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Disease Management Update
Apple Scab -- The End of the Primary Stage is Near
As you can see in the following chart, we are in the final phase of ascospore maturation in all but the cooler parts of the state. The 'final phase' extends from 700 - 800 DD. As a conservative estimate, the final ascospore release in commercially managed orchards can be assumed to have occurred when 900 DD have accumulated and there is a daytime rain of at least 0.1 inch when the temperature is at least 50F during the wetting period. If the temperature is below 50F or if the total rain is less than 0.1 inch, then only about 50% of the remaining, mature ascospores will be released.
However, you should not let your guard down for apple scab until you are sure that your fungicide program has been effective in preventing lesions from developing. That means a thorough scab assessment of cluster and terminal leaves plus fruit should take place between 9-17 days, depending on average daily temperature, after the final primary infection period (please see Table 4 on page 8 of the 2000-2001 New England Apple Pest Management Guide (NEAPMG) for an estimate of the number of days at different average daily temperatures). Note that unfavorable conditions for lesion development, such as very dry weather or a prolonged period above 80F, can delay lesion formation beyond the days indicated in the table. What is meant by a "thorough" assessment ? Check all the leaves (upper and lower surface) on a minimum of 10 extension shoots and 10 fruit clusters (leaves and fruit) on each of at least 10 trees per block . There is a significant risk of secondary scab if you find scab on one or more terminals or clusters. In addition to the standard materials used to suppress conidia production in the lesions (i.e., captan, dodine, SI fungicides), a new option is the strobilurin fungicides (i.e., Sovran, Flint). Please refer to the NEAPMG for details about how all of these materials should be used for best results.
Don't wait until the end of primary to check your orchard for lesions ! Checking your orchard on a weekly basis will tell you whether you have to switch your focus from ascospore maturity to suppressing secondary infections by conidia.
Powdery Mildew -- Critical Time for Management
As mentioned in the IPM Alert of May 16, 2000, now through the time when the apical bud sets on vegetative terminals is the key time to manage powdery mildew. You can get an idea of the extent of overwintering inoculum by walking around your trees and seeing how many primary infection sites there are -- these are buds in which the fungus has overwintered and where the fungus is now growing on the surface of new tissue. The picture on the left shows such a site; the picture on the right shows secondary infections in the form of patchy areas of mycelium on the surface of the leaves. In many situations, you will only find these mycelium patches on the underside of the leaf. Usually, only in the interior, shaded part of the tree will mildew be seen on the upper surface of the leaf.
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