Tomatoes seem susceptible to as many bacterial, viral and fungal diseases as humans do. Certainly the most serious is late blight, a fungal disease which has been spreading quickly across the eastern United States for the past two years. This is the same blight that caused the Irish Potato Famine in the mid-19th century.
In 2009, 26 Wisconsin counties reported late blight on tomatoes. So far this year, five counties scattered throughout the state have reported late blight.
Most tomato and potato diseases can be avoided by mulching under the plants so that soil-borne bacteria doesn’t splash onto the foliage, and allowing space between plants so that air can freely circulate. But late blight spores on infected plants are swept up in wind currents high into the atmosphere, where they can be carried up to 40 miles. The spores survive up there on rainy, humid, overcast days, and then fall into gardens and farm fields when it rains. There is really no way of preventing the spores from landing on your plants.
Because late blight is highly contagious and destructive, it is critical that we do our best to prevent the spread. This means inspecting plants every day as this disease spreads so quickly that it can develop overnight.
Symptoms include leaf lesions beginning as pale green or olive green areas that quickly enlarge to become brown-black, water-soaked, and oily in appearance. Lesions on leaves produce spores which look like white-gray fuzzy growth on the undersides of the leaves.
Stems can exhibit dark brown to black lesions with spores. Tomato fruit symptoms begin small, but quickly develop into golden to chocolate brown firm lesions or spots that can appear sunken with distinct rings within them. You might also see the spores, which appear as white fuzzy growth on the tomatoes themselves.
The time from first infection to lesion development and spore development can be as fast as seven days depending on the weather. High humidity, dew, wet weather and temperatures from 50 to 80°F encourage late blight.
If you find late blight on your tomatoes or potatoes, pull up and destroy the plants immediately. Also remove the plants next to the infected plants. Do not compost the diseased plants. They should be double-bagged and landfilled.
Late blight does not survive freezing in winter, but it can survive on plant parts that are kept warm in a compost pile or in storage. It is very important to buy fresh plants or seed for next year’s tomatoes and potatoes. Do not allow volunteer tomato plants from dropped seed or potatoes you missed digging up this year to grow next year. Do not use potatoes you grew this year as seed for next year.
Inform your neighbors if you discover late blight on your tomatoes or potatoes so they can be on the lookout for it in their own gardens. The only way to prevent an epidemic of late blight is if everyone does proper clean-up and disposal of their plants.
Leave a Reply