It's frustrating when the leaves on your prized heirloom tomatoes turn yellow or are munched away by hungry worms. Here are explanations and help for some common tomato-growing problems.

• Yellow leaves - Ozone and other air pollution can cause leaves to yellow. Some varieties are more susceptible to this damage than others. Experiment to find tomato varieties that will thrive in your garden.
• Wilt and die - A healthy plant that suddenly wilts and dies is probably a victim of verticillium or fusarium wilt. Discard the dead plants and replant in a different area of the garden since these fungal diseases can live on in the soil. Also look for resistant varieties with the letters VF after the name.
• Tomato hornworms - These large, striped worms often are spotted on stems or the backs of leaves. Pick them off or apply Bt (Bacillus thuringiensis) which is fatal to the worm but won't harm other beneficial insects. Look for it in nurseries or garden supply stores.
•  Cracked fruit - Irregular irrigation generally is the culprit here. Water stress also can lead to blossom end rot that leaves leathery patches on the fruit. Water deeply in a 2-foot radius around each plant and increase irrigation when temperatures climb.
• Blossom drop - Blossom loss occurs naturally when temperatures climb above 90 degrees during the day or drop below 55 degrees at night. When severe weather ends and temperatures adjust, healthy plants will blossom again and fruit production will resume.

Find a complete tomato growing guide on the San Diego Master Gardener Web site, www.mastergardenerssandiego.org. The new "Sunset Western Garden Book of Edibles" also is a detailed source of problem-solving info.