Hello — Thanks for the latest newsletter. Great info! I’ve been having tomato leaf blight on almost all my tomato plants. Is there any way to eliminate this? Thanks!
Blight is both a specific and a generic term for leaves that wither and die. Many diseases can affect tomato leaves. NCSU offers a collection of scary pictures that may help identify the particular problem that affected your plants.
This photo to the right shows early blight on tomatoes. Despite their similar names, early and late blight are different diseases. Early blight is soil borne and infects the tomato plants from soil that is splashed onto lower leaves and then it moves up the plant. In dry years tomatoes can often outgrow early blight so yield is not greatly affected. Organic and plastic mulch help avoid that initial infection from the soil. Removing yellow leaves with the characteristic target spot also helps, if you can take the time to do that at your scale. The main organic management method for early blight is vigorous plants. Correct pH, regular irrigation, and soil supplements matching the soil test results are some of the best ways to ensure vigorous plants. In most years early blight is a nuisance but will not threaten the success of your crop. Remember that tomatoes are heavy feeders and a long crop will need supplemental nutrition to keep the plant growing actively.
Late blight, pictured to the left, is a completely different disease. The first step in managing late blight is to get in the proper frame of mind. I suggest putting on the Jaws soundtrack. Late blight is the same disease that led to the Irish potato famine. One August I was anticipating my best tomato crop ever when ten days of rainy, misty weather moved in. A few days after I saw the first blight damage, the foliage was gone, the fruit were infected, and the crop was a total loss. That disappointing experience led to a SARE-funded research project on our farm and on Pat Battle’s farm in Celo. We tested compost tea, hydrogen peroxide and copper sulfate – all organically approved control methods. None worked outside but copper worked great under cover. In my view, copper fungicides are the only organic solution to late blight.
Copper fungicides have three problems. The first is that copper is a nutrient at low concentrations but it is toxic to plants at high concentrations. Excessive spraying over several years can damage your soil indefinitely. The second problem is that copper is a preventative fungicide. It does battle with fungal spores on the leaf surface. Once the invaders infect a leaf, that leaf is lost, so copper must coat every leaf before the fungal spores arrive. Unlike early blight, late blight is air borne. It blows in from other states and drifts down on the tops of plants. The leaves appear water-soaked and black. The third problem with copper is that it washes off in the rain.
Despite all this gloom and doom, organic tomato culture is not hopeless. First, not every year is a late blight year. When the weather turns dry in late August and September, late blight may never appear. Growing tomatoes under cover is another solution – one that has worked very well for us. Greenhouses and cold frames are expensive, but without direct rainfall, copper fungicides stay on the leaves and are very effective at preventing late blight.
Breeders at the NCSU Mountain Horticultural Crops Research and Extension Center have an active tomato breeding program. They isolated the genes that resist both early blight and late blight. Some of their crosses include heirloom tomatoes so our hope is that we will soon have heirloom flavor in a blight resistant tomato. A few blight resistant varieties appeared in the Johnny’s catalog this year. Growers are trialing others in hopes of having even more varieties in the next several years. (Please note that I referred to a breeding program. A genetic engineering to disease resistance approach is ruled out for organic production.) Check with Dr. Dilip Panthee at the Mt. Hort. Center if you are interested conducting trials for some of these new varieties.
For now, here are my recommendations for organic tomato growers:
- Use vigorous plants and mulch to manage early blight.
- Use drip irrigation to avoid long periods of wetted leaf surfaces.
- Invest in cold frames or greenhouses for “insurance” against late blight
- Plant no more tomatoes outside than you are willing to lose
- Find an acceptable copper spray on the OMRI list and use it weekly as soon as late blight is reported in the area.
- Use a mist blower to ensure good coverage deep into the plant foliage.
- Use plastic mulch to intercept copper overspray (your certifier will ask about this issue).
- Keep spraying even if late blight gets into your coldframe. (You may lose leaves but you can protect the new growth with weekly sprays.)
- Try out the new resistant varieties to see if they work for you.
- Rotate tomato crops to leave soil-borne diseases behind.
If you’re interested in reading more, Debbie Roos of the Chatham Co. Extension office wrote a great article on blight.
You can take the Jaws soundtrack off now. Tomato blights are manageable but we need to plan ahead to do it well.
Ask Tom © 2013 Tom Elmore & Organic Growers School
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Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.