Issue: January 5, 1998

Tomato plant diseases, planning for next summer

I've been having a tomato problem during the past four years and am hoping you can offer some suggestions. I've been a heavy duty gardener for over 30 years with over an acre in garden now, a garden writer to boot, but I'm really stumped. (I'm four years "new" to New Mexico's climate, so that may have something to do with it.)

I think I have a blight problem. It starts out with black spots on the leaves, then the tomatoes. The vine bears poorly, the tomatoes do not grow out, and production is really poor!

The transplants are well-started plants when put out, watered very well, sprayed with copper weekly, but still do not produce. I should be getting LOTS of tomatoes as I set out some 80 plants.

I rotate my crops, destroy the vines in the fall, plant several varieties, and NONE do too well. The other plants in the garden do great.

Natalie Goldberg, plant pathologist with NMSU's Cooperative Extension Service, states that early blight caused by the fungus Alternaria solani is a possibility for your problem. However, she said that this is usually not a big problem except in isolated occasions in gardens watered excessively. The fungus needs high humidity to cause a problem. Such high humidity conditions occur in those rare wet summers in New Mexico, or when the garden is irrigated too frequently and when the garden is irrigated in the evenings. The symptoms appear first on older, senescing, leaves and progress up into younger leaves, and eventually the infection is evident in the fruit.

Dr. Goldberg stated that another fungus, botrytis, is also a potential suspect. If this fungus were present, you would see spores on the leaves at the location of the lesions. These spores appear as grey fluffy growths. However, she stated that botrytis is less likely than early blight.

I also discussed the potential of copper toxicity with her. Copper in excess concentrations can cause injury to plants. As you are applying it frequently, and you didn't state what form of the copper was used, we had to consider the possibility that copper was also a potential culprit. Dr. Goldberg indicated that there are copper-based fungicides which should be no problem if used according to the directions. However, there are other forms of copper which can be purchased and could cause problems. She and I have seen problems with copper intended for killing roots in sewers also being used. It caused foliar injury to plants and defoliation.

You have already discussed this problem with your county Extension agent, but we recommend that next summer during the growing season you send samples to Dr. Goldberg at NMSU with the assistance of your county agent so that Dr. Goldberg can determine if fungus diseases are actually present. In the meantime, as you plan for next year's garden, show your county Extension agent the container and label from the copper material you are using. The county agent will consult with Dr. Goldberg to determine if the copper could be causing the problem.

Continue the excellent practice of crop rotation and destroying infected plants. This will help prevent the buildup of pathogenic fungi in your garden.

Good luck, and may your tomatoes produce a bountiful harvest next year.

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!