Failing fungicides force rapid revisions of apple disease control programs used by farmers in eastern United States
CALS Impact Statement
Because a fungal disease of apples has become resistant to many fungicides, apple growers needed to change quickly to new fungicide programs or risk losing their entire apple crop. By blanketing the eastern apple-producing areas of the United States and Canada with information on how to adjust apple scab control programs, major losses were averted in most orchards.
Apple scab is fungal disease that causes black spots on apple fruit and leaves. Severely affected fruit crack. Even fruit with small spots are unmarketable. For the past 40 years, fungicides were available that could stop apple scab development even when applied after the fungus initiated growth on fruit or leaves. Now the apple scab fungus has developed resistance to all of these fungicides in some orchards. In several cases, apple scab has caused complete crop failures because growers were still depending on fungicides that were no longer effective.
Over the past three years, Dr. Wolfram Koeller at the New York State Agricultural Experiment Station in Geneva has studied apple scab from more than 25 orchards where growers reported scab control failures. Orchards with scab-control failures consistently had apple scab populations containing a high percentage of individuals resistant to the demethylation inhibitors (DMI) fungicide group. Dr. David Rosenberger, from Cornell University's Hudson Valley Laboratory, used Koeller's data to convince apple growers that they needed to alter their fungicide programs. Rosenberger incorporated the fungicide resistance data and suggestions for alternative scab-management strategies into numerous extension articles and oral presentations. Since October 2002, Rosenberger made 25 oral presentations on fungicide resistance at fruit-grower meetings throughout New York State, Wisconsin, Michigan, Ohio, New Hampshire, Vermont, Connecticut, and Pennsylvania, as well as Ontario and Quebec. Total audience was more than 1,745 people. At the same time, Rosenberger authored or co-authored 16 separate extension articles describing how growers should adjust fungicide programs to compensate for fungicide-resistance problems. Those articles often were reprinted in local grower newsletters or included in regional extension websites, ultimately appearing in 42 different venues in the United States and Canada.
The widely disseminated warnings about fungicide resistance penetrated most apple production areas in eastern North America. If a number of apple growers had not been apprised of this problem and modified their fungicide programs, significant crop loss could have occurred over many of the 188,000 acres of apples that exist in non-arid production areas of the United States. Because eastern production regions were blanketed with this information, most fruit growers avoided the catastrophic losses that occurred on some orchards where fungicide-resistant apple scab destroyed the entire crop after growers continued to use the now-ineffective fungicides. One orchard in the Champlain Valley suffered near-complete crop loss for two successive years on several acres of McIntosh before we identified fungicide resistance as the cause of the scab problem. By using different fungicides the next year, that grower was able to completely eliminate losses to apple scab. The same scenario was repeated in Saratoga Springs, on Long Island, and in several Ohio orchards. In Ohio more than 100 acres of apples could not be harvested in 2004 because of severe scab. (Apple growers should gross at least $5,000 per acre if one assumes an average value of $10 per bushel and production of 500 bushels per acre.) Similar disasters in other orchards have been averted by alerting fruit growers to fungicide-resistance-issues and by convincing them to adopt more alternative programs before they suffer extensive crop loss. Effective and efficient use of fungicides benefits not only the fruit growers involved, it also reduces environmental impacts: growers who fail to control the early stages of apple scab in spring end up using much more fungicide throughout the season as they attempt to protect their crop from secondary infections.