Late blight in tomatoes and potatoes
The late blight pathogen was confirmed by the UW in an Adams County potato field on Aug. 10. UW-Madison Vegetable Plant Pathologist Dr. Amanda Gevens is advising potato and tomato growers in the Adams County area to increase scouting for late blight symptoms, remove and destroy any infected plants, and maintain weekly treatments with an anti-sporulant fungicide for infected fields and surrounding fields.
Gardeners are advised to monitor plants for signs of infection, including brownish-black water soaked leaf lesions, dark stem lesions or sunken golden- to dark brown spots with distinct rings on the fruit surface. Removal and destruction of infected plants is required if lesions are noticed. Composting will not generate sufficient heat to kill the pathogen and is not recommended.
Late blight is a fungal disease that was the cause the Irish potato famine in the 1800’s. Around two million people died from starvation and disease as the result of this disease in the 1840’s. It is estimated that a million people emigrated from Ireland also during the 1840’s and that another four to five million people emigrated from 1850 to 1900 as a result of continuing food shortages and a poor economy.
Late blight is a potentially destructive disease of tomatoes and potatoes. It is referred to as “water mold” since it thrives under wet conditions. Symptoms of tomato late blight include leaf lesions beginning as pale green or olive green areas which quickly enlarge to become brown-black, water soaked, and oily in appearance. Lesions on leaves can also produce pathogen sporulation which looks like white-grey fuzzy growth. Stems can also exhibit dark brown to black lesions with sporulation. Fruit symptoms begin small, but quickly develop into golden to chocolate brown firm lesions or spots that appear sunken with distinct rings within them; the pathogen can also sporulate on tomato fruit giving the appearance of white, fuzzy growth.
In Wisconsin the pathogen that causes late blight is not capable of overwintering in the soil. Plant material which remains green throughout the winter, infected potato tubers that are kept overwinter as seed for the following year or that may be in a compost pile or a cull pile that does not completely freeze can serve as a source of inoculum for the following year. New infections each year may also be caused by late blight spores from the southern states where it can overwinter which may be blown up into the Midwest by storms in the late spring or early summer. The disease may also be brought into the area by the movement of infected transplants that are grown elsewhere and trucked into the Midwest for sale in garden centers.
If you have tomatoes that are infected you should destroy the plants by pulling them or by cutting them off at ground level and then leaving them in place to die and dry up. Once the plants are dead the fungus will also die. Gardeners are advised not to place the infected vines in a compost pile as there is a chance the vines may remain green and not freeze overwinter which would allow those vines to become a source of inoculum for the following year.
Healthy fruit from infected tomatoes and healthy tubers from infected potatoes are safe for human consumption. Tomatoes or potatoes with disease spots on them are not recommended to be used for human consumption. The disease organism by itself is not harmful to consume. But tissue damage and a rise in the pH that occurs can create conditions that promote the growth of other potentially harmful microorganisms. It is especially important that diseased tomatoes should not be used for canning since the potential rise in pH could adversely affect the canning process.
A late blight tomato fact sheet is available for download at; http://waushara.uwex.edu/agriculture/.
