The incidence of fungal molds on high tunnel and greenhouse tomatoes has increased in recent years. Management of these diseases begins with an understanding of the difference between the two major types of fungal molds and their control strategies.
Gray mold is caused by the fungus Botrytis cinerea and is a common pathogen of nearly all plants, including tomato. Symptoms include light tan or gray spots on the leaves. These spots later become covered with a grayish-brown fungus. Ultimately the leaf collapses and dies. The fungus also can cause cankers on stems and kill flowers and fruit.
Leaf mold is caused by the fungus Fulvia fulva (Cladosporium fulverum) and affects only tomatoes. Because of its need for high humidity to become infective, leaf mold is almost entirely a disease of greenhouses and high tunnels. Symptoms include pale-green or yellowish areas on the upper surface of leaves. Lower leaves are affected first; younger leaves show symptoms later. At the same time discoloration appears, the fungus begins to grow on the undersides of the leaves in the same area as the discoloration. The mold that results is more deeply colored toward the center of the affected areas. As the disease progresses, the leaf spots turn yellowish-brown and the leaf curls, withers and ultimately dies. Leaf mold differs from gray mold in that the fungus appears only on the underside of the leaf.
There is genetic resistance for leaf mold which is the best way to prevent the disease. Most tomato varieties bred for greenhouse production (e.g. 'Trust', 'Quest', 'Geronimo') carry genetic resistance to leaf mold. Conversely, most tomato varieties bred for outdoor production do not have genetic resistance. This is of little concern outdoors since the environment is not conducive to leaf mold infection. Growing the same varieties in a greenhouse or high tunnel provides an ideal environment for the disease to become problematic, and it frequently does.
Both gray mold and leaf mold favor cool temperatures and high relative humidity greenhouse tomatoes are especially at risk. The cultural control of these two diseases involves keeping greenhouse temperatures warm (above 70° F) and relative humidity as low as possible. It is important to understand that the cooler air is, the less water vapor it takes to saturate the air. Therefore cool greenhouses normally have high relative humidity. On the other hand, keeping temperatures warm tends to lower relative humidity to a desirable point, at least temporarily.
Encouraging good air movement by adequate plant spacing and leaf pruning helps to lower the humidity around the leaf surface. Additionally, good sanitation practices including the removal of all plant debris between crops helps to reduce inoculum of the diseases but will not prevent them entirely.
For optimal control, an integrated approach should be followed. In addition to the previously mentioned cultural practices, fungicide use should be considered. Chlorothalonil is effective in gray mold control and copper compounds are also labeled; maneb, mancozeb, copper compounds and chlorothalonil are all labeled for leaf mold control. Growers should remember that most fungicides are preventatives, not curatives. Therefore application of fungicides before symptoms appear gives best results.
One grower in Central Missouri prevented leaf mold on Goliath in 2008 & 2009; Goliath is a greenhouse tomato without resistance. He applied Bravo (chlorothalonil) first in early April, and then about two weeks later. Some growers have noted that Bravo (also Echo & Equus) are labeled for field application only. A high tunnel is considered a field when fully vented, and a greenhouse when closed. Remember if vented, it should be left that way for the reentry interval (REI).
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REVISED: December 2, 2015