You have visions of homegrown tomatoes dancing in your head. . . Homemade tomato sauce canned and stacked neatly on shelves, stewed tomatoes in jars, tomatoes on salads and just taking a big juicy bite out of one straight out of the garden – you can almost taste it! You go out to your garden to do your checkup and daily watering and see something odd on one of your tomato plants. The leaves are watery and rotten looking and there is a brown lesion on the stem. What is going on? You guessed it: you may have tomato blight.
What is tomato blight?
Early tomato blight is caused by a fungus called Alternaria solani. This fungus can affect almost every part of the tomato such as the stems, leaves and fruits. It may not outright kill the plant but it will weaken it and the yield may be smaller.
Early blight loves damp weather and heavy dew, it can be in the soil, or it can overwinter in diseased plants from the year before if you didn’t clean up the vegetable garden promptly. Like with most pests and diseases, plants which are stressed or just in plain poor health are more susceptible.
Early tomato blight forms spots on the leaves, which then turn yellow and die. The spots may start out small and shrunken and as they get bigger they get longer. Spots which are on the stem near the ground can cause the stem to girdle. When it’s on your fruit, the spots show up near the stem. The spots will be dark leathery and sunken with concentric rings and it will affect your ripe and unripe tomatoes. Your plants may survive this attack but there won’t be as many tomatoes. While early blight is a nasty disease, it’s not as devastating as late blight.
Late tomato blight is just plain bad news all the way around. The name of this fungus is called Phytophthora , which means “plant destroyer”. It can ruin a whole crop and infect other plants quickly. It’s highly contagious and can spread from your garden to a neighboring gardener’s plot in the breath of a spore carrying wind. If a plant becomes infected, it has to be destroyed and taken off-site (not composted).
Late blight shows up as blue-gray spots which then turn brown, and it affects both the leaves and the fruit. The fruit will have brown greasy spots which are shaped irregularly. The spots on the leaves and fruit may have a cottony looking ring of white mold and the leaves will eventually fall off. Late tomato blight shows up in cool and wet weather in mid-to-late season and can kill a plant within a week.
California’s Humboldt County and the coastal Pacific Northwest has such cool, damp summers that our tomatoes often suffer from blight – though it’s by no means limited to the Northwest region. Gardeners across the country suffer from both early and late blight.
Methods to control blight
Plant blight-resistant tomato varieties
- ‘Iron Lady’
- ‘Jasper’ – red cherry
- ‘Lemon Drop’ – yellow cherry
- ‘Pruden’s Purple’
- ‘Red Currant’
- ‘Defiant PhR’
- ‘Jasper’ red cherry
- ‘Mountain Magic’
If you can find grafted versions of tomatoes that are bred for disease-resistance, all the better. Grafted tomatoes can be much more vigorous and better able to shake off stresses.
Preventing early tomato blight
- First of all buy your starts and seeds from a reliable source – don’t just take starts off friends or well-meaning neighbors.
- Make sure there is enough space between your plants for air circulation.
- Add a layer of mulch or use those red plastic tomato mulches they sell as a barrier between tomato foliage and soil, as the fungus can be carried in the soil.
- Check your plants frequently during wet weather or if they are stressed.
- If you see anything that even remotely looks like blight, begin a spraying program of alternating organic copper spray, and Serenade biological fungicide, both of which are safe to use on edibles. Don’t spray them together on the same week as there is some question as to whether the copper inactivates the Serenade. Do use caution with the copper – spray early in the morning to avoid harm to bees, and because it can build up in the soil and cause toxicity, don’t grow tomatoes in the same spot every year if you’re going to use copper frequently.
- Make sure your garden area is cleaned up promptly at the end of the summer gardening season and there isn’t any plant debris lying around.
- If you have an outbreak, rotate your crops even if you have to plant in containers.
Preventing late tomato blight
- Don’t overwinter your plants and remove any volunteer tomatoes and potatoes that come up. Potatoes can carry blight too.
- Use trellis and plant supports to keep your plant branches off the ground, and as noted above, use mulch!
- Don’t water your plants from above, and water early in the day so your plants are dry by nightfall.
- A product called Actinovate, which has the beneficial bacteria Streptomyces lydicus in it, can be used as a preventive measure.
- Oxidate, made of activated peroxide, is another safe and effective product to keep plants sterile and clean of blight.
- However, the above methods are for prevention. If late blight shows up, you need to pull out all the plants which are affected and put them in a trash bag. Don’t compost them as this highly contagious disease can infect the rest of your plants, and even your neighbors’ plants!
While in cool, moist climates, tomato blight can’t be totally beaten, by using preventive measures you can go a long way towards reducing the likelihood of outbreaks.
Tomato photo at top courtesy of J. Regan of Flickr (via the Creative Commons Attribution License)
Late blight photo courtesy of C. Paquette of Flickr (via the Creative Commons Attribution License)