Monday, July 11, 2011

How to Tame and Protect Your Tomatoes From Late Blight

By Kelly McKeon and Mark Scialla

Late blight on potato leaves /
Although the threat of frost and early blight is behind us, the dreaded late blight looms close to our region. Late blight, affecting tomatoes and potatoes, has been confirmed in Delaware, Virginia and on five farms in Long Island, NY. 

Late blight is the more ferocious cousin of early blight, the latter affecting leaves and fruit. Early blight can be controlled if spotted early, saving the life of the affected plant. Conversely, late blight is a very serious pathogen that causes blight, which kills tomato and potato plants. Late blight has been known to affect other members of the Solanaceae –nightshade- family, such as tomatillos and eggplant.

Luckily the weather here in RI has been warm and dry lately, but if it becomes wet and cool with frequent rains, our plants may suffer from late blight. The pathogen is able to produce millions of spores during wet and cool weather that allows it to spread rapidly. The spores can travel through land and air and soak down into the soil to affect potatoes causing rot before or after harvest.

Late blight is introduced to fields from infected potato tubers, transplants or compost piles, and is able to survive from season to season. The pathogen only survives on living tissue, so be sure to inspect your potatoes and transplants thoroughly before planting. If your tubers or transplants have blemishes or yellow leaves, they may be infected. Do not compost these rejected plants as it will not rid your garden of the pathogen. Destroy your rejected tubers and transplants.


Late blight causes dark lesions on the stem and leaves of tomato and potato plants. If you cut deeper into the lesions you will find a rot that extends into the plant. White powdery spots will form at the edges of the lesion. These spots are where the spores originate. The lesions may appear less than a week after infection, usually after wet and humid periods.

Dark, greasy lesions will also appear on the tuber of a potato and the fruit of a tomato. They may be brown or purple. Cutting into them will reveal a darker rot. 


Once plants are infected with late blight, they must be destroyed to avoid further contamination. To prevent late blight from occurring, follow these steps:

-       Carefully select where your tubers, seed and transplants come from

-       Dust your plants with an organic copper sulfate fungicide, a commonly accepted organic control
-       Apply the fungicide shortly after rain, and before a dry period - if it is applied too close to a rainfall event, the powder will be washed away before it will do its job. 

Tending your tomatoes

Other than fungicides, it is very important to keep your garden clean, and that means taming your wild tomatoes. For maximum production by each plant, tomatoes should be pruned and staked

Pruning is removing unwanted growth for maximum fruit yield.  Tomatoes have one main stem and horizontal branches.  Often, "suckers", will appear between the main stem and the horizontal branches. Suckers are easiest to remove when they are small - pinch them off with thumb and forefinger.  Larger ones can be cut with pruning snips or scissors.  The suckers should never be torn off because it will damage the main stem. Removing suckers allows the plant to focus growing energy into the fruit bearing parts of the plant. 

Staking can be done in many ways - the simplest is a single stake set alongside the tomato seedlings when they are first set into the garden.  As the plants grow they are tied at intervals to the stake. This will keep the plant upright, allow for air flow and facilitate harvesting. Upright plants produce better, are more disease resistant and are less prone to insect damage.

Stop by the community garden for our workshop and see a free hands-on demo of "Taming Tomatoes" on July 20 at 6:30 p.m.  

If you think you might have late blight or any other disease, send in a sample with $10 to the URI Outreach Center at 3 East Alumni Avenue in Kingston, RI – 02881. Address it to the Plant Clinic and Heather Faubert will positively ID the problem for you.

Or email her at

Check out these links for more late blight information and photos:

1 comment:

  1. I doubt that our tomato plants have this problem. I think that the problem is the overnight moisture in the Park. I have several tomato plants in my yard at home and they are doing great. I live only blocks from the park but all my tomato plants are brown and have no new growth on them. We have been there faithfully to water. Also squash plants have a white mildewy look to the leaves and are behaving like the tomato plants.