Late blight, early blight and brown leaf spot risk status in potatoes

Disease risk severity accumulations are widely variable for both late blight and early blight across Michigan.

As of Monday, June 10, 2013, there is great variability in the disease risk severity accumulations for both late blight and early blight (P-values) across Michigan. These are updated daily and stations across the state can be compared at the Late Blight Risk Monitoring website. The accumulations were initiated on May 1 and for many parts of the state, potatoes were only planted mid- to late May. It is possible to accumulate late blight risk based on emergence date by choosing your own emergence date. However, this is not available for P-value accumulations.

Our general recommendations for initiating late blight crop protection programs are when DSV values accumulate to 18. Early blight protection programs should be initiated at P-value = 300. Some areas have reached DSV = 18 and programs should be started if growers have not already done so and P-values are approaching 300 in some areas also. Current recommendations for fungicides for late blight, early blight and brown leaf spot can be found in the Michigan State University Extension bulletin E312, “Insect, Disease and Nematode Control for Commercial Vegetables” on page 114.

Early season applications of fungicides before secondary inoculum is produced often have minimal or no effect on the spread of early blight. Early blight can be adequately controlled by relatively few fungicide applications if the initial application is properly timed.

Although no late blight has been found in potatoes since early 2013 in Florida, there was a recent report on June 7 of late blight from Tennessee on tomatoes, although the genotype has not yet been reported.