Botrytis fruit rot, commonly called gray mold, damages an exceptionally wide range of hosts, including berries, ornamentals and vegetables. Although numerous troublesome pathogens exist, Botrytis cinerea often causes the most serious losses in strawberries. The fungus affects the flower petals, stalks, and the fruit and fruit caps.

Strawberries are a major fruit crop, and Botrytis fruit rot threatens this delectable, nutritious fruit.

How disease presents on fruit

Strawberry blossoms are highly susceptible to the pathogen. Moreover, this fungus typically targets the blossoms for the primary infection, which can remain dormant until the berries begin ripening. Then the gray mold develops. Beginning as soft light brown lesions, it rapidly enlarges on the fruit.

The infected berries become inoculum sources, causing secondary infections on adjacent berries. The disease can remain undetected until harvest. But as the green fruit matures, it becomes more susceptible, with ripe and overripe berries highly desirable to the pathogen. If berries remain on the plant, the effects of the fungi produce dried-up, mummified berries covered with a dusty, powdery mold.

This strawberry has succumbed to the ravages inflicted by the pathogen Botrytis cinerea.PHOTO BY SCOTT BAUER, USDA/ARS.

The gray mold fungus overwinters as small, black, inactive sclerotia or as mycelium in plant debris such as dead leaves. In early spring, the mycelium produces millions of microscopic spores. Wind spreads the spores, which then germinate on a moisture film; in only a few hours, infection occurs.

Temperatures between 70 and 80 degrees Fahrenheit, and moisture from dew, fog, irrigation or rain favor disease development.

Consider cultural controls to minimize disease

Cultural controls not only improve strawberry production, but can also minimize diseases, including gray mold. Growing in plastic tunnels has been observed to reduce Botrytis cinerea infections. Plastic mulch production, by preventing soil contact with berries and leaves, can also reduce diseases. Straw mulch also acts as a barrier.

Good soil drainage, plant spacing that facilitates air circulation and direct sunlight all help control fruit rot. Over-fertilization, especially with nitrogen, enhances overly dense foliage that interferes with both air circulation and drying. Rows in the direction of the prevailing wind promote faster drying in wet weather. Weeds should be controlled to increase field drying time and to reduce infection chances. Frequent harvesting – but not during wet periods – decreases disease development.

Importantly, culling and destroying diseased plants and fruit, and removing any fruit with signs of decay or rain damage help reduce the availability of inoculum capable of producing new infections. Because sanitation is so critical for good disease control, promptness is essential.

Since climatic conditions vary considerably among different regions and even locales within regions, growers should always consult with their local extension educators for production and pest control advice. Strawberry production, especially, is impacted by the weather in addition to the climate.

Follow recommendations on fungicides

Fungicides are typically recommended for commercial strawberry plantings. Used in conjunction with integrated pest management programs including cultural controls, fungicides must also be timed properly. State and regional production guides, such as the Mid-Atlantic region developed by Penn State, Rutgers, Virginia Polytechnic Institute and State University, and the Universities of Delaware, Maryland and West Virginia, include a spray schedule, which includes fungicide rotations with different FRAC codes. The Clemson University smartphone app “MyIPM” also provides useful nonchemical and chemical control information.

Fungicide recommendations indicate that control in the bloom period is especially important because at that time the fungus colonizes the flower parts and establishes the fungus within the young fruit. It then usually remains latent until the fruit starts to mature.

Plant pathology farm advisor Steven Koike in both Monterey and Santa Cruz Counties, California, said the Botrytis cinerea inoculum builds throughout their vast strawberry production and requires chemical battling, depending on the weather. “Along with integrated pest management, growers rotate the fungicides. There currently is no clear evidence of fungicides failing to control gray mold due to resistant strains. These experienced growers look for any sign of fungicide failure, and manage resistance with proper fungicide rotation.”

Professor and plant pathologist Guido Schnabel, Clemson University, said that Botrytis cinerea diversifies quickly and is often the first pathogen to develop resistance to new chemical classes of fungicides. Schnabel reported, “Resistance to commonly used fungicides is now common in conventional strawberry fields along the eastern United States. Growers battle resistance by rotating chemical classes and, by doing so, risk applying ineffective fungicides at key times. This increases the risk of disease outbreaks, leads to unnecessary selection of resistance, and often forces tighter spray intervals to compensate for fungicide failure.”

Dr. Schnabel added, “Alternatively, growers combine fungicides with multi-site activity with one or more single-site fungicides to spread the risk in hopes to achieve disease control. This may lead to excessive fungicide use.”

Services helps ID fungicide resistance issues

Dr. Schnabel’s lab at Clemson provides a service that identifies fungicide resistance problems in strawberry fields in the East. Using fungicide sensitivity assays, the lab has been testing Botrytis isolates for resistance to 11 commonly used chemicals over the last five years. This service is available for Southern state growers; for samples from other regions there is a charge.

With testing from close to 100 farms per year, the resistance to many of the fungicides used for gray mold ranges from 0 to 100 percent. “Only about 20 percent of our samples are clean,” Schnabel said. “Depending on plant source, other nearby small fruit crops, and spray history there is a good chance that some fungicides in any particular location will not do the trick for growers due to existing resistance. When those fungicides are applied at key times, there is no protection.”

Senior extension associate Kathleen Demchak, Penn State University, in cautioning growers to manage at-risk products, advised rotating with broad-spectrum fungicides while avoiding bees, and using cultural controls to minimize need for spraying. “Every spray avoided is avoidance of an opportunity for resistance development,” she said.

Captan and thiram should be at the core of disease management, and at-risk fungicides with greater efficacy should be added prior to major rain events at temperatures around 70 degrees Fahrenheit. To determine what at-risk fungicides are best to use, the Schnabel lab instructs growers to obtain 20 to 40 dead strawberry flowers early in the season from various locations of each strawberry field. If dead flowers are unavailable, send 150 healthy flowers, but dead flowers are preferable. Include samples of dead leaves.

If late in the season, collect 10 individual strawberries (again from the entire field) with fresh gray mold lesions. Do not collect from old mummies or discarded fruit on the ground. Using a fresh cotton swab for each berry, carefully rub one side of the swab on the youngest diseased portion of each berry without touching the fruit itself. The cotton will turn light gray. A minimum of 10 cotton swabs from 10 fruits should be submitted per location.

Mail the flowers or the 10 swabs together with your farm name, state, your name, phone number and email address to: Guido Schnabel, Clemson University, 105 Collings St/220 BRC, Clemson, SC 29634. For further information, email the lab at schnabe@clemson.edu.

COVER PHOTO COURTESY OF CLEMSON UNIVERSITY.