Straw bales set up and ready to be prepared - watched by Marlin

Reading this article reminds us all of the serious nature of Late Blight (though it is certain that things have moved on beyond this in the years since - diseases find ways to survive through mutations, of course). It also reminds me of my friendship with Tad, from whom I received from really interesting tomatoes, including Pale Perfect Purple, and the variety that I believe the bees crossed with Brandywine in my garden, leading eventually to Lucky Cross and Little Lucky (the variety was Tad, which was work in progress by Tad, and itself not stable when I grew it).

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Late Blight: New Strain Threatens Tomatoes and Potatoes

by Dr. Tad Smith (Senior Plant Pathologist, Rohm and Haas Co.)

A very troublesome disease known as late blight of potato and tomato is once again causing problems for the home vegetable gardener and commercial grower. The fungus responsible for this blight is known to scientists as Phytophthrora infestans. This is an appropriate name for this plant pathogen, as its name is derived from the Greek language (phyto = plant, phthrora= destroyer, and infestans refers to the infestation). It is the same fungus that caused the Irish Potato Famine of the 1840’s, which resulted in the starvation of approximately 1 million Irish and the immigration of millions of Europeans to North America.

For many years, commercial potato growers in the United States were able to control the well-established strain of late blight known as A1. By planting healthy potato tuber pieces and applying fungicides when weather conditions were favorable for the fungus, growers were able to maintain the upper hand against late blight. In the early part of 1994, it was obvious to plant pathologists that something had changed in the dynamics of the disease. A new strain was identified in Florida, and reports of severe outbreaks of late blight were surfacing in Georgia and the Carolinas. Soon the disease was reported in cooler areas of the Appalachian Mountains. Not only were commercial growers reporting tremendous damage in their potato and tomato fields, but home gardeners watched with horror as the foliage on their lush tomato plants died literally overnight. As the year progressed, reports of disease spread up the east coast to Maine, with scattered reports from the Midwest and western states.

What had happened? From a scientific viewpoint, all three parts of the plant-disease triangle had fallen into place during 1994; in other words, a virulent fungus had combined with cool and wet weather to destroy crops of susceptible potatoes and tomatoes. The virulent fungus was a strain of late blight not previously found in the United States. It was designated as strain A2, and was probably introduced into the United States from Mexico in the 1990’s. Free trade may be good for the economy, but it is a biological disaster. During this century, the United States has been invaded by numerous foreign plant diseases. Dutch elm disease, chestnut blight, and dogwood anthracnose are just a few recognizable examples.

During 1993, the disease became established in some potato fields, in the spring of 1994, diseased potato tubers were inadvertently transported and planted all over the United States, and the first part of the disease triangle was in place. As soon as the crop of tomatoes and potatoes began to grow, another part of the triangle was complete. The third part of the triangle was completed by the unusually cool and wet summer throughout much of the country.

The development of a late blight epidemic depends on weather conditions, particularly high humidity and cool temperature. The fungus produces spores most abundantly at 100% humidity and temperatures between 60 and 70 F. Plant leaves must be wet for spores to germinate and infect leaves. This process can occur as quickly as 4 hours. Within 5 days, one infected leaf can produce millions of new spores. Unless the air temperature rises quickly and the plant remains dry, the potato or tomato plant will become severely diseased, and an entire field can turn into a fetid mass of dead plant tissue.

The introduction of strain A2 into vast areas of the United States presents several problems. First, this strain is much more virulent than the old A1 strain. Strain A2 can kill plants much quicker, before a grower can react to the fungus. By the time a grower can apply fungicide to protect the crop, it may be already severely diseased. Second, even if the fungicides are applied in time, some isolates of A2 may be less sensitive to fungicides than strain A1. Finally, some isolates of A2 may be able to survive the winter in the soil by forming a survival spore through sexual reproduction with strain A1. These specialized spores, known as oospores, could infest thousands of acres of farmland. In the past, strain A1 would die during the winter as it could not survive without living plant material to serve as host and did not form oospores.

Symptoms of the disease appear at first as water-soaked spots, often seem at the edge of leaves. Initial damage caused by the fungus resembles frost injury. If the lesions dry out, the areas turn a grayish-brown color. Under moist conditions, a thin layer of white fungus can be seen on the underside of the leaves. Later the infected leaves die and hang from the plant in a rotten mass of dead tissue. If the weather turns dry, the affected leaves look as if they were hit by a blowtorch. If cool and wet conditions return, the disease will resume its activity and can kill the plants in a few days.

What can a home gardener do to protect his crop of tomatoes and potatoes? First, make sure that you purchase healthy potato tuber pieces and tomato transplants. Probably the best way to guarantee healthy plants is to raise your own. Tomato transplants are easy to raise from seed. True potato seeds are now available from Park Seed Co. and are no more difficult to grow than tomatoes. Probably the worst thing to do is to plant potatoes from the grocery store. In addition to late blight, they may contain other disease organisms. Second, prevent the growth of any volunteer potatoes and tomatoes, particularly if the disease was present last year. Infected potatoes can function as a reservoir for late blight.

Proper selection of the garden site can reduce the risk for disease. Select an area not previously planted to a vegetable garden that receives early morning sun. The early sunlight will quickly dry off dew and limit the length of a potential infection period. Do not water foliage late in the day so that leaves are wet during the night. When planting tomatoes and potatoes, leave sufficient room between plants to allow for good air circulation. This will also prevent the foliage from remaining wet longer than necessary.

At the end of the growing season, tomato and potato foliage and vines should be removed from the garden. This will eliminate some overwintering oospores that may have formed and also spores of other plant pathogens from the garden area.

As a last resort, fungicides may be useful for some home gardeners. Years ago, there were numerous fungicides available to the home gardener for control of various blights. Most of these chemicals have been removed from retail sale due to pressure from the Environmental Protection Agency. One of the few fungicides still available to the home gardener that is effective against late blight is chlorothalonil, sold under several trade names including Daconil. It is a broad-spectrum fungicide, with activity against most foliar fungal diseases of potato and tomato. This fungicide can prevent the disease, but once the disease is established, treatments cannot cure it. Hence the home gardener is faced with the task of spraying fungicides to protect the plants against a disease that may or may not appear. For many people, it is not worth the inconvenience to apply fungicides. If fungicides are used, the home gardener must follow all directions on the fungicide label. Please contact your local county extension office for current information pertaining to proper fungicide use for your area.

It is likely that home gardeners and commercial growers will have to adapt to more frequent and severe outbreaks of late blight. Use of disease-tolerant potatoes may help commercial potato growers, but there are no tolerant tomato varieties for the home gardener. The agrichemical industry is working hard to provide effective and safe fungicides for control of this and other plant diseases. Unfortunately, the registration process for a new fungicide is time-consuming and extremely expensive. It will be many years before a new generation of fungicides is available. In the meantime, sanitation, proper plant location, and plant spacing are the best tools to combat late blight and other plant diseases.

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Craig’s comments

I never experienced Late Blight when gardening in Raleigh, but believe it hit my plants here in Hendersonville later in the season - late August into September - ending things quickly. I hope you found something of interest in this article. It would be well worth perusing the latest info on Late Blight (this is 30 years old) - diseases can mutate quickly and adapt. As Jeff Goldblum says in Jurrasic Park - “Life finds a way”