Staff Writer
PRESQUE ISLE – The frequent showers and wet conditions as of late have created excellent conditions for the spread of potato late blight. Late blight has now been found in Caribou, Connor, Hamlin, Van Buren, Grand Isle, Madawaska, Fort Kent, Westfield, and Fryeburg. “There have also been confirmed cases in Saint André, New Brunswick and Drummond, New Brunswick,” said Dr. Steven B. Johnson, crops specialist and professor at the University of Maine Cooperative Extension. “When I go to a field to verify late blight, I take a look at how long it’s been there, what it looks like, and right now we’re still in the stage of the epidemic that it is of great value to physically destroy the area that’s infected … take it right out of the equation.”
Johnson said there are a couple of ways to do that.
“Some people use a chemical desiccant,” he said, “while others on nice, hot, sunny days pull the plants and either leave them, bury them in the soil or bag them.”
Potato late blight is caused by Phytophthora infestans, a fungus that overwinters in infected tubers, cull piles, and in infected volunteer plants.
“Potato late blight leaf lesions first appear as irregularly shaped, pale to dark?green, water?soaked spots,” said Johnson. “These usually appear at the tip of the leaves or on actively growing tissue. The lesions enlarge rapidly and form brown to purplish?black necrotic areas with indefinite borders. A yellowish?green halo will appear surrounding these lesions.
“In cool, wet weather, or other humid conditions, a ring or a surface of white fungal growth may appear at the edge of the lesion on the underside of the leaf,” he said. “The lesions are often covered with a glistening, white growth of fungus. In cool, wet weather, late blight may move rapidly from leaves to petioles and stems and infect an entire field in only a few days. A field heavily infected with late blight will have a distinct fetid odor associated with it. The odor is that of rapidly dying potato foliage, and is similar to that of vine killing. In dry weather, the water soaked halo disappears from the lesions and the dead tissue becomes brown and dry.”
Late blight spores will also infect tubers, usually at harvest time, but can attack tubers while they are still in the hill or while in storage.
“At first, the infected tubers have a brown to purple sunken discoloration of the skin,” said Johnson. “The margin between diseased and healthy tissue is not distinct. Brown, peg?like projections may penetrate into healthy tissue to various depths. In newly-harvested tubers, white tufts of fungal growth may appear at the lenticels, eyes, or on surface damage. Under cool, dry storage conditions, tuber lesions develop slowly as typical dry rots.
“Often, secondary organisms invade the diseased tubers. The fungus causing potato late blight overwinters in infected tubers, cull piles, and in infected volunteer plants. This fungus will not survive long in soil or away from association with a host,” he said. “To control potato late blight, growers should plant certified seed free of late blight, follow a protective spray program for late blight and cull piles and volunteer potatoes should be eliminated. Some potato varieties are more resistant than others.”
Officials with the Maine Potato Board reminded growers that finding late blight is nothing to be embarrassed about.
“It’s a fact of life in the potato business,” said Donald Flannery, executive director of the MPB. “It is what it is and we have to deal with it. If we look at it as something to be ashamed of, then it gets hidden and it gets worse and it’s harder to take care of.”
Growers who find late blight in their fields are asked to contact the University of Maine Cooperative Extension at 764-3361.
“If they find something that’s in question, farmers should call the Extension because that’s ultimately who’s going to confirm whether it is or isn’t late blight,” said Flannery. “That confirmation is key.”
Johnson agreed.
“The best control is early detection,” he said, “and then action from then on.”
Growers should keep their spray programs on what the University is recommending and use the Extension’s resources including the IPM hotline (207-760-9IPM).
James Dwyer, crops specialist for the University of Maine Cooperative Extension, said growers in all locations “need to be field scouting for potato late blight at this time.”
“If potato late blight is found,” he said, “please inform your neighbors; you would want the same courtesy.
“Scouting this past week has been challenging because of the frequent showers, and we were not able to survey some of our regular fields,” said Dwyer. “It is our policy not to have our staff scout fields when the foliage is wet. We encourage all growers to be scouting fields when the weather permits.”
