With the help of high tunnels, many growers can snatch an early start on their vegetable crops. However, these unheated structures have a growing environment and disease concerns similar to those in a heated greenhouse. Growers may be plagued by crop diseases, which normally aren’t encountered in field production.

Often, tomatoes are chosen for high tunnel production, as growers aim to be first on the market with this high-demand, high-value crop. Although many diseases of concern in high tunnel production – including molds that impact tomatoes such as white and gray molds – also affect other crops, one mold focuses on tomatoes exclusively.

Leaf mold of tomatoes is a fungal disease that, in many regions of the United States, isn’t normally found in the field. Growing under cover, however, provides the high humidity in which the fungus thrives. The disease occurs in the field in areas with warmer temperatures and high humidity. It can rapidly spread under wet and humid conditions.

“I have observed leaf mold in the field, but it is much more common in the greenhouse or high tunnel due to the higher humidity in the latter,” said Dan Egel, Extension plant pathologist, Southwest Purdue Agricultural Center. “Leaf mold is a problem in high tunnels because the humidity is higher in high tunnels and greenhouses.”

Caused by the fungus Passalora fulva, leaf mold damage can reduce yield. Although the fungus primarily attacks leaves of the plant, the damage indirectly impacts fruit production. Leaves show a yellow spot on the top side, and a green or grayish fuzzy mold appears on the leaf underside. When severe, the mold can be found on the top of leaves, too. As the disease spreads, spots join together and turn brown. Withered, dead leaves may stay on the plant. Yields will be reduced due to compromised photosynthesis.

“The disease causes chlorotic, round lesions,” Egel said. “Left unchecked, leaf mold can cause defoliation and loss of fruit quality, including smaller fruit.”

If infections are extreme, blossoms and fruit may also show symptoms, although infrequent, and yield will be directly impacted. Blossoms turn black, and infected fruit will form a lesion on the end. It is rare that the disease progresses beyond the leaves, but can do so if plants are infected at an early age, or with a high severity of disease pressure and conducive environmental conditions.

Disease process and control

The fungus responsible for leaf mold can remain for many years in soils and apparently can survive for many months on structures such as stakes, benches and other equipment found in environments where the fungus is present.

Spores, as well as sclerotia, will then germinate when environmental conditions are favorable. Sclerotia are “multicellular structures having a more or less regular morphology and serving as reproductive bodies. They are often hard, solid and able to survive relatively long periods of time in a dormant state,” according to a definition provided on a mycology website, written by David Malloch, research associate of the New Brunswick Museum (search Google for “mycology David Malloch” for more information).

The fungus that causes leaf mold can often be observed on the bottom of tomato leaves

The disease pathogen is spread via air, water, tools, contact and insects. Temperatures in the lower to mid-70 degree Fahrenheit range, accompanied by relative humidity of 75 percent or more, provide optimal conditions for the fungi to germinate.

Adequate air flow within the high tunnel, as well as between plants, helps reduce risk. Keeping relative humidity low, avoiding dew formation by keeping the inside night temperatures higher than those outside, avoiding irrigation via overhead sprinklers, and staking and pruning tomatoes to keep vegetation upright and less dense, are all recommended practices.

“Avoid putting plants too close together. It will cause the relative humidity to go up and fruit size will go down,” Egel cautioned. “I recommend about 20 inches between plants and 5 feet between rows.”

Removing the fungus from the field, high tunnel or greenhouse means disinfecting surfaces, tools and equipment, removing all crop debris, and rotating crops. Disinfecting the greenhouse or high tunnel – the structure itself – is advised at the end of each growing season.

“I recommend that growers use floor covering so that crop debris from tomato leaves that may have leaf mold doesn’t get into the soil,” Egel said. “Remove all plants from the greenhouse after the season is over – unless one will practice a three- to four-year rotation from tomatoes, which most growers do not.”

Fungicides can be used when conditions are ripe for leaf mold pathogens to germinate, in advance of disease symptoms. As with all fungicides, alternating between families with different modes of action, and applying only as directed at the advised intervals is prudent.

“There are fungicides that may reduce the severity of leaf mold. But check the label to see if the product is allowed in the greenhouse (states differ on regulations). Also check for pre-harvest intervals, reentry intervals and Agricultural Worker Protection Standard (WPS) requirements,” Egel said.

Variety selection

Passalora fulva can survive in plant seed, which can often be an initial source of introduction. There are resistant varieties of tomatoes, but there are different P. fulva strains, or races, and there is concern that the fungus can overcome resistance readily. Some cultivars have resistance to more races than do others.

“There are varieties of tomato with resistance to leaf mold. Growers should consult their seed reps, Extension resources and use their own experience before making a decision on varieties,” Egel said. “Growers should realize that any resistance can change.”

Leaf mold disease resistance is not the only factor to consider when selecting tomato cultivars. Along with days to maturity, type of tomato, taste, growth habits and resistance to other diseases, customer preference is a concern.

“Growers may prefer varieties that lack resistance,” Egel said. “The grower knows which tomato varieties their customers like. If those varieties happen to be the ones that are susceptible to leaf mold, the grower feels compelled to grow the variety over-resistant varieties.”

Cultivar resistance to leaf mold disease, cultivation practices that reduce the likelihood of disease germination and spread, disinfection of all surfaces, use of floor coverings to prevent spores and sclerotia from entering soils, crop rotation practices and appropriate use of fungicides all work together to prevent leaf mold infections. As with any plant disease, the right combination of factors can cause disease spread, and best management practices can prevent it.