Imagine two strawberry growers with side-by-side booths at a local farmers market. One has plump, vibrant red strawberries stacked on top of a bushel basket. The other batch is clearly attacked by anthracnose, speckled with brown and black spots. Which strawberries do you think will sell? It takes hard work and good agricultural practices to control the many diseases, such as anthracnose, that can result in bruised, dull produce.
Cause
Frank Louws, a plant pathologist at North Carolina State University (NCSU), indicates that the primary cause of anthracnose is when the inoculum rides along with infected strawberry transplants.
“If a grower has an anthracnose fruit rot (AFR) problem, they need to make sure no green strawberry plants survive over the summer,” Louws said. “The AFR pathogen does not survive over the summer in the soil; and therefore, the problem does not reoccur if strawberries are on the same land unless infected plants are replanted.”
Warm and wet conditions can promote anthracnose, according to NCSU. Louws stated that hot, rainy weather or frequent irrigation can cause anthracnose to increase rapidly, since the disease is favored by wet foliage and spread by raindrops.
Symptoms
Anthracnose (Colletotrichum) can occur on the fruit and the crown. Louws indicated some species of Colletotrichum include C. acutatum, C. gloeosporioides and C. fragariae. However, he said C. acutatum is more associated with anthracnose fruit rot.
“This one can affect all parts of the plant, but the most damage is caused when the fungus affects ripening fruit,” he noted. “A field can go from zero to 80 percent fruit rot within 14 days under favorable weather conditions.”
Louws says growers can spot anthracnose fruit rot as brown to black, water-soaked spots on red fruit.
“Firm, sunken brown to black lesions can develop over time depending on prevalent relative humidity at the time of disease development. Pink salmon, or orange-colored masses of spores may form in the lesion under humid conditions where lesion may appear less sunken and brownish,” he said. “Under dry conditions, lesions appear more sunken and black, and the entire fruit may dry up to be mummified. Buds, pedicels, peduncles and flowers of most cultivars are susceptible to C. acutatum. Flowers may also die and dry out.”
Louws noted that if infection occurs shortly after pollination, the developing fruit remains small, hard and is misshaped. The pathogen can also cause black lesions on roots, which can lead to widespread infections on young tissue of the plug plant and plant loss. These symptoms can often be misconstrued as Alternaria fruit rot, Phomopsis fruit rot, Rhizoctonia dry rot or hail damage.
Producers will see anthracnose crown rot, caused by the C. gloeosporioides strain, on the crown and runner plants, according to NCSU. On runners, they will see elongated, black lesions with perhaps a gray center. The runner plant may turn black and die. The anthracnose fungus can enter the crown and create reddish-brown discoloration in its center. Once it enters the crown, it may wilt the plant and cause the plant to die, and it can carry over in the crowns annually if infected late during the summertime.
Prevention
To help control anthracnose, growers can follow a prevention program that includes cultural practices, varieties and fumigation.
Rotation
Crop rotation is important in any disease prevention plan, but Louws says crop rotation is not as helpful for anthracnose. He indicates that in North Carolina, this disease rarely reoccurs for a second year in the same field, unless growers purchase contaminated transplants or their plants from the previous year are already infected.
For anthracnose prevention, “the most important method is to buy disease-free plants,” Louws said. “The problem is that these fungi can be hidden enemies, growing on strawberry leaves or other plant parts without showing symptoms. Therefore, they can be moved from the strawberry nursery to fruiting fields undetected. Then when the right weather conditions occur, the disease can become explosive. There is a whole system of starting with tissue cultured plants, field inspections and management practices to prevent or reduce risk of the pathogen in the nursery system.”
Varieties
Choosing varieties that help control anthracnose is a good start toward disease prevention. Extension specialists recommend using only certified plants offered by certified growers. This helps assure plants are true to variety origin and possibly free from diseases and insects.
According to NCSU, some varieties used in perennial systems that have resistance to this disease include: Rosanne (resistant), Titan (resistant), Apollo (intermediate resistance) and Earlibelle (intermediate resistance). Rosanne is also resistant to leaf spot, leaf blight and powdery mildew. Titan is very resistant to leaf spot and leaf blight. Apollo is resistant to leaf spot, leaf blight and powdery mildew. Earlibelle is very resistant to leaf spot and leaf blight and resistant to powdery mildew. There are no varieties available for the more common annual production systems, Louws said.
Chemical management
Another way to manage anthracnose is to use fungicides. Extension specialists recommend fungicide sprays when necessary before and during harvest. Applications are beneficial during early and full bloom. Louws advised growers to manage their fungicides well so they can prevent any development of resistant populations.
Some effective fungicides for Colletotrichum species are Quadris and Cabrio (or Pristine), according to the Integrated Pest Management (IPM) center at NCSU. They can be combined or rotated with Captan. Follow label directions.