Potential survival of potato volunteers in Michigan in 2021

Begin scouting for volunteer potatoes, which may serve as possible sources of late blight inoculum. Observed high risk of survival throughout the state.

Map of Michigan with pins at various locations.
Figure 1. Estimated risk of volunteer potato survival at MSU Enviroweather locations in Michigan based on hours of exposure below 27 F, 2020-21.

The tool for estimating potential survival of potato volunteers in Michigan and Wisconsin is now available on the Michigan State University Potato and Sugar Beet Pathology website. Epidemics of potato late blight are initiated from mycelium of Phytophthora infestans. This organism survives between successive growing seasons by overwintering in infected potato tubers intended for seed, or as volunteer tubers that are left in fields at harvest, or within discarded cull and rock piles.

It is difficult to estimate the probability that infected potato stems will emerge from an infected tuber and several factors can influence the fate of the infected tuber, temperature being one of the most important. Over the past several years of monitoring, it has been recorded that over-winter soil thermal conditions have been conducive for the survival of volunteer potatoes, which may act as potential sources of inoculum in the spring.

By clicking on the Help page, you can access additional information about volunteer survival, including the following.

Studies at MSU have shown that tubers of most cultivars appear to breakdown after exposure to 27 degrees Fahrenheit for about one day, according to Kirk 2003; Wharton and Kirk 2008. A model that predicts the likelihood of volunteer survival over the winter based on soil temperatures between Nov. 1 and March 31 was developed and is summarized below:

Hours below 27 F at:


2-inch depth

4-inch depth

> 120

> 120


> 120

< 120


< 120

< 120



All regions in Michigan experienced soil thermal conditions that placed them in the high-risk category for volunteer survival (Figure 1). Potato growers should implement integrated pest management (IPM) scouting programs early in 2021 and consider volunteer elimination programs in adjacent crops and non-potato crops if herbicides are registered.

Please visit the MSU Potato and Sugar Beet Pathology website for more information.

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