Enticingly fragrant strawberries, with their vibrant red color, attractive form, and succulent sweetness plus flavor-balancing tartness, delight the senses. And, strawberries pack plenty of nutritive goodness.
Strawberries yield the highest value per acre of any crop that can be grown in this country, according to the U.S. Department of Agriculture.
Yet several diseases, if not controlled, can destroy an entire crop. Of these, anthracnose fruit rot now threatens strawberry production throughout the United States. Three related species of the pathogen Colletotrichum cause fruit rot. They affect the entire plant – fruit, buds, blossoms, petioles, runners, crowns and foliage. Cornell University reported that C. fragariae is associated with crown rot and black leaf spot, C. gloeosporioides also causes black leaf spot, and C. dematium is linked to fruit rot. While each of the above Collectotrichum pathogens cause fruit rot, C. acutatum is the pathogen most often associated with this destructive disease.
Anthracnose fruit rot symptoms appear as water-soaked brown to black spots on both green and ripe fruit. The stems can also exhibit lesions that appear as dark brown or black sunken spots. Abundant conidia are produced on petioles, runners and fruit and dispersed through splashed or wind-driven rain. The entire plant typically wilts and dies when the crown tissue becomes infected and decayed. Warm, humid conditions favor disease development. Masses of pink, orange or salmon-colored spores then form on the lesions. In dry weather, the lesions become more black and the fruit dries up and mummifies.
The pathogen can infect the strawberry plant at any stage. If tips become infected, widespread infection of the young tissue of the plant plugs can result in significant loss. If the infection occurs shortly after flower pollination, the developing fruit becomes misshapen and remains small and hard. Even the roots can be infected if the pathogen washes down into the root zone; black lesions on the roots then result.
Because early control can be important, an accurate diagnosis of the symptoms can prevent confusion with other diseases, which can be alternaria or phomopsis fruit rot, rhizoctonia dry rot or even hail damage.
Integrated pest management guidelines at the University of California pointed out that the anthracnose pathogen can survive in soil for at least nine months without a host plant. Also, chickweed, fiddleneck and vetch are known hosts. Strawberry plugs planted in infested soil can become infected as soil-containing spores splash crowns or stems by rain or irrigation water. Contaminated soil on field equipment can spread the pathogen.
Root cause and how to treat
However, the primary source of anthracnose inoculum enters the fields through strawberry transplants.
Professor of pathology Frank Louws, who directs the NSF-Center for Integrated Pest Management at North Carolina State University, advised growers, “The most important factor is clean plants. The problem is that the ability to find the pathogen in a nursery setting can be difficult.” He also looks for hot spots in plug production. “If small, you can cull there and stop the problem.” He added, “Overhead watering should be discontinued. And, do not work in the fields when wet.”
In addition to Louws’ vital cultural management points, the North Carolina Extension Service suggested periodic scouting especially during warm and wet weather to detect anthracnose early. Removing and destroying infected and surrounding plants is advocated. Keep nitrogen at the required level. Excessive nitrogen favors fungal development. Also, use nitrate sources rather than ammonium forms of nitrogen, which is readily accessible to the pathogen. Use drip irrigation to avoid water dispersal. Keep equipment clean and restrict worker movement from an infested area to a clean area. Remove and destroy host weeds by burning or burying. Dead weeds can still produce spores, and herbicide-killing may even initiate spore production.
After harvest, destroy the plants if anthracnose fruit rot was present that year. Rotate production out of strawberries for two or three years to help rid any fields from inoculum debris in the soil.
Early fungicide applications limit pathogen buildup. Once symptoms occur, control becomes difficult. Since conditions differ regionally, growers should contact their local or regional extension educators for the latest recommendations. Educators can also advise which cultivars perform well or are anthracnose-resistant or tolerant. In addition, they can direct growers to nurseries with a plant certification program to reduce the risk of contaminated plugs.
Any fungicide program must be carefully managed to avoid fungicide resistance from developing. Alternating different modes of action, and minimizing use of specific materials must be followed. Local extension educators can supply current data, plus recommend production methods and cultural practices to control anthracnose fruit rot.
The National Institute of Food and Agriculture, USDA, has funded a study led by Dr. Mark Gleason at Iowa State University. This three-year project with field trials in Iowa and Florida will, among other tools, develop warning-system strategies to minimize fungicide spraying. C. acutatum is becoming resistant to strobilurin fungicides in some areas. Plus, the fungus can colonize plants without displaying symptoms. Thus, infected but healthy-looking nursery plants have been shipped nationwide, spreading the disease. Field days, web-based outreach and grower meetings will be forthcoming in this study.
Geneticist Dr. Kim Lewers, Agricultural Research Services, USDA, has been transferring anthracnose fruit rot disease resistance to future strawberry cultivars. She also has been evaluating low tunnels to extend the season for growers to provide more of this
PHOTO COURTESY OF FRANK LOUWS, NORTH CAROLINA STATE UNIVERSITY.