Our colleagues in Ontario reported late blight today on tomatoes in Chatham-Kent. That means that inoculum of the late blight pathogen is active in the region, and given the rainy/overcast, cool conditions of the last week or so, it is time to make sure tomato and potato crops are protected from this disease.
It is very important that both potatoes and tomatoes be scouted regularly (at least twice per week) for late blight. If late blight is suspected, it can be confirmed by bringing or sending a sample to the OSU Vegetable Pathology Lab in Wooster or the OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic in Reynoldsburg. We appreciate hearing from you if late blight is suspected so that we can confirm and alert others.
Growers need to maintain an effective fungicide program on tomatoes and potatoes. This should continue as long as rainy conditions, high humidity and/or heavy dews are expected. If late blight has not been observed and weather conditions are generally dry and warm, use a protectant fungicide on a 7-10 day schedule, depending on how fast the plants are growing. Good protectants are chlorothalanil (Bravo, Equus, Echo), mancozeb (Penncozeb, Manzate, Dithane), and to a lesser extent, copper-based products – use according to label instructions. Under cool wet conditions when late blight is likely, or if late blight has been found on the farm or nearby, use one of the following, tank mixed with one of the above protectants: Curzate (3 day PHI),Orondis Ultra (1 day PHI),Presidio (2 day PHI), Previcur Flex (5 day PHI), Ranman (0 day PHI), or Tanos (3 day PHI). Orondis Opti (3 day PHI), Zing! (5 day PHI) and Gavel (5 day PHI) are pre-mixes containing mancozeb or chlorothalanil. Note the Pre-Harvest Interval (PHI) for these fungicides ranges from 0 – 5 days. So far, Ohio late blight strains have been sensitive to Ridomil, so that is another option (but may be risky).
For organic producers, there is a good webinar on late blight and its management in organic potatoes and tomatoes originally presented in January 2014 and featuring leading researchers on late blight from New York, Pennsylvania, Wisconsin and Florida. They mention Actinovate AG, EF-400 and Zonix as products approved for organic that can be effective in reducing late blight – but will not completely control the disease. OMRI-approved copper fungicides (Champ WG, Nordox 75 WG, and others) generally rank highest for control among organic approved products. Actinovate, EF-400 and Zonix might be used in a program with copper. NOTE: It is critical to apply fungicides prior to infection – these fungicides (and most for conventional use as well) are not effective once plants have become infected.
If the disease is mainly found in one or a few foci, it is a good idea to remove and destroy the diseased plants. This is particularly important for organic tomato or potato growers. Home gardeners should consider spraying tomatoes and potatoes with a fungicide containing chlorothalanil, and should prune out diseased tissue. Pack up diseased plants in a plastic garbage bag and discard – don’t leave them in the field or garden or on a compost pile, where the pathogen can be released into the air and spread to other plants.