By Diana Alfuth
Pierce County UW-Extension Horticulture Educator
ELLSWORTH - Late blight is a destructive disease of tomatoes and potatoes, and it has recently been confirmed in Pierce, Polk and St. Croix Counties.
The disease is caused by a fungus-like water mold called Phytophthora infestans., which is the same organism that caused the great Irish potato famine in the mid 1800’s. It starts with pale green or olive green spots on leaves that quickly enlarge to become brown-black, water-soaked and oily looking areas. A white cottony mold can also form on affected leaves and stems. Tomato fruits develop large, often sunken, light to chocolate-brown, firm spots , sometimes with distinct rings, followed by rotting. Potato tubers with late blight develop reddish-brown discoloration under the skin, which may become sunken. Entire plants can collapse and die from late blight in 7 to 10 days if weather conditions are favorable. The patch will start to smell as plants and fruits break down.
The spores usually blow into an area by wind, or can be brought in on infected plants. It does not over winter in the soil, but can survive on plant material that remains alive over winter, such as seeds or potato tubers that are missed during harvest.
Plants that get late blight cannot be saved and should be removed and disposed of immediately to limit spread to other plants. You can pull entire plants, place them in plastic bags and leave the bags in the sun for several days to make sure the plants and the disease are totally killed, then place the bag in trash for pickup. Do NOT place the plants or any affected fruits on your compost pile
Healthy-looking fruits from affected plants are safe to eat or preserve, but if the tomato fruits or potato tubers show any symptoms, they should not be used.
To avoid problems in the future, be sure to dispose of any volunteer tomato or potato plants that sprout next year, as well as any nightshade weeds in the area. Do NOT use tubers from this year’s potato crop as seed potatoes, but instead purchase certified seed potatoes from a reputable supplier each year. Consider planting tomato varieties that show resistance to late blight, such as ‘Better Boy’, Golden Sweet’, ‘Green Zebra’, ‘Juliet’, ‘Roma’, ‘Sun Sugar’ and ‘Wisconsin 55’.
Fungicides can also be used to reduce late blight, but in most cases, fungicides are preventative, not curative, so they must be applied prior to the onset of the disease. If weather is dry and hot, conditions are not favorable to late blight so fungicides are most likely to be useful during periods of wet, cool weather. Be sure to follow all label instructions if you choose to use a fungicide.
For questions about late blight, contact Diana Alfuth, UW-Extension Horticulture Educator, at (715) 273-6781.