Late & early blights: dealing with these diseases of tomatoes, potatoes

by Steven Jakobi, Allegany County Master Gardener Volunteer

Gardeners love growing tomatoes, and losing them to disease can be disappointing. There are two blights to watch out for: late blight and early blight. These can affect potatoes as well.

Causes of late blight and early blight

There are two very different blight diseases that affect tomatoes and potatoes (and some of their relatives in the plant family Solanaceae).

Late blight, caused by the fungus-like water mold, Phytophthora infestans, is a devastating disease that can quickly wipe out many acres of crop. Late blight was found in Onondaga County (the Syracuse area) in the middle of June. It was identified in Chautauqua County last year.

Early blight, caused by the true fungus Alternaria solani, can be a significant cause of plant mortality and crop loss, but it usually doesn’t spread as fast.

To confuse the matter further, late blight often occurs early in the growing season when cool, wet weather conditions allow for the rapid spread of swimming spores, called zoospores, to spread from plant to plant.

Early blight, on the other hand, often occurs later in the growing season and is typically first manifested on older leaves and maturing tomato fruit and potato tubers.

However, both diseases can occur during any part of the growing season as long as environmental conditions are favorable for reproduction and spread of the pathogens.

Of the two diseases, late blight is more devastating. World-wide, the pathogen causes billions of dollars’ worth of crop losses annually. Historically, Phytophthora infestans was responsible for the deaths of millions of people from starvation. If your ancestors came to the U.S. or Canada from Ireland or Germany in the 1840s and 1850s, most likely it was because of the potato famine, brought about by the loss of that important and nutritious crop due to late blight. During World War I, more than half a million people starved to death in Germany because supplying the war effort took away all available control measures to fight the pathogen.

Symptoms of late blight and early blight

Fortunately, the symptoms of the two blights are fairly easy to tell apart.

Late blight symptoms progress rapidly and the entire plant may die in a day or two. Dark brown/green water-soaked lesions appear on the leaves. Discoloration usually begins on the margins of leaves and soon the entire leaf is covered and dies. White spore masses and cells of the water mold typically develop on the underside of the affected leaf. Purple/black spots on the leaf stalk and stem, and grayish green, greasy or water-soaked lesions on green or maturing tomato fruit become visible. The tomato fruit or potato tuber rots, has a foul odor and is inedible.

The disease can rapidly spread from plant to plant and to adjacent fields.

Early blight symptoms are likely noticed on lower leaves first. Leaf spots are dark brown to black, enlarging in concentric rings that give the lesion a target-like appearance. (This symptom is not seen in late blight infections.)

Affected leaves then turn yellow and either dry up or fall off. Stem lesions are usually black and may kill a branch or the entire plant. Maturing tomato fruit develops sunken, blackish circles that also resemble targets at first. These then enlarge and eventually merge. The spores of Alternaria produce a black, velvety texture in the centers of the spots. Underground potato tubers do not escape the disease because spores are washed into the soil from aerial plant surfaces. Tubers develop small, black, sunken spots that render the potato unusable.

See more photos of early blight here. 

See this decision tree to tell whether your plants have late blight or early blight. You’ll have to request a user account.

Preventing late blight and early blight

Choose disease-resistant varieties of tomatoes and potatoes that have been developed over the last few decades.

This won’t guarantee that your plants won’t become diseased. Different varieties of the fungi have different degrees of aggressiveness and may favor certain cultivars of the crops. The “US8” isolate of Phytophthora is a very aggressive pathogen of most potato cultivars, but only moderately aggressive with tomatoes. And since sexual reproduction and mutation create new genetic variants of the fungi, what may have been a pathogen-resistant cultivar crop a few years ago may not be so in the future.

Another way to try to prevent blight is through cleanliness, crop rotation and eradication of weeds. The pathogens for both blights can overwinter in plant debris, rotting fruit or infected tubers left in the field, so cleaning up can greatly reduce the number of spores and cells of the fungi.

However, that may not prevent next year’s epidemic because the organisms spread easily from field to field and farm to farm by wind, rain, machinery and human clothing when environmental conditions are favorable. This is especially true with late blight.

Controlling late & early blight

Early intervention is of paramount importance. You must apply fungicide before plants are infected because products available for gardens cannot cure existing infections.

The most effective preventative fungicides currently include chlorothalonil or copper-based products, such as copper sulfate, copper octanoate or copper soap. The copper fungicides are generally considered acceptable for organic farmers, but please check with the Northeast Organic Farmers Association (NOFA) for the most up-to-date standards.

A “biofungicide,” made from the bacterium Bacillus subtilis, is available under various product names (Plant Guardian, Natria Disease Control RTU, etc.).

Regardless of the type of material used, please keep in mind that the pesticide label is a legal document and you must follow the instructions enclosed with the container. To do otherwise is against the law.

The use of chemicals is not always necessary. If weather conditions appear to favor disease development, you could spray, and if the weather looks good, you could hold off. Disease can develop with 80-100 percent relative humidity and daytime temperatures of 60-80 degrees Fahrenheit for several days. Hot, dry conditions keep fungal development and infection in check.

For more information, contact the Cornell Cooperative Extension in your county or see more at a national clearinghouse on blight called USAblight.org.