A rarely observed disease of field-grown tomatoes, called "leaf mold", was found last week in a commercial field in west central Iowa. Leaf mold, caused by the fungus Cladosporium fulvum, causes white to yellowish spots with diffuse margins on upper surfaces of the older, lower leaves. The spots are so diffuse, in fact, that the yellowing often looks more like a nondescript mottling. The most distinctive symptom is on the underside of leaves, where patches of olive-green, fuzzy mold are found. Infected leaves curl up, wither, and may eventually drop from the plant. Occasionally, other aboveground plant parts, including fruit, can be attacked; fruit infections result in a black, leathery rot on the stem end. Only foliar symptoms were seen in the Iowa field.

This is the first time in nine years at ISU that I have seen this disease. Leaf mold is usually considered to be a disease of greenhouse tomatoes, which appears only rarely in the field. To infect tomato plants and spread in the field, the fungus requires very high humidity - in excess of 85% - for prolonged periods. These conditions occur frequently in some greenhouses with poor air circulation, especially at night, but less frequently in the field.

The pathogen survives in fields for at least one year in infested debris or as sclerotia (tiny resting structures produced by the fungus). The fungus can be spread by machinery and by workers.

The big question is: why did this disease appear when and where it did? Evidently, frequent rains and long dew periods in late June and early July were sufficient to create an epidemic in the field I visited. But where did the inoculum come from? The pathogen is known to be carried on seed, so a possible scenario is that contaminated seed gave rise to infested seedlings in a greenhouse, then the disease was moved to the field during transplanting. Alternatively, it's possible that the pathogen had overwintered in or near the field on infested debris created during a previous growing season, then spread onto the 1994 crop after transplanting.

Some familiar precautions will help avoid problems from leaf mold:

• Sanitation is a good start; clean up and destroy debris from infested fields and greenhouses to discourage overwintering of the pathogen. In the greenhouse, it is advisable to sterilize production areas once debris hs been removed, either with steam or with a disinfectant such as 10% bleach (one part bleach to nine parts water). In addition, buy high-quality seeds and transplants that have been certified disease-free.

• Rotation. Rotating away from tomatoes for 3 years, preferably longer, is a very important preventative for several other debris-borne tomato diseases as well as leaf mold. The most damaging epidemics of many diseases occur where adequate rotation has not been practiced.

• Control humidity. Regulate night temperatures to prevent the humid, terrarium-like conditions that favor spread of Cladosporium fulvum. Venting the house shortly after sunset, plus heating, will help to keep night-time humidity down. Good air movement during the day is essential, too. In the field, staking and pruning will improve air flow and can reduce humidity.

• Fungicides. Leaf mold can be controlled effectively with a labeled contact fungicide such as chlorothalonil (e.g., Bravo 720) or an EBDC (e.g., Dithane F45) applied on a preventative basis.

This article originally appeared in the July 22, 1994 issue, p. 119.