Tomato Late Blight

Tomato late blight is caused by the fungus Phytophthora infestans.  Phytophthora (pronounced Fi-top`-thor-uh), is a water mold and needs water to spread effectively.  Late blight spores, or fungus eggs, actually swim in water to spread.  Periods of rainy weather with cooler temperatures drive the disease to spread rapidly. Early to mid-August is the most prevalent period for this disease in the midwestern states, but early occurrence may appear in seasons with higher than normal heat and moisture.  Cool night temperatures associated with high-pressure periods and oftentimes full moon events can make things worse.  Heavy dews also spread the disease very quickly once it has infected. 

In the tomato late blight condition, both the lower and upper leaves may be affected with the appearance of water soaked areas that grow rapidly across the leaf.  Irregular, greenish-black blotches will soon appear, and the plant will look as if it has been frosted by exhibiting wilting and dying leaves.  Under severe conditions the underside of the leaf will begin to show white downy or dusty-looking growth.

Once into the fruit, tomato late blight symptoms appear as large, irregularly shaped brown blotches, and infection will be present on green or red fruit. Rotting fruit will be the next symptom of late blight as the disease spreads very quickly turning the tomatoes into a foul-smelling wasteland.  This disease also infects potatoes. In areas that become infected, you should rotate the crop in this area to something other than tomatoes or potatoes in the next year if possible.

Control measures for tomato late blight start early in the season with a plan.  High levels of plant sugars called brix are required for plants to resist the spread of late blight through the tissues.  Anything that helps build plant sugars and keeps energy levels high can be useful.  Very strong cell walls are required to hold the plant together if disease tries to strike, so compounds containing calcium will be useful. These compounds may be soil applied or foliage applied, but they must be applied throughout the season to be fully effective.

Products containing phosphite compounds will also help to control tomato late blight disease.  University researchers are confused as to why these compounds attract and wear out the fungus, but they do help.  Late blight will try to get oxygen and energy requiring compounds from the phosphates, but are not capable of extracting them.  The fungus basically commits suicide trying to power itself.  So far no resistance to this type of natural control has shown up.

Using products that contain bacillus subtillus, a naturally occurring good guy bacterium on the soil and on the leaf will also be useful in fighting tomato late blight disease. Foliar feeding strategies that include micronutrients help to keep the plant making food efficiently.  If all else fails this year, make sure to rotate tomatoes and potatoes away from these areas next season.  Plant a crop such as corn, which does not suffer with late blight, in those areas for a couple of years to help reduce the quantity of disease organisms in the soil.
To view a brief, field identification video about tomato blight diseases, click here.