Tomato Diseases: How To Fight Late Blight

by Matt Gibson

About Late Blight

Late blight is a fungus called Phytophthora infestans, which affects tomatoes and potatoes. In case you were wondering, late blight is the plant disease that is responsible for the famous Irish potato famine of 1845. Luckily, modern gardening has brought about a new age where we have the tools to fight back against garden diseases like late blight, and not only can we take steps to treat the garden to fight against localized diseases, but we can practice control methods to keep diseases from coming back.

There are a number of tomato diseases that don’t affect the harvest or touch the fruit of your tomato plants. Late blight is not one of those diseases, and is actually capable of taking out fields full of crops in just a few short weeks. Found on tomato plants throughout the United States, late blight attacks tomatoes late in the growing season and symptoms often do not appear until after blooming has occurred. 

Please note that late blight and early blight are two entirely different diseases.

Causes And Symptoms of Late Blight

Late blight first appears on lower, older leaves as small, wet, greyish-green spots. As the disease progresses, the spots darken in color and a white fungal growth occurs on the underside of the leaves. The disease eventually takes hold of the entire plant, and entire crops can be damaged before you can react.

Luckily, the disease does not overwinter in the soil. The spores of late blight enter the garden though a variety of other ways. They can be carried on the wind from nearby gardens. They can also re-enter the garden from infected transplants, seeds, the weed nightshade, or from infected potato volunteers that pop up randomly. Temperatures between 70 and 80 degrees F and wet, humid environmental conditions help late blight thrive and quickly increase its grasp on your tomato crops. 

Treatment and Control of Late Blight

Practice safe control methods to prevent late blight, such as practicing regular crop rotation, never planting tomatoes in the same location that tomatoes or potatoes were grown in the last two growing seasons. Plant resistant cultivars whenever possible. Remove all volunteer potato and tomato plants that sprout up in between seasons. Space all tomato plants out far enough from each other to allow for plenty of air circulation to help plants dry out faster when they get wet. When watering, try to perform the task in the early morning hours so that plants have a chance to dry out before the warmest part of the day. Try to avoid overhead watering methods, which get the stems and foliage of the plant’s leaves wet, opting instead for direct soil watering techniques, such as using drip irrigation, or soaker hoses. After harvesting, destroy and completely remove all tomato and potato plant debris immediately. Do not compost infected plants.

If you notice late blight early on in the disease’s progression, treat the plant with one of these recommended fungicide options:

• Copper-based fungicides – Use a copper-based fungicide (mix 2 ounces of fungicide with a gallon of water) every 6 or 7 days following a watering or heavy rain. Try to apply fungicide when at least 12 hours of dry weather is ahead, so that the treatment has time to kill the fungus.
• Monterey All Natural Disease Control – This ready-made mix of all natural ingredients that help to control most plant foliar diseases. However, this blend is not for treating ongoing fungal issues, but needs to be sprayed on the plant when it reaches maturity to prevent infections from occurring in the first place.  
• SERENADE Garden – SERENADE is an organic bio-fungicide that is non-toxic and will not harm bees and other beneficial insects.
• Organocide Plant Doctor – Organocide is a foliar spray that attacks fungal issues that are already at hand and prevents future fungal issues from occurring. Mix two teaspoons of organocide with one gallon of water and spray at young plants every one to two weeks in intervals to protect your crops from fungal infections. 

Commonly Asked Questions About Late Blight

How do you stop late blight on tomatoes?

Plant resistant varieties, practice crop rotation, water the soil and root system, not the plant and its leaves, which you should try to keep as dry as possible. Use organic spray before fungal issues appear. Pay attention to proper spacing needs from each variety of tomato you plant. Solarize your garden beds before the growing season if your soil has a history with blight issues.

If blight has already infected your plants, pull up infected plants immediately and discard them safely. Do not compost or leave any plant debris around to reinfect your plants. Spray a copper based fungicide on the remaining plants in your garden. Do other local farmers a favor too, and report the infection to your local extension agent.  

What is late blight on tomatoes?

Late blight is a fungal disease caused by the fungus Phytophthora infestans. First symptoms appear on lower, older leaves as dark, wet spots. The spots darken and white fungal growth forms on the underside of the leaves as the disease progresses. Without treatment, and especially in warm, wet environments, late blight will spread rapidly and eventually infect the whole plant. Entire crops can be destroyed by this fungal disease.

Can you eat tomatoes with late blight?

Though most plants infected by late blight will die rather quickly, you may be able to salvage a few tomatoes or potatoes from plants that were infected by late blight. If so, they are safe to eat, as plant diseases don’t affect human beings. This being said, if the fruit has begun to rot, or is discolored, or softened by disease, you should use your best judgement and play it safe if the fruit is compromised. Fruit can easily harbor bacteria and other compounds that could cause food-bourne illness. 

What causes late blight?

When the low-lying leaves of a plant get wet and are exposed to warm, moist conditions, they become susceptible to late blight. The pathogen Phytophthora infestans infects the low lying leaves, then works its way through the plant, choking off its nutrient and water supply, eventually killing the plant. Warm, moist environments and wet leaves are the common culprit.

What is the difference between early and late blight?

Early and late blight are two different diseases that come from different fungal strains and attack the tomato plant in distinctly different ways. The main difference is that late blight attacks the plant more rapidly than early blight. Plants affected by early blight can be saved if treated quickly, but plants affected by late blight should be pulled up and discarded immediately, treating only the remaining plants in the garden that were not yet affected by the disease.

Want to learn more about tomato late blight?

Gardener’s Supply Company covers How to Prevent Late Blight on Tomatoes

Gardening Know How covers Tomato Late Blight

Missouri Botanical Garden covers Late Blight of Tomato

The Free Range Life covers How to Prevent Late Blight in Your Garden