In 2009, potato growers throughout the Southeast, Northeast and Midwest were hit with a widespread outbreak of late blight. A big box retailer selling tomato plants was suspected of being the source of the inoculum, but once it started to appear it spread rapidly.
"We were expecting a bit of an onslaught in 2010," said Willie Kirk, plant pathologist with Michigan State University.
The genotype that hit tomatoes and potatoes in 2009 was largely the new US-22 genotype, which is Ridomil sensitive, Kirk said. US-22 is a little less aggressive on potatoes – it prefers tomatoes, he said – but it was more likely to survive because it was less susceptible to rot.
"That means US-22 is a more dangerous pathogen," Kirk said.
In Michigan, the foliar disease risk in 2010 was the highest since MSU started recording. The highest risk area was in Van Buren County, and the lowest was in the Thumb area. This season, all tablestock varieties were susceptible to late blight, even the more resistant varieties. Kirk said Ridomil Bravo worked well in infected fields with one application, and the Michigan late blight forcasting website was accurate and allowed growers to be proactive about their management.
Late blight was first detected in Kentucky in late May – although this find is circumstantial because it was found on tomatoes labeled from a greenhouse in Michigan. On June 21, late blight was found on potatoes in St. Joseph, Mich., and on tomatoes in Mecosta County, Mich., on Aug. 9. By late August, late blight had been found throughout the state, including on potatoes and tomatoes in the seed-growing area of Michigan.
Late blight is of particular concern because it tends to overwinter in tubers in the field. During winter 2010, soil temperatures in Michigan did not hit the freezing point for volunteers to die. Michigan has had volunteers every year since MSU started recording in 1995, and this season the first volunteers were found in southwest Michigan in April, even earlier than usual, Kirk said.
Seed growers responded by intensifying their crop protection programs, and many left tubers in the field for 21 days, which allows infected tubers to rot. Kirk recommends that growers looking for seed in 2011 purchase only seed that has been inspected by a state seed association. He also suggests using a seed treatment targeted for late blight – one containing mancozeb. Lastly, Kirk reminds growers to be community-minded, and if you suspect late blight, let Extension know immediately.