Potato late blight. See August 13, 2014 post for tomato late blight photos.
We confirmed late blight in heirloom tomatoes from an organic farm in Ashland County, Ohio yesterday. Although the hot weather we have experienced the past week does not promote late blight as much as cool weather – the cooler daytime temps, rain and cool nights of last week were very conducive for the disease. During hot weather the disease may continue to build slowly and then spread quickly with rain and cool weather. The disease was reported on potatoes on the OSU student farm in Columbus on August 1 and last week on tomatoes in Trumbull County, Ohio (see my August 13, 2014 post). It is fair to assume that the potential for late blight is widespread in Ohio at this time, and tomatoes and potatoes need to be protected with appropriate fungicides. I talked about fungicide programs for conventionally-produced tomatoes in my August 13 post – photos of symptoms are also posted there. This post expands on information for organic tomato and potato growers, who have a bit tougher job than conventional growers in keeping late blight at bay. 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. Scouting programs must be carried out in earnest now; scouts need to be sure to look within the tomato or potato canopy for the tell-tale dark brown to black stem and petiole lesions and leaf spots. Infected tomato fruits develop copper-colored irregular lesions that may cover half the fruit or more.
If possible, diseased plants should be removed from the field and destroyed. It is best to bag the plants and discard them. Infected fruit should not be allowed to remain in the field. If it is not feasible to remove individual plants from the field, they can be killed in place by cutting them at the base or by flaming.
For next season’s planning, consider including late blight -resistant tomato or potato varieties. There is a good deal of discussion on these varieties in the webinar mentioned above.