Devastation by fire blight (Erwinia amylovora) of a 2-year-old high-densityGala apple orchard in Michigan after a violent storm. Other orchardsin the path of the storm had similar losses.
Photograph courtesy Alan L. JonesDepartment of Botany and Plant PathologyMichigan State University, East Lansing 48824
Background: Truly damaging outbreaks of fire blight, caused by the bacterium Erwinia amylovora, are becoming increasingly common in Michigan’s high-value apple orchards. This year, it is orchardists in the west-central Michigan fruit belt who are experiencing devastating losses after a severe rainstorm on 31 May 1998 that had winds estimated up to 140 miles per hour. It was initially estimated that the storm had destroyed 60,000 trees and injured another 60,000 (The Great Lakes Fruit Growers News, 1998, Volume 27). Two weeks later, as predicted by a program for forecasting fire blight (MARYBLYT), fire blight symptoms began to show up on trees that had survived the storm. Then a hailstorm on 16 June further compounded the problem by spreading bacteria to fresh injuries. Young trees in high-density plantings are particularly vulnerable to infection. Cultivars exhibiting the greatest damage include Gingergold, Gala, and Jonathan. In early July, E. amylovora began to ooze from the rootstock of trees on Malling 9 and 26 rootstocks after the apparent movement of the pathogen through symptomless scion tissues into the rootstock. The presence of E. amylovora in the ooze was confirmed by isolation of the bacteria and identification by PCR. In August, many apparently healthy trees will exhibit discoloration of the foliage and then collapse and die as the pathogen girdles the tree.
APS publication number: IW00008
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