Tuesday, October 13, 2009

Late blight in tomatoes and potatoes in 2009

The first report in Michigan of potato late blight caused by Phytophthora infestans was on potatoes in southwest Michigan. The Kirk Lab at MSU confirmed the disease on tomatoes from home gardens in Lansing, East Lansing, Sturgis, Traverse City and Cass City in the following few days. Reports of late blight on tomatoes and potatoes in Montcalm and Mecosta Counties quickly followed and it was clear that an epidemic had broken out in many areas in Michigan. Home gardeners, organic and small growers were particularly badly affected. In 2009, an uncommon strain appeared on both tomatoes and potatoes. The strain is similar to US-14 (the established strain in most of the US is US-8). The disease is normally managed by preventing survival of its over-wintering stages in potato tubers by crop management and waste potato management and use of herbicides in rotational crops followed by regular applications of fungicides. Such management restricts epidemic development and usually excludes the disease from urban and rural areas.

At Michigan State University the Department of Plant Pathology runs a website that addresses the risk of this disease and describes measures for controlling initial sources of the pathogen and for controlling the disease should it appear during the season (www.lateblight.org). Late blight has caused considerable loss to growers in areas where the disease is uncommon, and to home gardeners. In Michigan, damage to tomatoes increased as the season progressed and put commercial potato producers at increased risk resulting in an increase in the application of crop protection products. Base-maintenance for potato late blight control for commercial growers would normally be about $100/A/season, but in 2009 this cost rose by about 50% due to increased frequency of applications and more use of expensive late blight containment fungicides. This is effectively an increase of about $2 million. In addition, growers have to protect tubers going into long-term storage with disinfectant-type products.

Whatever the source of the disease, at MSU we are developing educational programs for growers to help them manage this disease in both tomatoes and potatoes. Growers may be asking "Did I do something wrong and what can I learn from this to become a better manager/grower?" Fungicides are available but that does little to help organic tomato and potato producers as they have very little other than cultivar resistance to rely on, and little is know about this resistance with this new strain of late blight.

During August an educational pamphlet was developed in conjunction with the Michigan Potato Industry Commission and distributed by MSU extension and Bedford Farm Supply Services to Amish growers throughout mid-western Michigan who were severely affected by the disease. The pamphlet and further information can be found at your local MSU extension office orat the Michigan Potato Diseases website (www.potatodiseases.org).