VEGETABLE CROP UPDATE
TOMATO & PEPPER EDITION
Janice LeBoeuf, OMAFRA Vegetable Crop Specialist
 Tomato late blight
There has been a lot in the media lately about the late blight outbreak in the US Northeast. As infected plants were apparently shipped to major garden centres across that region, the disease became widespread across that region very early in the season.
Some areas of Ontario got significant rainfall this past weekend. Cloudy, rainy weather is favourable for the development of late blight, if spores are present. Late blight was found in July in a few potato fields in Ontario, but was quickly controlled. One tomato field in the London area was also diagnosed with late blight and control measures were taken. It is important to be able to recognize this disease, so that action can be taken quickly if it is found.
Descriptions and photos, as well as management information, can be found on the Ontario CropIPM site at http://www.omafra.gov.on.ca/IPM/english/tomatoes/diseases-and-disorders/late-blight.html.
Late Blight Symptoms (figures 1, 2, and 3)
Late Blight Occurrence and Management in Potatoes and Tomatoes in the Northeastern United States
Excerpted from an article by Margaret Tuttle McGrath, Associate Professor, Plant Pathology, Cornell University
On Long Island, NY, symptoms of late blight were confirmed on June 23 in a commercial field of potatoes and on June 24 on tomatoes in a near-by home garden. Appearance of symptoms in the garden suggested they were the result of inoculum spread from the commercial field. It also appeared that spread occurred in the commercial field. Based on these observations late blight likely had been on Long Island for about two weeks. Prior to this there had been a few reports of late blight from states to the south and west of Long Island. Then came a report of late blight on tomato plants at a retail store in Ithaca, NY, on June 24. This was followed by a flood of findings from Ohio to Maine as extension specialists started looking in stores. Then came more findings in home gardens, including on plants purchased up to two weeks earlier, and in plantings on farms. So far in 2009 late blight has been detected in South Carolina, North Carolina, Virginia, West Virginia, Ohio, Maryland, Delaware, New Jersey, Pennsylvania, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, Vermont, and Maine. Occurrence of late blight this year is unprecedented. There is no record of it being this widespread in the eastern U.S. this early in the growing season. And it has never been seen on tomato plants for sale in garden center stores.
This is an extremely destructive disease when not managed, quickly killing foliage and rotting tomato fruit and potato tubers. Late blight was the cause of the Irish Potato Famine.
Rainy, cloudy conditions have been providing favorable conditions for the pathogen to successfully be dispersed, including long distances and for infection. Clouds protect spores being dispersed in wind from the killing effect of ultraviolet radiation.
All tomato and potato crops [in the affected region – J.L.] are at high risk of developing late blight this season, especially if the rainy weather continues. All growers should assume their crops eventually will be affected and thus should be on a weekly schedule to both thoroughly inspect their potato and tomato plantings and apply fungicides.
Classic symptoms are large (at least nickel sized) olive green to brown spots on leaves with slightly fuzzy white fungal growth on the underside when conditions have been humid (early morning or after rain). Sometimes the lesion border is yellow or has a water-soaked appearance. Leaf lesions begin as tiny, irregularly-shaped brown spots. Brown to blackish lesions also develop on upper stems. Firm, brown spots develop on tomato fruit. Photographs are posted on the web at:
http://www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/lateblight_tomato.htm
http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm
When late blight is found in a localized spot in a field, promptly destroy all symptomatic plants plus a border of surrounding plants to eliminate this source of inoculum. Physically pull and drop affected plants, spray with herbicide, or disk. When disking is used the crop should first be sprayed with fungicide because of the potential to move spores on equipment especially while driving out of the field, and the equipment should be pressure washed afterwards.
Tomato late blight management in Ontario
Michael Celetti, OMAFRA
Managing late blight requires a proactive and integrated approach including scouting, cultural methods and fungicide application. The following are some tips to consider for managing this disease.
Crop Scouting:
Tomato growers should try to monitor their crops twice a week for the disease. If a few plants are observed to be infected, they should be placed in a sealed plastic garbage bag, removed and destroyed away from the field as soon as the disease is observed. Volunteer tomato or potato plants in nearby fields should also be destroyed since they may be a source of the disease. If a large area of plants are found to be infected it will be impractical to remove them from the field. Disking under large areas of infected plants and applying a fungicide with some kick back has proved to be successful in managing the disease while minimizing losses for some growers, provided the epidemic has been caught early.
Weed Control:
Good weed control particularly of host plants such as hairy nightshade is also important. Dense weeds also reduce drying within the crop which prolongs the leaf wetness period for infection and makes fungicide penetration into the canopy more difficult.
Fungicide Application:
Growers should be vigilant about keeping up with scheduled fungicide applications to reduce losses and inoculum from building up. There are several good fungicides registered on tomatoes for late blight control in Ontario (See OMAFRA Vegetable Production Recommendations Publication 363). Remember to rotate fungicides with different modes of action to reduce the possibility of selecting for resistant strains of the pathogen. If an application of a registered fungicide has not yet been made to the crop, it is a good idea to consider making an application soon before the disease appears. Remember fungicides work best against this disease when they are on plant tissue before the pathogen infects. Good spray coverage is also very important. Adjust water volumes, pressure and ground speed to ensure the fungicides are adequately covering lower leaves and stem. During wet cool periods, a fungicide should be applied every 5 - 7 days. If the weather conditions become dry, the spray intervals may be extended to every 12 -14 days.
Cull Pile and Volunteer Plant Destruction:
It is very important that growers burn or bury cull piles if they haven't already. Tomato and potato debris left on fields should also be cultivated deep into the ground to encourage decomposition and reduce the potential of unsuspecting infested debris from becoming a source of the disease.
Full article at http://www.omafra.gov.on.ca/english/crops/hort/news/hortmatt/2006/23hrt06a3.htm.
Visit the OMAFRA Vegetable Web Pages: ontario.ca/crops - click Vegetables |
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