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BUGS AND BUGABOOS: Tomato Patch Postmortem

By Vincent Lazaneo.

Now that the prime season for harvesting flavorful, vine ripe tomatoes has passed, it’s time to evaluate the performance of your tomato patch. If a plant you grew did not produce the quality or quantity of fruit you expected, you should decide whether to plant it again next year or try another variety. There are lots of tomatoes to choose from and keeping an underperformer can deprive you of the opportunity to discover a great new variety.

Planting tomatoes that are genetically resistant to common diseases and pests can improve your chances for a bountiful harvest. Many hybrid varieties are resistant to damage by root knot nematodes, which block water movement in roots through the formation of galls, and whose feeding wounds provide entry points for pathogenic fungi. Hybrid varieties are also often resistant to the soil borne pathogens that cause Verticillium wilt and Fusarium wilt; the letters VFN indicate such resistance. Some hybrid varieties are also resistant to tobacco mosaic virus and other pathogens. Before you buy a tomato plant or seed, search online to find out what disease resistance it has. Also check if it is an All-American Selection (AAS) winner. This award indicates the tomato performed well in different geographic areas of the US and it will probably grow well in your garden.

Heirloom varieties are very popular, but they are not resistant to the most common soil borne pathogens that damage tomatoes. Some nurseries now sell a few heirloom varieties grafted on a disease resistant rootstock. The grafted plants cost more but you may want to try one or two in your garden if you have grown heirloom varieties before and found them lacking.

Even the best tomato varieties are susceptible to some diseases and pests. Check your plants often to detect problems early so appropriate control measures can be taken. If a pesticide is needed, select the least toxic product that will work.

Gardeners often complain that the inner leaves on their tomato plants dry out and die. Affected leaves first appear near the base of the plant and spread upward as the season progresses. By the time fruit begins to ripen only the outer foliage on the plant may still be green. This malady is often caused by powdery mildew and sometimes tomato russet mites. Powdery mildew occurs every year in my garden, which is located near the coast. Damage on tomato leaves from powdery mildew spreads by airborne spores. Discolored spots and white powdery growth appear on the upper leaf surface. Affected leaves die, turn brown, and shrivel, but remain attached to the stem. To combat it, I only grow varieties with a strong indeterminate growth habit. The older foliage on these plants is still damaged, but they produce enough new growth to allow most of the fruit to ripen. Varieties with a determinate growth habit often lose most of their foliage before all the fruit matures.

Powdery mildew spreads by airborne spores, and infection can be delayed by applying a protective fungicide. Organic fungicide products sold to control powdery mildew in the home garden include Serenade® Garden Disease Control and Monterey® Complete Disease Control. Each product contains a different strain of naturally occurring Bacillus bacteria that destroys the disease pathogen. For best control, spray all leaf surfaces and re-treat weekly as plants grow.

Tomato plants can also be damaged by russet mites, which remove sap from surface cells on leaves, stems, and fruit. Affected leaves dry out and often have a greasy brown or russet color. Since a 14x hand lens is needed to see the yellowish, conical shaped mites, they often go unnoticed until plants are damaged. Russet mites can be controlled with products containing sulfur, such as Safer® Garden Fungicide, which is labeled to control powdery mildew.

Vincent Lazaneo is UC Urban Horticulture Advisor Emeritus. He has a master’s degree in horticulture and a teaching credential in vocational agriculture from UC Davis. In 1983, Vince began the Master Gardener program in San Diego. Vince frequently contributes, or has contributed to, the San Diego Union-Tribune, Mira Mesa Living, and other publications, and he enjoys growing specialty plants in his home garden, reading, hiking, and fishing.

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