-emulsified. This may partially account for reduced effectiveness determined for flowable products in earlier tests (Timmer and Zitko 1996). Inside fruit and top fruit receive lower deposits of Cu from most spray equipment. On the other hand these two areas receive either lower rain removal of Cu (inside fruit) or less exposure to spore dispersed from dead twigs (top fruit). Inside grapefruit grow faster and this causes faster dilution of Cu residues. All the calculations in the current graphic program are based on average deposits and dilution curves for different positions in the tree. 7

8 The end-point of Cu residue decline that determines the time to re-spray is based on the required Cu to suppress spore germination. With one exception, 1 mg/l of Cu suppressed spore germination (Table 1). Studies of other plant disease organisms suggests that 1 to 2 mg/l of Cu is generally sufficient to suppress growth of most fungi and bacteria (Mennkissoglu and Lindow 1991). Since the Cu residue is not uniformly distributed on the fruit surface (Albrigo et al. 1997), a safety margin of six fold was built in. Therefore the program has a danger default of 0.5 µg/cm 2, which would provide 6 mg/l in solution. Growers can reduce that default margin in order to extend the time between sprays. In doing so, they increase the risk of insufficient Cu on some fruit or fruit surface areas. TABLE 1. Effect of copper concentration in solution on the ability of conidia of Diaporthe citri to germinate. Cu Concentration 0.0 0.1 1.0 10 100 mg/l Sample Experiment time (h) 1 2 1 2 1 2 1 2 1 2 Colonies / Plate 8 TN z 68 TN 27 TN <1 0 0 0 0 24 69 30 55 28 6 0 0 0 0 0 48 36 27 4 22 0 0 0 0 0 0 72 52 24 24 12 0 0 0 0 0 0 96 27 26 7 16 0 0 0 0 0 0 z TN = too numerous to count Evaluation results year 2000 In grower testing, the program generally was useful as a guide for adjusting their schedule from a standard 21-day rotation. They could reduce or extend the time between sprays as the weather events (rains) dictated. For a given spray cycle, the program also provided growers

information to start the first block or grove in a different order, if rainfall was heavier in a particular geographic location. An evaluation trial in 2000 demonstrated that on the sampling date for Cu residues there were higher residues on grapefruit on a 21-day spray cycle compared to using either scheduling thresholds (Table 2). Also as expected, outside fruit had higher Cu residues than inside fruit. The low residue levels detected on the control plots may indicate that spray drift occurred into these plots. All treatments had acceptable melanose ratings at harvest in this year with low melanose pressure. The low melanose rating on outside control fruit compared to Cu-treated fruit probably indicates that Cu stippling contributed some to the melanose fruit counts in the Cu treatments. 9 TABLE 2. Copper residues and melanose ratings for grapefruit receiving either no Cu sprays, Cu sprayed at danger thresholds (THold) of 0.125 or 0.25 µg/cm 2 or Cu on a 21-day calendar schedule in the spring of 2000 in an Indian River District grove. Treatment Inside Fruit Outside Fruit Spray Threshold z Cu µg/cm2 Melanose Rating y Cu µg/cm2 Melanose Rating Control 0.09a 2.0c 0.06a 0.7a.0.125 THold 0.11ab 1.5b 0.30b 1.2b.0.25 THold 0.17b 1.3b 0.24b 1.2b 21-Day Calendar 0.30c 0.8a 0.77c 1.3b z. Sprays were 2.24 kg metallic Cu in 1169 L water/ha y Ratings were a 1 to 7 scale with 3 or greater unacceptable for domestic fresh fruit. Evaluation results year 2001 In a 2001 test, in the Central Ridge Citrus District (Haines City), the Cu values were close to predicted levels in three of four measurements of inside fruit (Table 3). Outside fruit had higher Cu residues than inside fruit in only one of two comparisons, but these trees received

