Question of the Week
Answer: Apple scab, caused by the fungus Venturia inaequalis, causes grayish-brown spots on leaves and hard, rough ("scabby") lesions on fruit. As the lesions progress, they often crack, opening the fruit to other problems. Rain, duration of leaf wetness, and temperature determine infection periods. If initial infections are not controlled, they will lead to secondary infections later in the season. Secondary infections begin when summer spores (conidia) develop in lesions on leaf and bud tissues, to be released during wet periods and disseminated throughout the tree. Secondary infections blemish and deform the apples and will also weaken the tree.
The use of scab-resistant varieties is the best long-term strategy for organic growers. The number of primary and secondary infections in a year depends on the amount of rain. The warmer the weather, the more quickly conidia development follows primary infection. If the grower is relying on protective-type fungicides, including all organically acceptable fungicides, then trees should be treated whenever there is a chance of primary infection. Apple scab can be controlled on susceptible varieties by timely sprays with fungicides. For the organic apple grower, there are three commonly used materials: sulfur, lime-sulfur, and Bordeaux mixture. Potassium bicarbonate (the trade name is Armicarb) and potassium phosphonate (Resistim), in combination with sulphur, have been shown to be very effective at controlling the fungus. Trees must be sprayed or dusted diligently before, during, or immediately after a rain, from the time of bud break until all the spores are discharged. Because the scab fungus overwinters on fallen apple leaves, growers can largely eliminate the primary scab inoculum and control the disease by raking and destroying the fallen leaves. Neem has also demonstrated some efficacy in managing scab.
You can learn much more about apple diseases and pests, as well as organic production, by consulting the following ATTRA publications:
Apples: Organic Production Guide
Mention of specific brand names is for educational purposes only and does not constitute endorsement by NCAT, ATTRA, or USDA.
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