Late Blight on Tomatoes


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An Article Written by Nancy DuBrule-Clemente

Last summer, factors converged to create the perfect situation for an infestation of late blight, a devastating disease of tomatoes and potatoes. Everyone has been asking me what steps they can take this spring to prevent this disease from infecting their crops in 2010. I have read articles, talked to scientists, and studied this problem in depth all last fall and winter.

Here is what I have concluded:

1. The pathogen that causes late blight (Phytophora infestans) only overwinters on live tissue. All parts of the tomato plant die in our freezing winters. Still, I hope that everyone pulled up their plants last year and threw them away in garbage bags. I even tossed the straw mulch beneath my plants as they contained infected leaves and rotten tomatoes that had dropped.

2. The main way that this pathogen will reappear in your garden will be from overwintering potato tubers. I love to grow my own potatoes, and when I harvest, I comb the soil for every teeny tiny baby potato I can find. However, I always miss a few. If you see ANY potatoes coming up from last year’s plants, immediately remove them and throw them away in a garbage bag.

3. This year, don’t allow volunteer tomatoes to remain in your gardens. Bag them and throw them away.

4. Plant only certified seed potatoes. Not just organic, but certified to be disease free. (Natureworks carries them, of course).

5. Buy organic tomato seedlings from a reputable source (that would be Natureworks!). Know your grower, know your supplier, buy local.

6. If you buy potatoes at the store and find rotten spots inside, don’t compost them. Throw them away.

7. Rotate your crops. This is a basic tenant of food growing, but it is essential this year. Tomatoes, potatoes, eggplants, and peppers are all members of the nightshade family. Do not grow them in the same spot this year. Move them to where you might have had peas, beans, broccoli, chard, squash, cucumbers- all crops in different families. If you don’t have enough room for this kind of rotation, consider container gardening this year.

8. Organize your tomato growing so that the plants are staked, open pruned, and growing with good air circulation. Don’t overcrowd them and don’t let all the suckers grow, especially if you see that cool, wet weather is upon us.

9. Spray your plants with Serenade, an organic product that is very helpful in preventing fungus spores from developing. I am definitely using Serenade on all my plants weekly this year. It will make a huge difference.

10. Check your plants daily and look for any early signs of any fungus. Treat your plants with liquid copper if the conditions for late blight seem favorable. (Last year, this product was impossible to get in late summer. We have a large supply of it in stock right now.) Immediately remove any suspect leaves with rubber gloves and throw them into the garbage. Clean all of your pruning tools with rubbing alcohol after use. Spray your stakes, raised beds, leftover mulch, and other related equipment with a 10% hydrogen peroxide solution to kill other types of fungus spores that do overwinter on non-living material. Common sense garden sanitation
and constant vigilance will pay off.

11. The most exciting new development in preventing late blight is the use of mycorrhizal and beneficial bacteria as a soil drench and foliar spray. We now carry a product, Actinovate, that is used specifically for this purpose. This 100% water soluble powder contains a high concentration of beneficial bacteria that suppress and control a wide range of fungus problems in the soil and on the plants, including the late blight pathogen. Additionally, the Actinovate microbe helps to build vigorous and very healthy root systems that make the plant much better equipped to fight off diseases. Use in when starting seeds, water it into the soil when you transplant, and spray it on the foliage. A second product, Actino-iron, has equal fungus fighting properties and also provides a complex, slow-release iron that is available to the plants over many months.

12. Focus on building a healthy soil. This is the key to healthy plants. Use compost, fresh brewed compost tea, organic fertilizers, and mineral powders. This article was originally published in the March 2010 issue of Natureworks Gardening Chronicles.

In an effort to provide horticultural information, these educational documents are written by Nancy DuBrule-Clemente and are the property of Natureworks Horticultural Services, LLC. You are granted permission to print/photocopy this educational information free of charge as long as you clearly show that these are Natureworks documents.