by Linda Carloni, Master Gardener and ABG Board Member
Your full-size tomatoes have started to ripen. Your cherry tomatoes may be ready to eat now. Nothing tastes like a tomato from the garden! But while tomatoes are one of the most popular foods to grow in your garden, they are also prone to many maladies – insects eat parts of them, fungi kill the plants or make them less productive, and environmental factors plague them. Here’s an introduction to some of the most common maladies, including what the problem looks like and ways to manage it.
Verticillium/Fusarium
Plants infected with the Verticillium or Fusarium wilts turn yellow and then brown, starting with one side or branch. The problem then spreads, eventually killing the plant. Both are caused by a fungus, which persists in the soil for a long time. To see if you have one of these wilts, cut an affected plant at the base of the stem. In an infected plant the central water-conducting tissue will have browned, compared to the healthy ivory of uninfected plants. Verticillium attacks a wide range of plants; fusarium attacks just tomatoes. It may require a lab test if you want to tell the two fungi apart.
Once your tomato plant has one of these wilts, there is really no way to save it. The best way to manage the problem is prevention, through planting resistant varieties the next year. If a variety is resistant to verticillium, it will be labeled V; for fusarium resistance, the label is F (sometimes followed by a number.) In Alameda, many of us need varieties that will grow without a lot of heat, meaning with days-to-maturity ratings of 55 to 75. Finding both disease resistance and low days-to-maturity in a single variety can require some research! Good places to start looking: seed companies (I like Johnny’s and Territorial, but there are lots of good options) or the lists of plants available for sale at various counties’ Master Gardeners’ plant sales. A 2005 UC article here is a great resource. This article specifies the UC California climate zone – A, B or C (in Alameda, we’re in C), provides days-to-maturity and lists the disease resistance symbols for each variety; unfortunately, since the publication is now 15 years old, it doesn’t include many popular newer varieties.
To try to avoid getting one of these wilts, rotate crops, so that you don’t grow tomatoes or other plants susceptible to the wilts in one place every year, to avoid build up of the fungus in the soil. It’s recommended to plant susceptible plants in a given place only every three years – that can be a challenge for the gardener with limited space in the sun!
Powdery Mildew
Powdery mildew is well-known for attacking peas and zucchini, appearing first as white powdery spots on both surfaces of leaves. It attacks other vegetables as well, and this year my tomatoes have a bad case. One of the powdery mildews that affects tomatoes causes yellow patches on leaves, but little powdery growth, so it looks quite different from powdery mildew on your peas. Leaves infected with powdery mildew may also twist and otherwise distort, or may gradually turn completely yellow, die, and fall off. Severely infected plants may have reduced yields, shortened production times, and fruit that has little flavor. Alameda gardens can be very susceptible to powdery mildew, because moderate temperatures (60° to 80°F) and shade encourage the disease.
Once again, the best management tactic is prevention. Fungicides, including some less-toxic horticultural oils, can be effective, but must be sprayed no later than the first sign of the disease. Once the mildew is spreading, it’s very hard to control.
For the future, practice prevention by planting resistant varieties where available, planting in full sun, pruning or staking to provide good air circulation, and avoiding excess fertilizer – slow-release fertilizer is good. And keep a close eye, to pounce on any early signs quickly.
Hornworms
One of the most frustrating tomato pests is the hornworm. The hornworm moths lay single eggs on the tomato leaves. When the caterpillars hatch, they munch on your tomato leaves and fruit. The caterpillars get large, but their color and stripy pattern make them remarkably hard to see.
For this problem, management can help immediately. Leaf or fruit damage and large black or dark green droppings are likely signs of hornworm caterpillars, so look on the plant stem above the droppings or in the vicinity of the damage. When you locate them, they can be managed by picking them off and cutting them in two. Rotating your crops and breaking up and turning over the soil will destroy pupae in the soil and help prevent repeating the hornworm life cycle the following year. For more on the hornworm click here.
Blossom End Rot
Plants with blossom end rot have small, light brown spots at the blossom end of immature fruit. As the fruit ripens, the spot expands into a dark leathery lesion.
Blossom end rot is not caused by a disease, pest or fungus – it results from water balance issues in the plant, leading to a low level of calcium in the fruit. It is more common on sandier soils, like we have in Alameda, and is aggravated by high soil salt content or low soil moisture. Some varieties are more affected than others, but no variety has been found to be resistant. To reduce the problem:
- Monitor soil moisture to make sure that the root zone neither dries out nor remains saturated.
- Add compost and other organic matter to increase the soil’s water-holding capacity. Use mulch to reduce evaporation.
- Follow recommended rates for fertilizers. You may want to be sparing with manures in the tomato bed – they tend to be high in salt.
- Take care when weeding not to damage the small feeder roots near the plant.
Unless soil tests clearly indicate calcium deficiency in your soil, applying calcium to the soil or leaves will probably not reduce the problem.
Sunscald
Tomatoes with sunscald have tan to light brown leathery patches on the side of the fruit that is exposed to the sun. It’s caused by overexposure to the hot sun. When we have warm days, it can definitely be an issue in Alameda if you have a nice sunny tomato patch. To deal with the issue, you can immediately begin providing partial shade during the hours of most intense sunlight. To prevent in the future, keep your plant vigorous with appropriate fertilizer and water to produce an adequate leaf cover, and avoid too much pruning.
With some advance planning for prevention and frequent checking of your garden, you can definitely minimize your problems with these tomato maladies.