Successful use of fungicides requires careful attention to application timing. Thus if rain threatens, it is important that a fungicide be applied just as soon as the tips of the green leaves can be seen. A second spray may be needed 10 to 14 days later, once the blossom cluster is visible but before it has opened. As the season progresses, the surfaces of the fruit and foliage become more resistant to infection, but extended wet, foggy weather can bring enough wetness to cause an infection period from secondary spores that develop on leaves and fruit. If no scab infections are evident one month after petal fall, however, secondary infections will probably not be a problem.
Several fungicides are available for the control of apple scab.
These include fixed copper, Bordeaux mixtures, sulfur, liquid lime sulfur, captan, soaps, and summer oils. Do not apply captan within 3 weeks of an oil spray. Copper or Bordeaux sprays are generally used only from green tip to full bloom to reduce the risk of fruit russetting, a chemical burning of the fruit skin. In some years, however, russetting may occur even when these materials are used only before full bloom. Apply sulfur before scab spores germinate in spring and repeat treatments weekly as long as conditions are favorable for infection.