02 August 2011

Solanaceous Diseseases

Because of the wet season we have had earlier this season, your vegetable gardens have been susceptible to many diseases for which you should be aware of.  Be on the lookout for the following: Septoria, early blight and late blight, which affect solanaceous plants such as tomatoes, eggplant and potatoes; Powdery and Downy Mildew, which affects vine crops such as cucumbers, zucchini and summer squash, winter squashes, pumpkins, etc., Alternaria and Downy Mildew (different species of mildew than the first) which affects cole crops such as cabbage, cauliflower, broccoli, brussel sprouts and kale. 

Septoria is a fungus which causes leaf spot on tomatoes (most commonly) but can also affect eggplant and potatoes.  It is found all over the world where tomatoes are grown and can occur at any stage of plant development.  The disease can occur on the stems and undersides of older leaves on plants ready to set fruit or can also occur on young seedlings.  The symptoms of Septoria look like spots with tan or grey centers and brown margins.  As the fungus progresses, the spots may coalesce and enlarge and then form pycridia (brown pimple-like structures which are the fruiting bodies of the fungus) as the pycridia mature, spores are produced and released, thus further spreading the fungus via wind, cultivation, pickers who handle the plants or spread the spores as their clothing touches them.   It takes about 2 weeks from initial symptoms to the production of more spores.  Warm, humid conditions favor the development of this disease and left untreated, heavily infected leaves will yellow, dry up and fall off the plant, resulting sun-scalding of the fruit.  Treatment can usually begin by mid-July, unless conditions are favorable and symptoms appear earlier. Treatment is similar to that of blight.

Although the fungus does not inhabit the soil, it can remain from year to year on infected leaves and weeds left in the field or garden soil.  If you have been dealing with infected plants, it’s best to remove them and all associated debris from your garden in plastic bags and discard them in the trash.
Early Bight is a disease most commonly found on potatoes, but can also affect tomatoes and other solanaceous crops.  If left untreated, it can spread rather quickly and lead to severe defoliation and low yield of your crop.  Symptoms of disease can first appear in early to mid-July, when warm, wet conditions typically occur here in New York.  Lesions on leaves can first appear within 2 or 3 days of infection and most commonly on older, less vigorous, lower leaves.  Lesions appear as dark brown, leathery spots with concentric rings giving a “target spot” effect.  The spots can enlarge to about 1/2” in size and are generally bound within the leaves veins, but can coalesce and produce more spores which are then spread as we discussed earlier with Septoria.  Lesions on tubers tend to form more slowly and will cause the tubers to become corky in texture.   Tuber rot may not become sever until later in the storage season, but can pre-dispose the tuber to secondary infections.

Late Blight lesions occur about 3 or 4 days after infection and will produce more spores if favorable conditions are present;  wet leaves with moderate temperatures ranging from 60°-70°F and lasting ten or more hours.  Both tomato and potato fruits are susceptible to the disease and stem lesions are capable of producing spores for a longer period of time.
Healthy plants will be more resistant to disease, therefore it is important to keep fertilizing your plants and during rainy seasons, a preventative spray program is recommended.  Please know what you are using on your plants for treatment of any disease.  We do NOT recommend using anything systemic, as it will eventually end up on your plate.  For all diseases, it is wise to consult your local Cornell Cooperative Extension office for the latest information on resistant varieties and disease management. 

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