The Potato Volunteer Survival website is now estimating potential survival of potato volunteers in Michigan in 2017. 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, or 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 over-winter soil thermal conditions across Michigan have been conducive to the survival of volunteer potatoes and acting as potential sources of inoculum in spring. In each year since the monitoring established, there have been reports of volunteers in areas where no and low survival was predicted.
Potatoes that are left in the field at harvest are known 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 diseases.
Studies at Michigan State University have shown that tubers of most cultivars appear to break down after exposure to 27 degrees Fahrenheit for about one day, thus eliminating the threat of surviving Phytophthora infestans. Researchers have developed a model 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 degrees for more than 120 hours between Nov. 1 and March 31 at 4- and 2-inch depths, the risk of tuber survival is considered low, indicated by a green marker pin.- If tubers were exposed to temperatures below 27 degrees for less than 120 hours at a 4-inch depth and greater than 120 hours at a 2-inch depth, there was a moderate risk of tuber survival, indicated by a yellow marker pin.
- If tubers were exposed to temperatures below 27 degrees for less than 120 hours at a 4-inch depth and less than 120 hours at a 2-inch depth, there was a high risk of tuber survival, indicated by an orange marker pin.
All regions of Michigan experienced soil thermal conditions that placed them in the high-risk category for volunteer survival despite the severe 2016-17 winter. This situation should alert potato growers to the high risk of potato volunteers having survived the winter.
All growers should be implementing their integrated pest management scouting programs early in 2017 and considering volunteer elimination programs in adjacent crops and non-potato crops if herbicides are registered.
Source: Michigan State University Extension