Marin IJ Articles

Why Are My Tomato Plants Failing?

At this time of year, many clients contacting our Marin UC Cooperative Extension/Marin Master Gardener office inquire about failing tomato plants showing severe wilt and poor fruit production. This seems to be especially true this year, perhaps due to our cool, wet spring. There are several possible culprits for these observed problems.

There are reports of a Phytophthora fungus, particularly noted in West Marin. It expresses itself as a water-soaked appearance of leaves and stems, gray green or yellowed at first, starting at the leaf margins, then spreading rapidly. In some cases a grayish white fungal growth may be present on the undersides of dying leaves. Fruits remain mostly immature with hardened areas or bumps and may have a white downy marbling. This white fuzzy appearance gives this disease its common name of downy mildew, but it’s also frequently called Late Blight. This disease thrives in cooler, moist conditions and is less a problem in the drier, warmer areas of our county.

If that wasn’t enough, we have another commonly reported problem of tomato plant failure caused by either of two fungal diseases: Fusarium wilt and Verticillium wilt. Although a lab culture is required to distinguish between the two, this step is unnecessary as the symptoms are almost identical and the treatments the same. The first signs of the fungus attack are usually yellow blotches on the lower leaves. The leaves may wilt, die and drop off. The disease symptoms progress up the stems and the plant becomes stunted with the fruit failing to mature to expected size. Only the top leaves stay green. Fruit remains small, develops yellow shoulders, and may sunburn because of loss of leaf cover. Diagnosis involves making a vertical slice through main stems, just above the soil line, and observing a brown color in the conducting tissues. This discoloration can be traced upwards as well as downwards into the roots. Of course, this diagnostic step is fatal to the plant, so you would only employ it when the plant is so stressed that it is unable to ripen edible fruit. In less damaged plants, you can check the stems and roots at season’s end.

What can I do?

For these disorders there really is nothing you can do at this time to correct the problem. The Phytophthora spores over winter on living tissue, so it’s very important to thoroughly clean up all tomato plants and plant materials of any other members of the nightshade family. Next year, plant your tomatoes in the sunniest spots in your garden and space them well to assure good air circulation. Also avoid any overhead watering as this adds to the humidity that supports disease development.

Fusarium and Verticillium are soil borne fungi present in most of our local soils. Fusarium attacks only tomato plants while Verticillium can attack over 200 plant species, especially tomatoes, potatoes, eggplant, strawberries, and black raspberries. Infection enters the feeder root hairs and grows rapidly up the plant’s vascular or water-conducting vessels interfering with the normal upward movement of water and nutrients.

Fusarium and Verticillium spores persist in soils for long periods. If susceptible plants are available, the fungus gains in strength and becomes a serious problem. Control is achieved through a variety of methods including selection of resistant varieties, cultural best practices or some remedial techniques to reduce fungus vitality. Cultural practices include long rotations (4-5 years) with non-related crops, well-drained soils, and soil moisture kept at the minimum.

By far the most feasible and economic control is the use of Fusarium and Verticillium-resistant tomato cultivars, of which there are many with varying maturities and excellent fruit qualities. The seed package and seedling labels usually will indicate resistance by placing the letters V, F and/or N in the descriptions. V stands for Verticillium wilt, F for Fusarium wilt and N for resistance to nematodes. Home gardeners are exploring an exciting universe of tomato varieties. Our local nurseries and farmer’s markets offer an increasing number of interesting cultivars, and spring gardening catalogues devote many pages to their tomato offerings. However, many of these are not resistant to these fungi. Remember to check on resistance by reading the descriptions or by asking the vendor.

For soils that are already severely infected, another tool for management is solarization. This harnesses the sun’s energy as heat trapped beneath plastic sheets spread on the soil surface during mid-summer. Water the soil and place clear (not black) plastic sheets at least 1.5 mil thick on the soil, holding them in place with weights such as small stones or boards. Heat build-up in the soil is lethal to the fungus. The best times to apply this technique are during the warmest periods of our summer—generally July and August. The sheets should be left in place for eight weeks.

Although it is disappointing, we have to tell tomato growers that there is nothing that can be done this year about failing plants. However, there are things you can do to help with next years’ plantings. For more information you can visit the UC Davis IPM web site at www.ipm.ucdavis.com or call your UCCE/Master Gardener office at 415-499-4204.