Late Blight On Potatoes, Tomatoes Confirmed In The Northeast
July 1, 2009 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. This was followed by a flood of findings from Ohio to Maine as extension specialists started looking in stores. Additional findings in home gardens were reported, as well, including on plants purchased up to two weeks earlier.
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, which is the main place that it has been found.
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 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:
www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/lateblight_tomato.htm and vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm. Home gardeners can learn more about the situation by going to http://blogs.cornell.edu/hort/?s=late+blight+disease.
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. The herbicides diquat and paraquat are good choices for applying with a hand sprayer. Gramoxone is effective but dangerous; there is no antidote in the event of accidental exposure. 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.
Conventional Fungicides
Begin a fungicide program with products specifically for late blight in this field and other fields near by. These products have translaminar activity and thus provide better coverage than contact, protectant fungicides. A five- to seven-day spray interval is recommended when weather conditions are wet and cool. It can be extended to 10 days under hot, dry conditions.
Alternate among fungicides in different chemical groups (as indicated by FRAC Code) to manage resistance. The late blight pathogen has demonstrated ability to develop resistance; Ridomil fungicides are no longer recommended because of resistance. Include in each application a protectant fungicide like maneb, mancozeb or chlorothalonil, or triphenyltin hydroxide for potatoes. This is important for resistance management and ensuring effective control, and is specified on the label and thus is a requirement. A spray program with just protectant fungicides applied regularly starting before late blight begins to develop can provide adequate control, but this is challenging to achieve when plants are actively growing and conditions are very favorable for disease development, as has been occurring this spring.
Curzate (FRAC Group 27 fungicide) at 3.2-5 ounces per acre (3.2 ounces for potatoes) or Tanos (also contains cymoxanil, active ingredient in Curzate) at 8 ounces is a good choice for the first application because these fungicides have some kickback activity, thus they can suppress some established lesions. The maximum kickback is about two days when it is cool, declining with increasing temperatures to about zero above 80 degrees F. Cymoxanil has little residual activity, therefore, five days later apply another fungicide.
Revus Top (Group 40 + 3) is a new fungicide that has excellent activity for late blight. It gets into plants fast, in about 30 minutes, then slowly moves in the plant providing good residual. It is labeled for use at 5.5 to 7 fluid ounces. It does not need to be applied with a protectant fungicide. Unfortunately, the U.S. inventory of this product has been used up. However, the manufacturer has responded to the situation and prepared a supplemental label for another fungicide, Revus (Group 40), which is not labeled presently for use on tomatoes and potatoes.
Previcur Flex (Group 28) has some systemic activity, which is an important attribute even though it is not as systemic as Ridomil. It was the only fungicide rated good for symptoms on stems and also for protecting new growth in a bulletin from the University of Maine; it is not known how effective many of the other products are on new growth that develops after the application. The product was not rated as highly as other late blight fungicides for leaf symptoms (good versus excellent). It is considered a good choice for an application made right before rain, as the product is rainfast in 30 minutes. It is labeled for use at 0.7-1.5 pint (1.2 pints max for potatoes). According to the manufacturer, Previcur Flex provides best control when applied in blocks of two applications alternated with two applications of other fungicides.
Other fungicides to consider including in the fungicide program are Gavel (Group 22) at 1.5-2 pounds, Forum (Group 40) at 6 fluid ounces, and Ranman (Group 21) at 1.4–2.75 fluid ounces plus Omega (Group 29) at 5.5 fluid ounces for potatoes. Gavel is the only late blight fungicide formulated with a protectant.
Group 11 fungicides (Headline, Quadris, Reason, etc) and Group 33 (phosphorous acid) fungicides are not considered as effective for late blight as the other products.
Good fungicide coverage is critical. Pathogen spores can be moved on equipment and workers, therefore spray and work in affected fields last and clean equipment between fields.
As soon as harvest is complete or the field is abandoned, apply an herbicide like diquat to kill plants and/or disk down the field.
Management In Organic Crops
Apply fungicides preventatively, using a five-day schedule when conditions are favorable. Copper is effective for protecting a crop, but copper has been found to be ineffective when used as the sole practice for controlling late blight once it has started to develop. Other OMRI-listed fungicides labeled for late blight include Sporatec, Sonata, Serenade Max, and Companion. It is important to scout regularly and promptly destroy affected plants when found to reduce the amount of inoculum in a field. It is recommended that plants with symptoms be physically pulled up plus a few border plants, preferably on a bright sunny day when possible, then tarp the plants; spores will be killed by sunlight and also heat under the tarp. Scout daily thereafter for a few days to see if more plants develop symptoms. Clean after working in infested fields to avoid moving spores on equipment and workers. As soon as harvest is complete or the field is abandoned disk down field.
The specific directions on fungicide labels must be adhered to. They supersede these recommendations (above), if there is a conflict. Check state registration and organic approval before using a product. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.
Some of this information on management was provided by Dr. Steve Johnson, University of Maine Cooperative Extension, and Dr. Tom Zitter, Cornell University.
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