Q: The heirloom tomatoes I planted looked healthy at first, but now they have many yellow leaves. What’s the problem?
— Pam Clark, San Luis Obispo
A: Heirloom tomatoes have become popular, both for flavor and also for unusual shapes and colors. However, some of these old-fashioned tomatoes may not be as disease resistant as modern hybrids. They can also be more finicky about soil and climate.
Caused by soil fungus, Fusarium and Verticilium wilts are common tomato diseases. Either can cause yellowing of leaves as can soil nematodes. They are difficult to control, and you may lose your plants. Tomato varieties resistant to these diseases are labeled with the letters VFN. Tomatoes are also susceptible to other diseases caused by fungi and to several viral diseases.
If tomatoes succumb to disease this year, next year you might find success by solarizing garden soil or planting in containers; avoiding replanting (for several years) where tomatoes last grew; rotating all crops; and keeping the garden clear of plant debris. Assure that tomato roots do not sit in soggy ground and buy varieties well-suited to your microclimate.
Aphids could also cause yellowing leaves. If you find these tiny sucking insects, they may be controlled with a jet of water or insecticidal soap. A number of insects feed on tomato leaves and fruit, but frequent inspection usually gives a clue to culprits. Bt (Bacillus thurengiensis), a safe pest insect pathogen, is helpful against many tomato caterpillars. Integrated pest management practices give alternative methods of controlling pests, without resorting to harsher chemicals. Hand picking can also be a good control method.
Despite all the problems tomatoes could develop, with persistence gardeners can have delicious homegrown fruit. Call your Master Gardeners about soil solarization or for help in diagnosis of tomato disease symptoms, or go to www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/DISEASES.