Potato disease update

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The main diseases to appear in Michigan fields in 2008 have been Rhizoctonia stem canker, bacterial soft rot and seed-borne fusarium dry rot. Spring and early season conditions have been ideal this year for the sporadic appearance of these diseases although Snowden has been particularly affected by soft rot issues. The disease has not been confined to seed from any single seed lot or grower, but has been a varietal issue. We are investigating this further. In 2008 – 2009, we intend to start a fusarium dry rot survey to revisit the species and fungicide sensitivity of these species and isolates to determine if there has been a population change, since the last work was done in Michigan about 25 years ago. Some states are reporting an increase in discoveries of potato dry rot caused by fusarium graminearum (the wheat head scab pathogen). This pathogen may be sensitive to fungicides that we do not currently recommend and there may be opportunities in the future to better manage this disease.

Foliar diseases such as early blight and brown leaf spot are starting to appear now and first reports of early die. Growers have been challenged by frequent rain this year and conditions have been ideal for late blight in many areas, especially the southwest and southeast growing areas of Michigan. To date, we have only sampled one suspected plant from southwest Michigan, but this did not turn out to be late blight. With the prevalence of volunteer potatoes throughout Michigan in 2008 and the continuing conducive conditions for late blight development, it will be prudent for growers to remain on at least a seven-day schedule at the highest recommended rate of protectant residual fungcides such as chorotahlonil (Bravo, Equus, Echo) or EBDC fungicides (Manzate, Penncozeb, Dithane, Polyram). In addition at this time when white mold sclerotia are likely to germinate and produce air-borne ascospores, white mold fungicides should be applied such as Endura and Omega. Omega is also an effective late blight fungicide.

Full recommendations for management of these diseases and others can be found at http://www.potatodiseases.org/index.html. Daily updates of potato late blight risk are available at http://lateblight.org for all the MAWN sites in Michigan and updates at Extension meetings held throughout the state.

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