10 weekly overhead irrigations of 12.3 mm. This may have washed off more of the outer Cu residue compared to inside fruit than the program accounted for when assuming natural rainfall. Lower fruit Cu residues were observed when a copper fungicide was applied at the lower concentration in two of three comparisons. At harvest the corresponding fruit had less severe melanose ratings where the product at a lower concentration was used (Table 4). Apparently in this year also, Cu controlled some melanose compared to the control but the higher rate of Cu may have led to some stippling, which is almost indistinguishable from melanose by a grader. TABLE 3. Comparison of Cu residues on grapefruit sprayed with different formulations. Residues and predicted values (µg/cm 2 ) for the dates of sampling are presented when the Cu spray scheduling program was running for inside fruit (Grapefruit near Haines City, FL, 2001). Date Kocide - DF 2.24 kg z Cu (µg/cm 2 ) Kocide - 2000 1.7 kg Cu (µg/cm 2 ) Inside Outside Predicted Inside Outside Predicted 5/17/01 2.1 1.9 2.6 0.5 2.5 0.7 6/28/01 5.3-2.6 2.9-3.1 z Sprays were 1.7 or 2.24 kg metallic Cu/1169 L/ha, - = no data TABLE 4. Melanose rating for grapefruit treated with different two Cu formulations and sprayed at a 0.5 µg/cm 2 danger threshold (Haines City, FL, 2001). Melanose Ratings - % in category Treatment None Slight Moderate Severe Scab % Control 3 22 a 25 44 c 6 a z Kocide-2000 z 1.7 kg Kocide - DF 2.24 kg 5 40 b 30 23 a 2 b 5 26 a 31 36 b 2 b Copper fungicide sprays of 1.7 or 2.24 kg metallic Cu/1169 L/ha In 2001 in the Indian River District more extensive sampling was done with Cu residues being higher than predicted for two of three dates when re-spraying was done at a low danger threshold of 0.25 µg/cm 2. Cu residues were lower than predicted in three of four cases when re-

spraying occurred at a 0.33 µg/cm 2 threshold, but higher than predicted for one of two dates in the 21-day spray schedule plots (Table 5). In every case, inside fruit had lower residues than outside fruit. For all conditions, actual values were near the predicted for three cases and greater than the predicted three times and less than predicted three times. Most values were within two fold of the predicted, but one value was less than 1/3 rd of the predicted. This case could have led to unprotected fruit if an infection event had occurred. 11 TABLE 5. Copper residues, actual and predicted (for inside fruit) values from the expert system (µg/cm 2 ) for the sampling dates indicated (Grapefruit, Indian River District, 2001). Sample Date 0.25 DangerT z 0.33 DangerT 21-D Calendar I y O P I O P I O P 5/2 1.5 1.8 0.9 1.3 1.9 0.8......... 5/4......... 0.9 1.5 3.5......... 5/22.................. 1.8 2.3 2.3 5/31......... 2.5 4.2 3.5......... 6/4 2.0 3.5 0.9......... 1.1 1.7 0.7 6/8......... 1.1 1.7 2.2......... 6/26 0.3 0.8 0.9.................. z Sprayed with 2.24 kg metallic Cu/1169 L/ha when danger thresholds was 0.25 or 0.33 µg/cm 2 Cu or sprayed every 21 days, not all treatments tested on a sampling date. y Cu residues on inside (I) and outside (O) fruit, and predicted values (P) for inside fruit. In this same 2001 test in the Indian River there were no differences in mean score nor % fruit scored greater than two (unacceptable for export marketing) for interior fruit between any of the treatments (Table 6). Exterior fruit had almost three times more fruit with ratings greater than two across all Cu treatments compared to the control. This is a clear indication that the

12 higher Cu levels lead to more stippling on outside fruit and this blemish was confused with melanose by the graders. TABLE 6. Melanose rating (1-7 scale) and % fruit with a >2 rating (> 3 not acceptable for fresh domestic market, less than 2 required for export). Sprays scheduled by danger thresholds of 0.15, 0.33 µg/cm 2 or sprayed every 21-days (Grapefruit, Indian River, 2001). Treatment Interior Fruit Mean Score % Fruit >2 Score Exterior Fruit Mean Score % Fruit >2 Score Control 1.3 12.5 1.1 a 6.7a 0.15 Threshold z 1.4 15.8 1.6 b 19.2b 0.33 Threshold 1.2 6.7 1.7 b 22.5b 21-Day Calendar 1.3 10.8 1.9 b 25.0b NS NS z Sprayed with Cu at 2.24 kg metallic Cu in 1169 L/ha solution Evaluation results year 2002 In 2002, two trials were run using different metallic Cu application rates comparing a 21- day calendar schedule versus the expert system set at the default threshold of 0.35 µg/cm 2 (Tables 7 and 8). In this high potential melanose year, treatments reduced the average ratings from about five to less than three in most cases. For comparisons of either the 1.46 or 1.9 kg Cu/ha application, the melanose ratings were slightly better for the 21-day calendar schedule, but the 21-day schedule was significantly better only at the 1.46 or 1.9 kg rate. No differences occurred at the higher 2.24 or 2.46 kg applications between the CuSSRS and the 21-day scheduling.

