Dear Tom –
How do you still have tomatoes in the fall? My plants
went down with late blight.
For years the only reliable organic solution to late blight (Phytophthora infestans) was spraying copper and growing in a greenhouse or coldframe. The tomato breeding program at the Mountain Horticultural Crops Research and Extension Center (MHCREC) in Mills River offers new hope for outside organic tomato production – late blight resistance in tomatoes. Refer to the link below for more on this disease.
Copper fungicides have very effectively prevented late blight for us in the past. Both copper sulfate (the fungicide, not the herbicide) and copper hydroxide work well and some of these materials are listed by the Organic Materials Review Institute. The problem with copper is that washes off in the rain.
While I am not a plant pathologist I have been told that copper works by dissolving in the free water on the leaf of a tomato plant, such as after dew forms in the evening. Our misty mornings extend this wetted period allowing fungal organisms to get a good start without copper on the leaf but the copper in the solution kills the emerging fungi. When the dew dries in late morning the copper is deposited back on the leaf. Copper is a protectant so it must be on the leaf when late blight spores arrive. Rainfall washes the dissolved copper off the leaf surface and removes it as a protectant.
In contrast to early blight which is soil borne, late blight is air borne and can travel hundreds of miles on the wind. The cool mornings of early fall are prime late blight season and a week of rain (like last week) will often bring it on.
This year’s Johnny’s Selected Seeds catalog offers two varieties that contain late blight resistance – Mountain Magic from MHCREC’s breeding program and Defiant introduced by Johnny’s with an acknowledgement of NCSU cooperation. We are trying both of these varieties this year to see how they do outside. We are also trying several experimental lines from NCSU. These new lines have both early blight and late blight resistance as well as flavor genes from heirloom plants. Many of you know Dr. Randy Gardner who recently retired. Dr. Dilip Panthee is carrying on his work.
I hasten to add that this breeding program is done the “old fashion way” of crossing two tomatoes with favorable characteristics and observing the offspring. This approach is in contrast to genetic engineering which is not allowed in organic production.
We have high hopes for these new varieties opening up organic production of field tomatoes in WNC. Most tomato fans know that WNC farmers grow the best tomatoes. Now we may also be able to grow a reliable organic crop outside the greenhouse.
Ask Tom © 2013 Tom Elmore & Organic Growers School
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Author: Tom Elmore
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.