Eric Sideman's Pest Report
8/3/02

LATE BLIGHT ALERT

Tomato and potato growers should be on the lookout for late blight. I found late blight prevalent in a field of tomatoes in Cumberland County. Late blight is noted for being highly destructive in potatoes. It can also be a very serious disease on tomatoes. The consistently cool and rainy weather in southern Maine two weeks ago was ideal for late blight. Now the symptoms are evident and tomato growers should scout their fields for the disease and control it. The late blight pathogen has been responsible for numerous epidemics on tomatoes and potatoes since the Irish potato famine.

Symptoms

Usually the first symptoms are indistinct water-soaked spots on the leaves, which when conditions are right enlarge rapidly into pale green to black lesions that cover the whole leaf. In moist weather the undersides may be covered with a gray to white mold. (If you do not see the mold, you can take some leaves and put them in a plastic bag overnight to see the moldy growth). Quickly the infected foliage shrivels and dies. Petioles and stems are also effected and shrivel. In Tomatoes, black areas form on the young fruit and quickly the whole fruit turns black.

Disease cycle

The pathogen survives the winter only in plant tissue that does not freeze. In the south it can live in tomato debris and tomato seedlings bought from the south can be a source of infection. In the north the pathogen survives in potato tubers. Cull piles and volunteer potatoes are the major source. In the spring the fungus begins to grow in the tissue of the sprouting potatoes and produces sporangia, which can be wind blown long distances. The sporangia germinate on potato or tomato leaves and in cool weather produce masses of zoospores (swimming spores that can move in a film of water). Cool, damp weather followed by warm weather is ideal for late blight. Sometimes the damage comes on so quickly it looks like frost damage.

Control

Cull potatoes should be destroyed by composting or freezing. Eliminate volunteer potato plants. If you have infected tomato plants make sure the plant debris freezes this winter.

Fixed copper sprays will control the spread of the disease. New tissue must be covered. If late blight is seen in your area it would be wise to be on a 7 day spray schedule.

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)