TABLE 7. Melanose rating for grapefruit in the Indian River, 2002 using a one to seven rating scale with a three or greater rating not acceptable for fresh domestic market. Treatment Melanose Avg. Rating Interior fruit Melanose Avg. Rating Exterior fruit Control 5.06 z 4.83 1.46 kg Calendar y 2.65 2.57 1.46 kg CuSSRS 3.14 3.25 2.24 kg Calendar 2.59 2.59 2.24 kg CuSSRS 2.53 2.50 z All treatments better than Control, 5 % P level Calendar better than CuSSRS @ 1.46 kg, 5 % P level, paired comparisons y Cu sprays of 1.46 or 2.24 kg metallic Cu in 1169 L/ha, scheduled by a danger threshold of 0.35 µg/cm 2 or sprayed on a 21-day schedule 13 TABLE 8. Melanose rating on a one to seven scale with a three or greater rating not acceptable for fresh domestic market for grapefruit treated in an Indian River grove in 2002. Treatment Melanose Avg. Rating Interior fruit Melanose Avg. Rating Exterior fruit Control 5.1 z 5.1 1.9 kg Calendar y 2.7 2.8 1.9 kg CuSSRS 3.1 3.3 2.47 kg Calendar 2.5 2.9 2.47 kg CuSSRS 2.5 2.7 z All treatments better than Control, 5 % P level, Calendar not better than CuSSRS, 5 % P level, paired comparisons. y Cu sprayed at 1.9 or 2.47 kg metallic Cu in 1169 L/ha, using a danger threshold of 0.35 µg/cm 2 or sprayed on a 21-day schedule. A comparison of the number of sprays required for the three test years was as follows: in 2000, four sprays were required for all schedules, but the expert system schedules resulted in the 4 th spray being applied 20 and 22 days after the last spray on the 21-day schedule. Presumably these provided Cu protection for a longer period after the last sprays in June. In 2001, one to two fewer sprays were applied using the CuSSRS, with the reduction of two sprays occurring when the threshold was reduced to 0.15 µg/cm 2. At least once during the season, this lower threshold left the fruit vulnerable to melanose if a rain had occurred before the next spray was applied since the danger threshold provided only an average of 2 mg/l Cu. In 2002, which was a high

14 rainfall year, all treatments received six sprays. At the same time that fewer sprays were scheduled in moderate to dry years by the CuSSRS, slightly more melanose occurred at lower rates of Cu application (1.46 or 1.9 kg). We did not have a very wet year in order to test if the CuSSRS would schedule more sprays and provide better melanose control than a 21-day calendar program. Early season sprays of Cu with oil have very low spray burn potential, even if applied by ultra low volume applicators. When greasy spot season arrives, growers tend to increase the oil content in the spray mix and air temperatures are higher. These conditions greatly increase the potential for spray burn (Albrigo et al. 1997). Spray burn results from a combination of increased Cu uptake due to oil and increased diffusion because of warmer temperatures. Addition of other materials to Cu sprays, particularly if acidic mixes result, greatly increases the likelihood of spray burn (Albrigo and Grosser 1996). Using lower rates of Cu (2.2 kg/ha), if 37.4 or 46.7 L/ha of oil are included, diminishes the chance of spray burn while giving reasonable disease control (Albrigo et al. 1997). Packout rates were often in the 80 % range. Cu application rates must be kept low if emulsified products are used because of increased spray burn (Timmer and Zitko 1996). These formulations apparently increase uptake, perhaps in a similar manner to spray surfactants (Albrigo and Grosser 1996). Split applications over the period of melanose susceptibility generally provide better control than a single application of the total amount of Cu (Timmer et al. 1998). In some years of low incidence, split applications do not significantly decrease the disease incidence over a single application. The current program builds on the usual improvement from multiple sprays and gives some guidance to scheduling those sprays.

