Issue: August 2004

Protect Tomatoes from Burning Summer Weather

To grow robust, juicy tomatoes in New Mexico, gardeners need to protect plants from scorching August heat.

Tomatoes generally thrive in full sun and warm weather, and some newer varieties can even withstand intense evening heat. But when nighttime temperatures remain above 75° F, blossoms often drop off tomato plants and fruit production stops.

Too much nitrogen fertilizer can result in overly vigorous vine growth and blossom drop. And intense sunlight can cause sunscald spots on tomato fruit. Sunscald blisters eventually turn grayish-white, forming a paperlike surface.

To protect fruit, try draping cheesecloth over the vines in mid to late summer, or train vines to a cage to shade developing fruit. Training the vines makes it easier to pick tomatoes, and it reduces fruit rot that can develop when tomatoes touch soil.

Avoid water stress. Underwatering plants during bloom can result in blossom end-rot—a leathery, brownish rot that occurs on the blossom end of the fruit. Excess nitrogen, calcium deficiency and salt accumulation in soil can aggravate the problem. Mulching soil around the cage with straw will reduce evaporation and ensure a more even supply of moisture to the plant.

Fluctuations in soil moisture can also cause split fruit or growth cracks. Hot, dry weather can lead to tough, leathery skin on the tomato fruit. Alternating moist and dry soil conditions may result in bursts of growth in fruit, causing the stiff leathery skin to crack. Mulching the soil will even out soil moisture and reduce weed problems by shading the soil and preventing weed seed germination.

Common tomato diseases include Verticillium and Fusarium wilts, nematodes, and curly top virus. The first two are soilborne fungal diseases that enter the roots and plug the water-conducting vessels, causing the plants to wilt. Brown discoloration of the vascular tissue near the base of plants will confirm infection. Pull out infected plants and throw them away. Do not recycle them as compost, because the disease can spread to other areas of the garden. To control both diseases, plant resistant varieties.

Root-knot nematodes can be a major problem, especially in sandy soils. These microscopic worms feed on roots, causing plants to become yellow and stunted. Roots will develop obvious swelling or galls. To discourage nematodes, plant resistant varieties, or add large amounts of compost or other organic matter to sandy soils.

Curly top is a virus spread by the sugar beet leafhopper. Leaves of stunted plants will curl upward and twist, gradually becoming stiff and leathery. Veins will generally turn purplish. Infected plants can be pulled out and recycled as compost, because the disease is not soilborne and is spread only by the beet leafhopper.

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For more gardening information, visit New Mexico State University's Cooperative Extension Service publications world wide web site at

George W. Dickerson, Ph.D., is is a horticulturist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.

Also Please join us on Southwest Yard & Garden, a weekly garden program made for gardeners in the Southwest on:
KNME-TV Albuquerque at 9:30 p.m. Saturdays,
KENW-TV Portales at 10 a.m. Saturdays,
and KRWG-TV Las Cruces at 11:30 a.m. Saturdays (repeated at 1 p.m. Thursdays.)