Potential survival of potato volunteers in Michigan
Climatic changes and increased disease tolerance are creating favorable conditions for potato late blight epidemics as more volunteer potatoes are surviving the winter.
Many interacting variables including meteorological factors such as climatic change and increasing tolerance of Phytophthora infestans to colder temperatures represent a serious situation for the potato and vegetable industry in Michigan. From 1950 to present, climatic conditions in Michigan have been becoming steadily more conducive for the initiation and development of potato and tomato late blight epidemics. At the same time as P.infestans populations are developing increasing tolerance to colder temperatures, winters in Michigan are becoming warmer which favors over winter survival of potatoes left behind in the field after harvest (volunteer potatoes) and waste (cull) potatoes.
Epidemics of potato late blight are initiated from mycelium of P. 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. With the recent trend for warmer winters, more volunteers and cull pile potatoes are surviving the winter and acting as sources of inoculum in the spring. 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.
Michigan winter soil temperatures from the soil surface down to a depth of 45 cm have been monitored for several years and show that the soil at 15, 30 and 45 cm depths does not experience freezing for extensive periods at any of the locations monitored between the coldest winter months in Michigan (the lowest was between 27° and 29oF for 5 days at MSU’s Southwest Michigan Research and Extension Center). We have now established that meristematic tuber tissue breaks down after exposure to 27°F for about 120 hours (5 days) in several commonly grown potato varieties, as does the pathogen that causes late blight (P. infestans).
Using a model with the following simple rules and soil temperature data from MSU’s Enviro-weather down to at least 4” depth, the risk of volunteer survival was calculated for Michigan regions for 2011 (Table 1). The rules were; 120 h of exposure from 1 Nov 2010 through 31 Mar 2011 at 4” depth and 2” depth = low risk; <120 h of exposure from 1 Nov 2010 through 31 Mar 2011 at 4” depth and > 120 exposure at 2” depth = moderate risk; and <120 h of exposure from 1 Nov 2010 through 31 Mar 2011 at 4” depth and < 120 exposure at 2” depth = high risk of volunteer survival.
Most regions experienced soil thermal conditions that placed them in the high risk category for volunteer survival. Only two regions were in categories other than high risk, Munger in Bay County had a low risk of volunteer survival and Bath in Clinton County had moderate risk, the weather station in Bath is situated on muck soil. 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 (IPM) scouting programs early in 2011 and considering volunteer elimination programs in adjacent crops non-potato crops if herbicides are registered.
Table 1. Average soil temperature at 2 and 4” depths from November 1, 2010 to March 31, 2011 at various Enviro-weather locations in Michigan and estimated risk of volunteer survival based on hours of exposure below 27oF
|Average soil temperature||Hours below 27oF||Risk of volunteer survival|
|Location of station||2” depth||4” depth||2” depth||4” depth||Risk|
|Freeland Bay County||33.4||33.1||0||0||high|