Preventing Tomato Late Blight

Until last year, I was innocent of the ways of tomato blights. My effortless introduction to growing tomatoes just the season before had not prepared me for the speed and efficiency of airborne fungi. Surely growing tomatoes was as easy as planting a few indeterminates on flimsy cages and sitting back to wait.

Then early last July, a vigilant gardener spotted late blight on a few tomato plants in a corner of the Interbay P-Patch where I garden. Not since 1998 had our P-Patch dealt with this ravenous killer. Dire warnings of its blitzkrieg-fast, unstoppable spread seemed utterly unbelievable.

The years had lulled us into a sense of complacency. Our tomatoes were left to sprawl and crawl and crowd together in the cool Seattle nights. Surely a few dark smudges half a garden away could not affect my plans for San Marzano sauces, my Seattle Marinaras.

But alas the warnings were true. The Phytophthora infestans fungus that causes late blight thrives in our cool, damp weather and spreads rapidly by airborne spores. It overtook our garden within two weeks. The fungus can also affect potatoes, and was responsible for the infamous Irish potato famine of the late 1840s.

You can identify late blight by looking for dark (almost black) blotches and smudges along stems and leaves. Fruits also begin turning grey or black from the top and harden as they discolor. A different fungus, Alternaria solani, causes early blight, which appears as dark bull's eye leaf spots with lighter, often yellowish halos.

Late blight spores are ever-present in the soil, and once it successfully infects a plant, it's nearly impossible to control in an organic garden. About all you can hope to do is prevent an outbreak. Here are some tips on to help you keep late blight and other tomato fungal diseases from getting a foothold.

• Mulch your soil thickly with leaves or burlap to prevent soil containing spores from splashing onto leaves.


• Leave adequate space between plants for good air circulation. Mature plants should not be allowed to intertwine.


• Keep leaves dry. Apply water directly and gently to the soil, not from overhead. Water in the morning so damp leaves can dry during the day.


• During damp weather, loosely cover your tomatoes with plastic to keep the rain off, but don't wrap them so tightly that condensation forms.


• As your tomatoes grow, begin clipping off the bottom-most leaves to encourage air circulation. Once your tomatoes are about two feet tall, remove all the leaves from the bottom six inches. Keep removing leaves up to about two feet from the ground as the plant grows. Continue to thin out leaves above two feet to encourage air circulation.


• Keep your tomatoes well staked or in sturdy cages. This keeps them up off the ground where soil would be more likely to splash onto leaves.


• In the fall, remove all infected foliage from your garden and dispose of it at home. Do not compost infected plant material.


• Rotate your tomato plantings so you never plant tomatoes or potatoes in the same spot within four to six years. The longer between plantings, the better.


• Don't allow volunteer tomato seedlings to grow. They tend to be weedy and close to the ground, providing easy access to splashed soil.


• Add a bit of lime to your soil. This helps plants remain healthy enough to recover from minor infections, improves fruit formation, and helps prevent blossom-end rot.


• Don't work around wet plants, when you're likely to spread fungus and spores from plant to plant on your clothes.

Copyright © 2006 Brian Ballard. All rights reserved.