What's wrong with my tomatoes?!

* Watch out for sunburn. Both tomato fruit and leaves can burn when exposed to too much afternoon sun. The solution: Create some temporary shade to shelter the plants during the hottest part of the day.

* Tomato flowers won’t set when temperatures stay hot (over 90 degrees) for prolonged periods. The pollen dries out. But indeterminate varieties will produce more flowers with new growth.

* Heat isn’t the only weather factor that can affect tomatoes. Lack of wind or breeze also can affect tomato set. Tomatoes generally are wind pollinated. To help with pollination, gently wiggle the tomato’s cage or trellis to shake up and spread the pollen.

* Blossom-end rot, which shows up most in hot weather, is tied to inconsistent soil moisture. Keep plants evenly hydrated. (Mulch helps with this, too.) Don’t let plants dry completely out.
Fungal woes Other tomato problems can be traced to fungal disease. The most common are fusarium wilt and verticillium wilt, both caused by fungi in the soil. Fusarium wilt attacks the plant’s vascular system. Its symptoms include yellowing, stunting and death of seedlings, and yellowing and stunting of older plants, say UC master gardeners. Infected plants wilt readily (as the name implies), and lower leaves yellow and dry. When a stem is cut open, the interior looks brown and rotting, not white and healthy.

Verticillium wilt, which can attack more than 350 plant species, looks very similar to fusarium wilt in tomatoes. Lower or older leaves develop V-shaped yellow patches that gradually turn brown, according to the master gardeners. As the leaves die, the plant loses its foliage protection and is more susceptible to sunburn. The developing fruit can get scorched.

The master gardeners note that verticillium wilt “seldom kills tomato plants but reduces their vigor and yield.”

The fungi that causes both wilts lurks in the soil (quite possibly forever). The best way to avoid this problem is to plant wilt-resistant varieties. The V or F attached to hybrid names indicates resistance. (Unfortunately, many heirloom tomato varieties don’t have that trait.)

These tomato leaves show evidence of septoria leaf spot. It was the only variety in this particular garden to show this (unusual to Sacramento) problem so likely had low resistance. The fruit was unharmed but the plant was doomed. This particular cherry tomato plant subsequently was removed and discarded, not composted.

Another fungal disease showing up this summer: Septoria leaf spot. The foliage becomes covered with black or dark brown polka dots, then yellows and dies. This is much more common in Southern states, where summer humidity is as high as the temperature. But it can happen in California, too, when the humidity spikes as we’ve seen during recent monsoonal-like spells. Overhead watering or sprinklers that wet the foliage in the afternoon also can contribute to this problem.

If caught early, remove the diseased leaves and discard. (Don’t compost.) If possible, improve air circulation around the plant so foliage can dry quickly. Mulch helps here, too; it reduces splashing water that could carry fungal spores to lower leaves.
Watch out for viruses, too Not all foliage problems are caused by fungi. Some are viruses, spread by pests.

Curly top – beet curly top virus – is spread by the beet leafhopper, which overwinters in the foothills and descends on the Central Valley in spring. Tomatoes have no resistance to curly top and can become infected by leafhoppers feeding on their stems.