The Apple Scab Epidemic of 1993

Without a doubt, 1993 has been the worst year for apple scab on apples in over a decade. Some orchards nearly 100% fruit injury in susceptible blocks. How did we get into this mess? Here are some of the contributing reasons:

  1. Too much rain. You already knew that, didn't you? Scab thrives on prolonged rainy spells in spring and summer. Rain has prevented many growers from getting into their orchards in a timely way, and has washed off part or all of fungicide sprays prematurely. As a result, the leaves and fruit have been unprotected for extended periods.
  2. 1992 weather. Last season was extremely wet in July, which may have contributed to a buildup of scab on leaves, especially if summer fungicide sprays were not adequate. Midsummer to late summer wet periods are insidious because they often do not infect fruit but do infect leaves. The result can be a good- looking crop but a "time bomb" for the next season in the form of infected leaves that allow large amounts of the fungus to survive over the winter.
  3. Susceptibility. Some of our main crop varieties, including MacIntosh and Red Delicious, are highly susceptible to scab.

What kind of strategy is best for dealing with an orchard that has been hit hard by scab? It's wise to continue protectant summer sprays this season, just to suppress additional secondary cycles of the disease and avoid adding to the amount of overwintering fungus in your orchard. Prune during the dormant season to insure good spray penetration. Next year, a rigorous protectant schedule of fungicides is advisable, starting at bud break. If you use contact fungicides exclusively, kep the interval between sprays short, especially during primary scab season (ending at about second cover). If you use eradicant fungicides such as Nova or Rubigan, tank-mix them with contact fungicides and don't extend the spray interval past 10 days during the primary scab period. Evaluate the orchard's recovery by counting the number of scabby apples out of 100 on each of 5 trees per block; if 1% or less of the apples have symptoms in the 1994 harvest, your cleanup has been pretty successful. Of course, a little assist from dry weather wouldn't hurt a bit.

You may also want to try some of the newer scab-immune varieties that have come into the nursery trade. Paul Domoto and I have established 9 plantings of 13 different scab-immune varieties around Iowa this year, including 3/4-acre, replicated plantings at the Hort Farm and the Castana Research Farm, so you may want to keep tuned as we learn more about the suitability of these varieties under Iowa conditions.

This article originally appeared in the July 28, 1993 issue, p. 130.


Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 28, 1993. The information contained within may not be the most current and accurate depending on when it is accessed.