Issue: July 21, 2007

Summer tomato problem

I live in Los Lunas, NM. I have several tomato plants purchased from nurseries. They have done well until just about the time the first fruit ripened. Then, they suddenly begin to yellow, wilt and die. I realize there are many potential causes.

The garden soil is clay-loam or silty clay-loam (valley soil-not sandy), with composted horse manure worked in. I water frequently, perhaps too much. I fertilize with miracle grow.

The most susceptible seem to be 'Roma', 'Beefmaster' and 'Health Kick' (looks like a 'Roma') tomatoes. The healthiest plant so far is a yellow pear miniature. It shows no sign of decline and is about 4 ft. tall and about the size of 2-3 bushels baskets.

The 'Health Kick' plant is the most recent to show decline. The older leaves curled inward and turned yellow, beginning from the edges. Soon the entire plant turned yellow and declined rapidly.

What varieties can you recommend for next year?

Rex W

Answer:

You are correct that there are many potential causes for tomatoes to begin to decline. Some tomato plants stop producing fruit when the temperatures kill the pollen and prevent fruit formation. The plants themselves may also show some yellowing and reduced growth. This usually ends when the monsoon rains bring moisture and cooler temperatures. However, I don't think this is the problem with your plants.

Curly top virus and other viruses can also affect tomatoes. The plants seem to be doing well, then the leaves begin to curl (twist so that the bottom is on top), with purple veins revealed. The color of the plants change with the new growth becoming yellowish, and the plant ceases growing and no longer produces fruit. Any fruit formed is safe for humans to eat. If the plants are affected by tomato spotted wilt virus, the fruit will not ripen properly and you will not want to eat them.

Finally, and probably your plants' problem, is fungal disease. This develops as you have describes. The leaves become distinctly yellow and then turn brown and black. This often begins at the bottom of the plant and works upward through the plant. It will kill the plant and end your harvest. I suspect this problem because of your frequent watering in a heavy soil. When it rains, the disease spreads to new plants as soil is splashed onto the foliage of nearby susceptible plants and as soil washes from one area to another.

The fungus is soil borne. That means it resides in the soil and infects the plants from that location. Crop rotation (not growing tomatoes or related plants in that location for 3 years) is one way to help reduce the buildup of this disease in the soil. Mulch to protect the plants from splashing soil will also help. Adding compost to the soil adds beneficial organisms that compete with the disease fungi. Soil solarization or other pasteurization techniques can also help. Do not put these diseased plants in the compost or turn them into the soil.

The viruses do not persist in the soil, so they do not create the same problems that the fungi cause. They overwinter in weeds, and are spread by insects. Control of the two viruses described above is by managing weed problems during winter and spring. Other viruses require different management strategies.

NMSU Extension Circular 572: Vegetable Variety Recommendations for New Mexico Backyard and Market Gardens will help you choose plants for next year. This publication is available at http://www.cahe.nmsu.edu/pubs/_h/ or your local NMSU Cooperative Extension Service office.

As you have observed the smaller fruited (pear and cherry) tomatoes tend to continue to bear through the summer heat and may be a wise addition to your garden along with the larger fruited types.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!