Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: THURSDAY, July 13, 1995 TAG: 9507130022
SECTION: CURRENT PAGE: NRV-7 EDITION: NEW RIVER VALLEY
SOURCE: JOE HUNNINGS
DATELINE: LENGTH: Medium
Late blight, which until several years ago was rarely a problem, has become more difficult to control. Complicating the picture is the development of new, tougher strains of the fungus, plus some strains which are resistant to fungicides that used to control them effectively.
During the Irish potato famine 150 years ago, Late blight caused the death of more than one million of Ireland's eight million people. Another 1.5 million fled to the United States and Canada because of the blight, and many area residents can trace their beginnings in the U.S. to this plant disease.
The fungus that causes Late blight is called phytophthora infestans, Latin for "devastating plant destroyer." After infection the fungus grows within the plant tissues, breaking them down so it can use the nutrients within them. The breakdown of the cells causes the blight symptoms.
The blight starts as small, yellow lesions on the leaves of the plant. These change to irregular, greasy-appearing grayish areas. These areas expand rapidly during periods of moderate temperatures (60 to 80 degrees F.) and moist conditions. A white downy mold appears at the margin of the affected area on the lower surface of the leaf. These spores can be carried hundreds of miles by the wind to land on healthy plants. This is the pathogen's asexual reproductive mechanism.
If conditions are favorable, blight can spread throughout fields in days, and turn a lush canopy of tomato and potato plants into rotting foliage. On tomato fruit, the blight begins as grayish-green, water-soaked spots enlarging to indefinite size and shape on green fruits. Affected areas become dark brown, firm, wrinkled and have a relatively definite margin.
Blighted potatoes in storage turn purplish and shrunken on the surface, with a corky, reddish rot on the inside. One infected potato can quickly spoil thousands.
P. infestans also has a sexual reproduction cycle. Sexual reproduction for P. infestans requires the presence of both the A1 and A2 mating types.
Until recently only the A1 mating type had been found in the Virginias. However an A2 strain was isolated from a garden near Blacksburg last year.
When the A1 and A2 mating types fuse, they create oospores.
These sexually produced spores have thicker protective walls that allow them to survive adverse conditions. Oospores can live in soil, stems and tubers, which means they can survive during winter in the soil and infect the next season's crop.
Sexual reproduction also creates a greater genetic diversity for the pathogen's population, making it more adaptable to changing conditions. As a result, strains of the fungus of both mating types are starting to show resistance to fungicides.
In the United States, researchers have found strains of the fungus that are resistant to metalaxyl (Ridomil), a systemic chemical that once was successful in combating blight.
Gardeners have several options for managing blight. While the new strains resist metalaxyl, they are sensitive to other fungicides. These fungicides include external chemical protectants, such as mancozeb, chlorothalonil and copper fungicides. These protectants have been around for years and provide a fungicide layer on the outside of the plant.
Early preventive sprays, when conditions are conducive to Late blight spread, are important. Also, gardeners should rotate use of chemicals to avoid development of resistant strains of the pathogen.
In the past it has been important to recommend sanitation to control the fungus from getting started, by planting healthy seed tubers and destroying infected seed tubers. However, if the fungus can survive in the soil for several years, as can oospores, traditional management strategies of killing the plant and destroying the infested tubers will not work.
The long-term solution is the development of resistant potatoes and tomatoes. In the meantime be on the look-out for this famous disease - the blight of the Irish potato famine.
Joe Hunnings is the Virginia Cooperative Extension agent for agriculture in the Montgomery County Extension Office in Christiansburg. If you have questions, call him at 382-5790.
by CNB