Anthracnose of strawberry—Colletotrichum acutatum
Anthracnose is a fungal disease that can kill strawberry plants and cause fruit to decay and become inedible.
Identification
The most obvious symptoms of anthracnose are dark lesions on strawberry fruit, petioles, and runners. Infected plants may remain undersized and develop yellow foliage. Anthracnose lesions generally are dark brown to blackish, lens-shaped, and sunken. Infected blossoms and fruit darken and as disease progresses fruit can become dry, hard, and brown overall.
Wilting, collapse, and death of plants can occur, but in California this is relatively uncommon on strawberries grown as annuals. Wilt and death of the plant occurs after crown tissue becomes infected and develops decay. This damage resembles plant death caused by Phytophthora crown rot of strawberries. However, with anthracnose the diseased, discolored crown tissue is generally cinnamon colored to red. When Phytophthora is the cause of crown rot the diseased tissue is more chocolate brown colored. In addition, the petiole and runner lesions caused by anthracnose are not produced by Phytophthora spp. Anthracnose lesions can develop orangish sporulation whereas the spores of Phytophthora are not visible to the naked eye.
Life cycle
Colletotrichum acutatum can survive in soil for at least 9 months without host plants. In addition to strawberry, several weeds are known to host this pathogen including chickweed, fiddleneck, and vetch. If strawberries are planted in infested soil, they become infected when soil containing spores is splashed onto crowns, fruit, or stems by rain or irrigation water. The disease also commonly originates from infected nursery stock. In addition, anthracnose inoculum can come from contaminated equipment, soil, or nearby weeds.
Fruit decay caused by anthracnose may be common depending on environmental conditions. If infected plants are present, decay can develop after periods of warm, rainy weather or if plants are irrigated overhead. Fruit at any stage of ripeness can be affected. Small, sunken, oval-to-round brown spots (on green fruit) or black spots (on red fruit) develop and may expand to cover most or all of the fruit surface. Under high humidity or plant wetness, salmon- or orange-colored spores can form on the lesions of the fruit, petioles, and runners. Well-decayed fruit tissue becomes dry and firm.
Damage
Anthracnose can cause fruit to decay and become inedible. Sometimes anthracnose kills entire strawberry plants. The disease is more of a problem in coastal growing areas, where plants are irrigated overhead, and when they are grown as perennials or planted in the same soil where strawberries previously grew.
Solutions
Where anthracnose has been a problem, control weeds near growing areas. In locations with hot summers, solarizing soil during late spring through summer can control weed propagules and inoculum of anthracnose and some other soilborne pathogens. After forming the planting beds, solarize beneath clear plastic for 30 to 45 days during hot weather. Increase effectiveness of solarization by first incorporating into soil the residue of a cruciferous crop, in particular broccoli or mustards.
An alternative to solarization is to each year relocate where strawberries are grown. This generally avoids planting in soil infested with inoculum of anthracnose and certain other strawberry pathogens such as leaf spots.
Obtain pathogen-free nursery stock from a reputable commercial supplier. Before planting, use running water to thoroughly wash all soil from plants. This can reduce disease development in crowns and fruit. Growing strawberries as annuals also reduces the development of anthracnose and leaf spot diseases of strawberries.
Use drip or furrow irrigation instead of overhead sprinkling. Water plants early in the day so soil surfaces and any wetted plant parts dry more quickly. No fungicides available to home gardeners are likely to be effective at preventing or controlling anthracnose of strawberries.
Adapted from Improvement of Soil Solarization with Volatile Compounds Generated from Organic Amendments, Integrated Pest Management for Strawberries, Soil Solarization: A Nonpesticidal Method for Controlling Diseases (PDF), Soil Solarization for Gardens & Landscapes, and Pest Management Guidelines: Strawberries, University of California Statewide Integrated Pest Management Program (UC IPM). |
Anthracnose decay lesions on strawberry fruit.
Circular, sunken, anthracnose lesion close-up on a strawberry fruit.
Two, dark, anthracnose lesions on a strawberry runner.
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