Perhaps a better compromise to achieve adequate fungal protection and decreased spray burn potential would be to use higher application rates early, 2.8 3.36 kg/ha, when dilution has its biggest effect combined with a moderate danger threshold, perhaps 0.25 -.35 µg/cm 2,, to increase the time between early sprays. By mid- to late April, as air temperatures and fruit uptake of Cu tends to increase, a lower rate of Cu, 1.9-2.24 kg/ha, will provide an extended period of Cu protection while decreasing the chance of fruit Cu burn. A comparison of the early and later spring dilution curves can be seen after the first compared to after the last spray in Figure 1. Further, eliminating oil from these mid-spring sprays can also decrease spray burn potential (Albrigo et al. 1997). The approach of higher Cu concentration early and lower ones later also would reduce the number of sprays required in many years. Continued work will emphasize temperature effects on growth rates of fruit and practical ways for growers to use the CuSSRS in their spring disease control for fresh fruit production. 15 Conclusions A CuSSRS was developed based on collected experimental data that predicted post-spray Cu residue from sprays of copper fungicides and depicts the residue dilution as fruit grow and weathering events occur. Warning and danger thresholds were established based on required Cu++ ions to suppress fungal spore germination. This CuSSRS was used by growers and tested for verification of actual Cu residues and melanose control during three years. Predicted Cu residues were usually within two fold of actual Cu values. Cases of over or under predicting residues were about equal. The program did not improve disease control over a standard frequent spray program, but results were similar to the calendar date scheduling procedure in most cases. The program did save one or two sprays per season, particularly in low spring rainfall years. Growers found the CuSSRS most useful for 1) adjusting a calendar schedule to a

16 shorter or longer interval depending on rain events and 2) to start spraying blocks in a different order if rains occurred in one particular geographic area rather than uniformly over all grove areas.

17 References Albrigo, L. G. and Grosser, J. W., 1996, Methods for evaluation of spray chemical phytotoxicity to citrus, Proceedings of the Florida State Horticultural Society, Vol. 109, pp.52-57. Albrigo, L. G., Timmer, L.W., Townsend, K. and Beck, H.W., 1997, Copper fungicides- Residues for disease control and potential for spray burn, Proceedings of the Florida State Horticultural Society, Vol. 110, pp. 67-70. Alva, A.K.. and J.H.. Graham. 1991. The role of copper in citriculture. Advances in Agronomy 1:145-170. Beck, H. S., Albrigo, L. G. and Soonho, K., 2004, DISC Citrus planning and scheduling program. Proceedings 7 th International Symposium Computer Modeling Fruit Tree Res. & Orchard Management, Copenhagen, Den., Acta Horticulturae (in press). Mennkissoglu, O. and Lindow, S. E., 1991, Chemical forms of copper on leaves in relation to bactericidal activity of cupric hydroxide deposits on plants, Phytopathology Vol. 81, pp. 1263-1270. Timmer, L.W. and Zitko, S. E., 1996, Evaluation of copper fungicides and rates of metallic copper for control of melanose on grapefruit in Florida, Plant Disease, Vol. 80, pp. 166-169. Timmer, L.W., Zitko, S.E. and Albrigo, L.G., 1998, Split applications of copper fungicides improve control of melanose on grapefruit in Florida, Plant Disease, Vol. 82, pp. 983-986.

18 Figure 1. Interface for the Copper CuSSRS. The graph at the bottom shows Cu deposition over time. A yellow band (light gray) near the bottom of the graph indicates a warning level, and a red band (darker gray band at bottom) indicates dangerously low levels of Cu concentration. The information at the top describes the block information for running the model. Figure 2. Details on the copper spray applications and rainfall screen. Each event (spray or rainfall) that occurs is recorded for a particular date and is incorporated in the graph when draw is activated (see Fig. 1).

19 Figure 3. Spray volume effect on initial residues of Cu on grapefruit at different heights in the tree and inside versus outside in the canopy of a mature (4.2 m high) grapefruit tree in the Indian River District. Figure 4. Example of spray schedule timings and the estimated residue changes against rain events for the CuSSRS. The danger threshold was set at 0.33 µg Cu/cm 2 (dark gray band) and resulted in 5 sprays (Cu peaks) being required through early July.