Yes. Fungicide applications are an essential component of effective scab management, even though a cultivar may be resistant to the scab fungus. The fungicide applications contribute to the preservation of the plant’s resistance genes, which are subject to continuous evolutionary pressure from plant pathogens. For scab-resistant cultivars in particular, the most critical period for fungicide applications is during the primary infection cycle in the spring. Spores from over-wintered leaves on the ground are the “offspring” of the previous year’s infections and have the potential to infect even scab-resistant cultivars if they contain the right combination of genes. Using fungicides on scab-resistant cultivars can help prevent infection by any newly virulent strain of the fungus.
Apply fungicides in early spring from green tip and continue on a seven- to ten-day schedule until petal-fall (seven days during wet weather, ten days if dry). After petal fall, scout trees to confirm that no primary infection occurred. If no scab lesions are observed and primary scab control was successful, no further fungicide sprays for scab are necessary, although fungicide applications to manage other diseases may be necessary (i.e. powdery mildew, cedar rusts, summer diseases and rots).
Good horticultural practices can minimize, or even prevent, scab. Used in combination, these practices will ensure leaves dry quickly after a wetting (such as rain or heavy dew) to minimize conditions favorable for scab development. These practices include:
• Selecting sites that provide more than six hours of sunlight per day
• Spacing trees adequately
• Following proper pruning practices to open the tree canopy
In addition to the standard cultural practices listed above, growers should not plant scab-resistant varieties together with scab-susceptible apple varieties. It is possible that when the scab pathogen infects scab-susceptible cultivars, it may mutate to become able to infect scab-resistant cultivars. Segregating Vf-resistant and susceptible cultivars can help growers reduce the likelihood that a mutated scab offspring could occur.
For more information on managing scab-resistant apples, see Publication BP-76-W, Managing Scab-Resistant Apples (www.extension.purdue.edu/extmedia/BP/BP-76-W.pdf).
This answer was adapted from an article by Dr. Janna Beckerman, Purdue University.