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Fruit Notes |
Testing Various Methods of Timing Summer Fungicides |
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Fruit Notes |
Kathleen Leahy Thomas Clark Ezekiel Goodband |
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Fruit Notes |
The summer diseases, sooty blotch and flyspeck are the cause of most of the fungicide use after June in New England orchards. Using an improved understanding of the biology of these diseases, and specifically the wetness-hour model developed by Turner Sutton and his colleagues , has allowed growers to reduce fungicide usage in North Carolina and other mid-Atlantic states. We wanted to test this model to see whether it is viable in New England conditions. In a nutshell, the method developed by Brown and Sutton in North Carolina is based on the biology of the sooty blotch and flyspeck fungi. It allows the first summer fungicide to be delayed until 200-250 hours of wetness (counting only wetting periods of 4 hours or more) after the last scab fungicide. With our relatively dry summers, we generally reach this threshold in early to mid-August. Thus, using this method, we could, in most years, save a couple of fungicide applications and have greater flexibility in timing summer fungicides with respect to insecticides and miticides/summer oils. We also decided to try a somewhat less radical method, based on work by David Rosenberger in New York, showing that fungicide retention during the summer is such that fungicides need not be re-applied until 150 hours of wetness (no minimum threshold) having occurred since the previous fungicide. This would still provide a measure of flexibility in summer fungicide use. Materials & Methods The two cooperating orchards were located in the Connecticut Valley, one in Walpole, NH and the other in Deerfield, MA. In these orchards, the test blocks were divided into three ½ acre plots, and each plot was treated according to one of three summer fungicide programs: 1) the ‘standard' program of a fungicide application every three weeks through June, July, and August; 2) the ‘New York' program of waiting until 150 hours of wetness had accumulated since the most recent fungicide application; and 3) the ‘North Carolina' model of waiting until 200-250 hours of wetness had accumulated after the primary infection period for these diseases. A modified hygrothermograph unit was used to measure the wetting periods in both orchards. The weather units were set out prior to blossom. The time of petal fall was noted (May 10 at Clarkdale Orchards; May 11 at Alyson's Apple Orchard) and wetness-hours were accumulated beginning 10 days after petal fall (May 20 and May 21, respectively). Trees in both blocks are on M.7 rootstock. The Clarkdale block consisted of Golden Delicious and McIntosh, about 15 years old, with rows running east-west, and the Alyson's block was McIntosh and Empire, about 12 years old, with rows running north-south. This was an unusually wet year in the Northeast. In a more typical year we would not reach the ‘North Carolina' threshold until early August, but in 1998, we reached this threshold by early July. Over 100 hours of wetness occurred during a single week in June. The specific threshold dates were: at Clarkdale Orchards, the New York threshold of 150 hours from the last apple scab fungicide was reached on June 22, and the North Carolina threshold of 200 hours was reached on July 3. At Alyson's Apple Orchard, the New York threshold was reached on June 24, and the North Carolina threshold on July 2. Because the New York threshold at Clark's was reached at the same time that he was planning a ‘standard' spray, both the ‘standard' and ‘New York' plots were treated on the same date. The first summer fungicide applications were made on June 22, June 22, and July 9 at Clarkdale for the standard program, NY program, and NC program, respectively. The first summer fungicide applications were made on June 24, July 2, and July 16 at Alyson's for the standard program, NY program, and NC program, respectively. At Alyson's, Mr. Goodband decided to test a yet more radical approach than what we had originally proposed. The fungicide application on June 24 actually coincided with the NY threshold, and the July 2 date was when the 200-hour NC threshold was reached. The application on July 16 was at 336 wetness-hours from 10 days after petal fall, and 262 wetness-hours from the previous fungicide. Once the first summer fungicide was applied in each section, that section thereafter continued to receive a regular fungicide program. Results Because of the early accumulation of the threshold level of leaf wetness hours, sampling (50 fruit per section per week) began in mid-July instead of early August as planned, but no symptoms were seen even in check trees until mid-August. When flyspeck symptoms began to appear (virtually all the summer disease seen this year was flyspeck), they appeared first in the most lightly-sprayed plots. Despite the early wetness, there was not a significant level of flyspeck except in the check trees until mid-September. As can be seen from Table 1 and Table 2, there were few significant differences in summer disease occurrence except between the treated and check trees, but there was a trend toward somewhat higher disease in the ‘radical' plot at Alyson's. When McIntosh fruit were harvested in this plot, a random survey of fruit showed over 40% with flyspeck, which is not commercially acceptable. It was clear that this ‘radical' option did not provide acceptable control of flyspeck. Because of the advanced season and the dry conditions in August, the fruit in the other two plots were harvested before a proper survey could be done, but it was evident that control was satisfactory in these two plots. At Clarkdale, there was very little difference between treated plots (the intended check trees were sprayed so the data were not included), and a harvest survey of 100 McIntosh per plot showed no significant difference nor any trend between treatments. Once the McIntosh had been harvested, it became evident that there were differences in flyspeck occurrence between varieties in both orchards. This effect had been masked by the random zigzag pattern through the plot adopted when sampling fruit in the regular weekly sessions. At Alyson's, the Empires had much less flyspeck than the McIntosh, whereas at Clarkdale, the Golden Delicious had much more flyspeck than the McIntosh. These differences were presumably not owing to varietal susceptibility, since the literature indicates that apple varieties are essentially identical in their susceptibility to flyspeck. We believe that the differences were owing to the more open growth habit of Empires vs. McIntosh at Alyson's, generating a less favorable (lower humidity) environment for the organism, whereas at Clarkdale the strikingly high level of flyspeck throughout all three treatments indicates that this entire row did not receive a summer fungicide application - perhaps the last scab fungicide on June 9. Mite sampling in the test plots did not show any difference in either pest or predator mite numbers. This was as we expected, since the first year of reducing fungicide is unlikely to produce a dramatic effect on the mite population. Discussion In general both the New York and North Carolina methods of timing fungicide applications seemed to work very well, under unusually challenging conditions for this region. It is clear, though, that extending the number of wetness-hours beyond the 250 recommended by the North Carolina model poses an unacceptable risk to the crop. The economic savings from reduced sprays overall were less than $10 per acre, but even this amount could be significant, given the slim profit margins in apple-growing at this time. The major benefit of adopting a reduced summer fungicide program, however, is more likely to be increased flexibility in time management and the ability to incorporate summer oils into the spray program without risking phytotoxicity from fungicide/oil interactions. In addition, it is possible that this practice might be helpful for beneficial organisms in the orchard. |
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