Managing Leaf Mold in High Tunnels

Over the past 20 years, Penn State has performed a number of tomato variety trials. A study from 2014-2015 specifically evaluated tomato varieties for their susceptibility to Leaf Mold. The data is still applicable today.

About the Pathogen

Leaf mold (also known as both Brown and Gray Leaf Mold) is caused by the fungal organism Passalora fulva (previously Fulvia fulva). It is a different fungus than the pathogen that causes Gray Mold (Botrytis cinerea). High tunnels are an ideal place for Leaf Mold to grow as it prefers 85% or higher humidity and temperatures between 50 and 90°F. It is most often a high tunnel disease but has been seen in field plantings. It is known to be highly mutagenic, which means it can produce new strains rapidly, so tomatoes that are resistant to current strains may be susceptible to new strains.

This makes management challenging so the limited array of fungicides that we currently use must be coupled with practices that reduce the conditions that cause the disease. Current tomato varieties exhibit a wide variety of susceptibility, tolerance, and resistance. Air currents, regular plant maintenance activities, and harvesting all spread the spores rapidly from plant to plant.

It is also known as a human allergen, so those with asthma and related mold sensitivities should exercise precautions such as using dust or allergen masks and thoroughly wash after handling infected plant tissue.

Tomato Varieties

Conclusions from the 2015 study:

• Charger and BHN 589 are very susceptible to the strain(s) that grew in the high tunnel in 2015 at the Penn State SE Agriculture Research and Extension Center.
• The two indeterminate varieties Big Dena and Frederik were quite resistant, with no spots seen on any plants in any of their plots.
• Red Mountain, Primo Red and the four varieties that we grew on grafts had little to no leaf mold (Primo Red, Scarlet Red, BHN 589, and Red Deuce). Both Red Deuce and Red Morning have been reported to experience little to no Leaf Mold in 2015.

Some seed catalogs provide leaf mold resistance information in variety descriptions. In general, it is thought that grafting tomatoes provides no control of leaf mold.

Practices that Reduce Leaf Mold

Everything that improves circulation to reduce humidity deep in the plant canopy can reduce Leaf Mold infection. Remember, this disease requires 85%+ humidity, so improving circulation to lower humidity will greatly reduce infection.

• Remove all leaf tissue below the lowest fruiting cluster. These leaves contribute little to nothing to plant growth or fruit quality but are excellent places for the disease to get started. Deleafing needs to be done regularly as the harvest proceeds. Not only is circulation improved, but these lower leaves are often where the infection starts, so de-leafing can remove inoculum.
• Prune tomatoes to remove any unwanted growth. This will improve pesticide coverage as well as improve circulation.
• Use currently recognized resistant varieties. Remember that this list will constantly evolve as new strains of Leaf Mold develop and varieties are released.
• Increase between row and in-row spacing. This is not easy as most high tunnel growers are generally striving for the highest plant populations, but a lower density planting will have better circulation.
• If your tunnel is only passively ventilated through roll-up sides, consider adding fans and louvers in the gable end wall peaks. Also, increasing the height of the roll-up sides will improve ventilation. Most high tunnel frame manufacturers offer taller ground posts to push the point where the side walls bend inward higher.
• Rotate fungicide modes of action and use the few known materials that reduce Leaf Mold. Please refer to the tomato chapter of the Mid-Atlantic Commercial Vegetable Production Recommendations for spray recommendations.