Tomato leaf mold is a fungal disease caused by Passalora fulva. It primarily attacks the leaves, beginning with pale green or yellow patches on the upper surface. On the underside, corresponding patches develop a distinctive olive-green to brown, velvety growth as the fungus produces spores.

It thrives in high humidity and prolonged leaf wetness, which is why it can become particularly troublesome in greenhouses or densely planted tomatoes with poor airflow. Spores spread through air, splashing water, tools and plant handling. Severe infections cause leaves to curl, dry up and die, reducing the plant's ability to support and ripen fruit.

How to Fix It

Remove infected leaves and dispose of them rather than composting them. Remove tomato debris at the end of the season and clean stakes, cages and tools before reusing them.

Reduce humidity around the plants. Increase spacing and airflow, prune crowded foliage, water at soil level and avoid wetting the leaves. Resistant tomato varieties are especially useful where leaf mold is a recurring problem.

Some Bacillus- or Trichoderma-based biological fungicides may help suppress infection when used preventively, but they won't restore infected leaves. For the home garden, airflow, dry foliage, sanitation and resistant varieties are the most dependable controls.