Managing Leaf Mold in High Tunnels

High Tunnel acreage in PA and the Mid-Atlantic continues to grow due to improvements in tomato quality, the NRCS cost-sharing program, and substantially earlier harvests.
Managing Leaf Mold in High Tunnels - Articles

Updated: August 8, 2017

Managing Leaf Mold in High Tunnels

Leaf Mold on determinate, high tunnel tomato plants.

For the past 13 years, we've been evaluating a wide variety of tomatoes under field conditions and since 2009 planted the trial in the Haygrove Super Solo tunnel at the Penn State Southeast Agriculture Research and Extension Center (SEAREC). The past program has identified, Scarlet Red, Brandyboy, Primo Red, Red Bounty, and Red Deuce as premier varieties that can generate sufficient yields of high quality fruit to justify production under intensive high tunnel conditions. This program has also identified several varieties that minus leaf mold susceptibility are excellent in high tunnels. One observation from the 2014 growing season was the pervasiveness of leaf molds across many varieties. Identifying varieties that can be managed for leaf molds using a single weekly applied fungicide program as well as those with resistance or tolerance to Leaf Mold, Fulvia fulva or Passiflora fulva, was the focus for our 2015 program.

For 2015, the trial targeted determinant fresh market red slicer types grown under High Tunnel conditions. The trial and subsequent tastings were held at the Southeast Agricultural Research and Extension Center in Manhiem, PA. Tomatoes were specifically evaluated for their susceptibility to Leaf Mold.

About the pathogen

Leaf mold (also known as both Brown and Gray Leaf Mold) is caused by the fungal organism 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 in that 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 that 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

As of this time, we are still analyzing the full season of data from our high tunnel variety trial. However, some conclusions are clear:

  • 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.

The usefulness of grafts to manage this disease is far from settled. An article by Judson Reid, Cornell Extension, claimed that grafting provided no control. This could be explained by the strain(s) present in earlier work, differences in the rootstock, growing conditions or scion varieties.

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 infection starts, so deleafing can remove inoculum.
  • Prune tomatoes hard 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.
  • Use a fungicide strategy that changes rapidly and uses the few known materials that reduce Leaf Mold.
  • 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.