By Willie Kirk and Lee Duynslager, Michigan State University, Department of Plant, Soil and Microbial Sciences; Phill Wharton, University of Idaho; Kathleen Baker, Western Michigan University; and MSU Enviro-weather.
The Late Blight Risk Forecasting website is now estimating potential survival of potato volunteers in Michigan in 2014. 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, and several factors can influence the fate of the infected tuber, temperature being one of the most important.
Over the past three years of monitoring, it has been recorded that over-winter 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 each year in areas even where no and low survival was predicted.
By clicking on the colored tabs on the map on the homepage of the Late Blight Risk Forecasting website, you will be directed to an explanation of the question: What is volunteer survival?
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.
Epidemics of potato late blight can be initiated from mycelium of Phytophthora infestans, which survives winter in infected volunteer potatoes. 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. We 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 Farenheit for more than 120 hours between Nov. 1 and March 31 at 4- and 2-inch depths, then 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, then 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, then there was a high risk of tuber survival, indicated by an orange marker pin.
Most regions experienced soil thermal conditions that placed them in the high-risk category for volunteer survival despite the severe 2013-14 winter. Only Stephenson, Michigan, in Menominee County, and Bay County were at low risk. Grand Junction, Michigan, in Van Buren County; Richville, Michigan, in Saginaw County; Bath, Michigan, in Clinton County; Linwood, Michigan, in Bay County; and Verona, Michigan, in Tuscola County, had moderate risk of potato volunteer survival.
This situation should alert potato growers to the high risk of potato volunteers surviving the winter, and all growers should therefore be implementing their integrated pest management scouting programs early in 2014. Michigan State University Extension encourages growers to consider volunteer elimination programs in adjacent crops and non-potato crops if herbicides are registered.
This article was published by Michigan State University Extension. For more information, click here. To contact an expert in your area, click here or call (888) MSUE4MI.