A lot of gardens are probably just about ready to start bearing the fruits of their labors. When it comes to tomatoes, one of the primary concerns farmers and gardeners have each year, especially here in the NE, is blight. Most people don’t know what it is. Below we’ve pasted the two most common types of blight (there are more and other fungi that can infect your crops).
Though the explanations are helpful, the bottom line of blight is – it destroys your crop.
Early Blight, caused by the fungus Alternaria solani, can affect seedlings but generally is observed on older plants. On seedlings, dark spots develop on cotyledon leaves, stems, and true leaves. Spotted cotyledon leaves may be killed, and spotted stems may be girdled. On established plants, dark brown spots with dark concentric rings develop first on oldest leaves. Spotted leaves may die prematurely, resulting in substantial early defoliation, fruit sunscald, and poor fruit color (see the section on Tomato Fruit Rots for fruit symptoms). The disease-causing fungus overwinters in residue from diseased plants, where it can persist for at least one year. The fungus also is seed-borne and can be introduced on seed and on transplants. Disease occurs under a wide range of weather conditions. It is promoted by heavy dews and rainfall and is severe on plants of poor vigor.
Late Blight, caused by the fungus Phytophthora infestans, affects both tomatoes and potatoes. On tomatoes, symptoms appear on foliage and fruit. Irregular greasy-appearing grayish areas develop on leaves. These areas expand rapidly during moist conditions and a white downy mold appears at the margin of the affected area on the lower surface of leaves. If the white fungus growth is not observed, leaves with suspicious spots can be put into a polyethylene bag containing a moist paper towel (to supply moisture) and held for one day to promote appearance of this diagnostic sign. (Fruit symptoms are described in the section on tomato fruit rots.) The disease-causing fungus overwinters in southern frost-free areas, on winter-grown tomatoes and potatoes, and in northern areas in potato cull piles and in potato “seed.” It may be introduced to tomato fields on transplants or may be wind-borne from diseased potato and tomato plants in nearby fields. Disease development is promoted by cool wet conditions.
Here is some information and tips for avoiding or minimizing the damage blight causes:
It is difficult to save a tomato plant once it has been infected with the blight – even if you spot it early and remove the infected leaves. And one infected plant is all it takes to ruin your whole crop of tomatoes (and potatoes), so here is some advice, courtesy of the University of Maryland’s Home and Garden Information Center and Cornell University’s horticulture department.
Blight winters over on living host material, such as the potatoes that might still be in the ground in your garden. Remove them and dispose of them in plastic garbage bags.
Remove any volunteer potato plants in your garden or compost pile. They can carry the virus.
Buy healthy plants from local growers. Learn what blight looks like and examine the plants for any signs. If you see something suspicious, alert the seller and the state extension service.
If you are planting potatoes, don’t use leftovers from last year’s garden or anything from a grocery store.
Blight loves cool, damp conditions and cloudy days. (UV rays kill the spores carried by the wind.) So water your plants in the morning and do it at ground level, not from above.
Inspect your plants at least once a week. More often if the weather is cool and wet.
If you find late blight in your garden, let the local extension service and your neighborhood gardeners know. Act quickly.
If you are forced to remove your plants, do it on a bright sunny day to kill of the spores that may blow around.
Late blight shows up on other plants, almost unnoticed: hairy nightshade, bittersweet nightshade, tomatillos and petunias!
If you wait until late blight shows up to use a fungicide, it will be too late. It has to be done as a preventative measure and the University of Maryland isn’t recommending that just yet since fungicides can actually lead to other diseases.
Once you begin spraying, the treatments must be regular. Use chlorathalonil or copper-based products and follow the directions carefully.
Consider planting potato and tomato varieties that have shown some resistance to late blight, such as such as ‘Allegany’, ‘Elba’ or ‘Kennebec’ potatoes and ‘Black Plum,’ ‘Matt’s Wild Cherry,’ ‘Yellow Currant’ and ‘Yellow Pear’ tomatoes.
“Mountain Magic’ and ‘Plum Regal’ have also shown disease resistance and their seeds should be widely available in 2011.
