Two common diseases on apple trees are cedar apple rust and apple scab. Though some apple varieties are resistant to these diseases — including Liberty, Jonafree, Redfree, Freedom, Williams Pride and Enterprise — most varieties are susceptible. For a listing of the disease resistance of various cultivars, go to: http://extension.missouri.edu/explorepdf/agguides/hort/g06022.pdf
Fungicide sprays during April and May are critical to preventing disease on susceptible varieties. With the warm spring this year, we may have to start sprays in March. The first spray should go down when leaves appear. A fungicide that is available to homeowners and very effective for control of apple scab and cedar apple rust is myclobutanil (Immunox). There are several formulations of Immunox but only one is labeled for fruit. Check the label.
Sprays should be done on a 7- to 10-day schedule to keep the protective chemical cover on the rapidly developing leaves and fruit. An insecticide will need to be added to this mixture after petal drop to prevent damage from codling moths that cause wormy apples. Methoxychlor or malathion can be used as an insecticide. In order to protect bees, DO NOT use any insecticide during bloom. Check the malathion label before purchase as not all labels list fruit.
Although gardeners may continue to use myclobutanil throughout the season, certain other fungicides are more effective on summer diseases such as sooty blotch and fly speck. Consider switching to Captan or to a fruit spray mixture about June 1. A spreader-sticker can be added to the fungicide-insecticide chemical mixture to improve the distribution and retention of the pest control chemicals over the leaves and fruit. A hard, driving rain of about 1 inch or more will likely wash chemicals from the leaves and fruit. In such cases, another application should be made. You can find information on controlling insects and diseases on fruit trees in our publication titled “Fruit Pest Control for Home Gardens” at http://www.ksre.ksu.edu/bookstore/pubs/c592.pdf Below is the spray schedule I use. Sprays are applied ever 10 days until 2 weeks before harvest.
Leaves Appear: Immunox
Petal Drop: Add malathion to the Immunox and so the mixture is Immunox + malathion
June 1: Replace Immunox with Captan. The mixture is now Captan + malathion Stop spraying 2 weeks before harvest. (Ward Upham)