Gardeners hope for blight-free tomato crop

Posted May 14, 2010, at 6:10 p.m.

Students working in the Eastport Schoolyard Garden spent several hours this week transplanting seedlings from germination trays to cell packs, moving them along a production path that leads to garden planting in early June, in the last two weeks of school. It will be an act of faith, planting those seedlings of tomatoes, peppers, cucumbers, summer and winter squash, tomatillos, and other veggies in their new raised beds, then walking away with hopes of a fall harvest.

Most of my students are first-time gardeners and have no memory of last year’s tomato and potato misery. My efforts to inform them about the fungus disease called late blight, about how it combined with an unusually cool, wet summer to ruin our hopes for a single ripe tomato, resulted in audible thuds as heads hit desks in the last period of the day. All they want to do is play in the sun.

I tell them that by growing our own tomato transplants we can at least start the season free of the blight. The pathogen can survive the winter only within living plant tissue and does not infect the seed, so our seedlings will be blight-free, at least in the beginning. This would be true of any tomato plants grown from seed by local nurseries.

Late blight occurs every year in Maine, but last year was different. Infected plants were distributed through large retail stores across the Northeast during June. By July, outbreaks of the disease were reported from Ohio to Maine. This record-breaking epidemic, combined with the cool, wet growing season and the exceptionally contagious nature of the disease, created the perfect storm. Spores of the fungus traveled on the wind to infect all tomatoes, even the homegrown.

Late blight also attacks potatoes, and tubers infected with late blight last year could have survived the winter in several ways. They may have been left in the ground at harvest, several inches deep in the soil, or disposed of in a compost pile that did not fully decompose and did not freeze. Potatoes that did freeze or that fully decomposed will not carry the pathogen into spring.

What should gardeners do this year? First, like my students, grow your own tomato transplants from seed or purchase them from a reputable local grower, someone who grows their transplants from seed. Avoid buying tomato plants from the large retail stores — they may be purchasing their plants from the same growers that introduced the dis-ease last year.

If you also are growing potatoes, purchase certified disease-free seed potatoes from a reputable source. Ask suppliers about the source of their seed potatoes and whether the fields were inspected for late blight last year.

Carefully inspect last year’s potato bed and any compost or cull piles for volunteer potato plants that might have survived the winter. If you find potato plants, pull them out and either burn them or put them in the trash — do not compost them. If the tubers were infected and survived, then the late blight could infect the leaves and stems and pro-duce spores when weather conditions are favorable.

In all of this there is a good argument for avoiding overhead watering, keeping leaves and stems dry by using drip irrigation or soaking rather than overhead sprinklers. If you must water overhead, do so early on a sunny morning so that leaves can dry quickly, reducing fungus spore germination, and never water late in the day when the leaves do not have time to dry before dark. Also, allow enough space between plants for air to circulate and carry spores away from the plants.

Learn to recognize the symptoms of late blight. You can find excellent photographs at the following website: http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm. Unfortunately, if you find late blight, your only recourse is to immediately destroy the infected plants.

Gardeners are hopeful creatures. I have forgotten last year’s misery and am eager to try again, hoping for a warmer, drier season. My students are hoping they will eat fresh tomatoes in early September.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@shead.org. Include name, address and telephone number.

PHOTO COURTESY OF CORNELL UNIVERSITY

Late blight cankers on tomato stems. Plants with these symptoms should be destroyed as soon as noticed to minimize spread of the disease.

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