Since potatoes are long-lived they can harbor late blight spores.

This year my garden was exposed to the late blight epidemic, and I lost most of my tomatoes. As I plan next year’s garden, what do I need to do to prevent another outbreak? -Karen

Late blight (Phytophthora infestans) is like the “flu” of vegetable gardens – it’s airborne, waterborne, and fast spreading, and it can wipe out tomato and potato crops in just a couple of weeks. Late blight generally comes in new waves every year, usually from:

• Infected transplants at garden centers.
• Infected potatoes that survive the winter underground.
• Cool, wet weather.

Unfortunately, even the most careful garden can be infected by spores blowing in from elsewhere, but following these tips can prevent bringing in (or harboring) the spores in your garden:

• Transmission: Late blight can only survive in living plant tissue – it doesn’t live in the soil, and it doesn’t live on seeds, tools, stakes, or cages. Make sure to harvest all your potatoes to prevent harboring the disease underground over the winter.
• Planting: Start your tomatoes from seed, to avoid buying infected transplants. If your potatoes were affected, start fresh this year with seed potatoes that are certified disease-free. Throw away any “volunteer” potato vines.



• Fungicides: Treating with fungicides can be used as a preventative measure on healthy plants, but late blight is not treatable once it strikes.
• Act Quickly: If you see signs of late blight, pull and throw away the affected plants immediately to prevent spreading of the disease. It won’t get better!
• Composting: Some experts recommend against composting the plants, but others feel it’s OK if done correctly. The key is to make sure the material is completely dead. Infected plant material and spores will usually be killed in a hot compost pile (over 115° F). To make sure, you can solarize infected plants before composting. Potatoes are tougher, however, so you may want to cut them up and spread them on the ground where they’ll freeze, and plow them in next spring.
• Prevention: Use smart gardening practices to prevent fungal diseases. Water without wetting foliage, allow for plenty of air circulation, and make sure your veggie garden gets morning sun. Even though late blight doesn’t live in soil, practice crop rotation to prevent other diseases.

Further Information

• Late Blight of Potato and Tomato (Maryland Cooperative Extension)
• Late Blight: Frequently Asked Questions (Cornell University)
• Can I Compost Plants Killed By Late Blight? (Penn State Master Gardeners)

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