The following is from Noah Rosenzweig, Luke Steere and Lee Duynslager, Michigan State University, and plant and soil scientists.
The Late Bright Risk Forecasting website, used for estimating potential survival of potato volunteers in Michigan, is now available for 2016 risk estimation. Epidemics of potato late blight are initiated from mycelium of Phytophthora infestans, which survives between successive growing seasons by overwintering in infected potato tubers intended for seed, as volunteer tubers 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. Several factors can influence the fate of the infected tuber, temperature being one of the most important.
Over the past five years of monitoring, it has been recorded that overwinter soil thermal conditions have been conducive for the survival of volunteer potatoes and acting as potential sources of inoculum in the spring. In each year since the monitoring was established, there have been reports of volunteers in areas even where no and low survival was predicted.
Potatoes that are left in the field at harvest and emerge the following growing season are referred to as volunteer potatoes. In areas where winter soil temperatures are not cold enough to kill tubers left in the field, they can survive the winter and become a serious weed problem the following spring. In addition, volunteer potatoes that survive the winter can harbor pests and pathogens.
Epidemics of potato late blight can be initiated from mycelium of Phytophthora infestans, which survives over winter in infected volunteer potatoes. Studies at Michigan State University have shown that tubers of most cultivars appear to breakdown after exposure to 27 degrees Fahrenheit for about one day.
We have developed a model on the Late Bight Risk Forecasting (www.lateblight.org/volunteer-risk.
php) website that predicts the likelihood of tuber survival over the winter based on soil temperatures at 2 and 4 inches between Nov. 1 and March 31.
• If tubers were exposed to temperatures below 27 F for more than 120 hours between Nov. 1 and March 31 at a 4-inch and 2-inch depth, then the risk of tuber survival is considered low (indicated by a green marker pin).
• If tubers were exposed to temperatures below 27 F for less than 120 hours at a 4-inch depth and greater than 120 hours at a 2-inch depth, then there was a moderate risk of tuber survival (indicated by a yellow marker pin).
• If tubers were exposed to temperatures below 27 F for less than 120 hours at a 4-inch depth and less than 120 hours at a 2-inch depth, then there was a high risk of tuber survival (indicated by an orange marker pin).
Almost all regions experienced soil thermal conditions that placed them in the high-risk category for volunteer survival despite the severe 2015-16 winter. Only Sandusky, Michigan, in Sanilac County was at low risk of potato volunteer survival.
This situation should alert potato growers to the high risk of potato volunteers surviving the winter. Michigan State University Extension advises all growers should therefore be implementing their integrated pest management scouting programs early in 2016 and considering volunteer elimination programs in adjacent crops and non-potato crops if herbicides are registered.