Apple Scab Ascospore Capture at Threshold Level
Posted: April 12, 2011
Scab infections may be prevented by applying fungicides at regular intervals throughout the growing season. The object is to provide a protective coating that will inactivate any spores landing on the fruit and foliage. It is critical to control scab early in the season from bud emergence through the second spray after blossom petals fall (second cover period). If scab infection can be prevented during the time all the ascospores are discharged from the fruiting bodies in the fallen leaves, the disease cycle is broken and no further source of infection remains for the rest of the season. However, if the cycle is not controlled, and leaf and fruit infection does occur, then conidia are produced on these lesions and scab will remain a constant threat all season whenever wet weather occurs.
Scab-control fungicides may be protectants or dual-action materials that combine both protectant and eradicant properties. Protectant fungicides, to be effective, must be present on the leaf or fruit surface before the spores land. Materials of this type are captan, ferbam, thiram, and wettable sulfur. If the germ tube has already penetrated the leaf or fruit, these materials will not stop infection. In contrast, an eradicant fungicide can be applied to infected surfaces after the infection has occurred and will stop the infection process. This property of certain fungicides is quite useful, as the grower may apply the fungicide a short time after the beginning of a rain period and still prevent infection. Procure, Rubigan, and Rally will control scab up to 96 hours after infection has occurred, but they have few protectant properties. Detailed recommendations are available in the Pennsylania Tree Fruit Production Guide.