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The apple scab fungus (Venturia inaequalis) overwinters on scabby crabapple leaf debris on the ground and scabby crabapple fruits still on the tree. Over the winter the sexual stage of the fungus develops on this plant tissue and is the source of fungal inoculum that first infects crabapple foliage and fruits in the spring. In fact this is happening right now as leaves and flowers emerge. Some infections occur on foliage before flowers are out. Many plant health care managers, that use fungicides to keep scab down to a dull roar on susceptible crabapple taxa, find that 2-3 sprays (two weeks apart) starting with petal fall does an adequate job. There are a number of different fungicides used, but propiconazole, thiophanate-methyl or combinations of the two are common. One of the horticultural challenges with this approach is that there is about a 3 week spread from when the earliest crabapple types flower and proceed to petal fall and when the latest flowering crabapples first flower and proceed to petal fall. In other words, the early-flowering 'Spring Snow' flowers about three weeks earlier than the late-flowering 'Golden Raindrops.' Obviously this phenological reality means that if you time your crabapple scab sprays for petal fall, you have to make a judgment of when most of the crabapples on your customer's properties are at petal fall. Otherwise, one would have to make a number of almost daily individual sprays for different crabapples as they reach that phenological point. As with most horticultural practices, usually some compromises are necessary. Horticulture, like politics, is the art and science of the possible!
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Last Updated ( Thursday, 24 April 2008 18:04 )
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BYGL 
