Submitted to: North American Strawberry Growers Association Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 21, 2007
Publication Date: December 15, 2007
Citation: Smith, B. J. Developmental Stage and Temperature Affect Strawberry Flower and Fruit Susceptibility to Anthracnose. Pages 55-57. In: Takeda, F., D. T. Handley, and E. B. Poling (ed.). Proc. 2007 North American Strawberry Symposium. North American Strawberry Growers Association, Kemptville, ON Canada. 2007. Interpretive Summary: Anthracnose diseases of strawberry including flower blight and fruit rot caused by the fungi Colletotrichum fragariae and C. acutatum are often severe in strawberry production fields. The fungi spread rapidly through a field during wet harvest seasons, sometimes causing a total crop loss. Symptoms caused by the two species are very similar; however, C. acutatum is more often associated with fruit rots while C. fragariae is more often associated with petiole and stolon lesions and with crown rot. Strawberry growers and researchers need to understand the anthracnose disease cycle better so that more effective disease management strategies may be developed. The purpose of this study was to determine when flower and fruit infections are likely to occur in the field. The effects of temperature, flower position, and flower and fruit developmental stage on anthracnose disease development were evaluated. The optimum temperature for anthracnose infection and the most susceptible developmental stages for infection were determined. The results demonstrate why anthracnose fruit rot is often devastating in production fields in the southeastern United States. Favorable infection temperature and rains are common during harvest. Open flowers, pink and red fruit are very susceptible to infection; therefore, one favorable infection period often results in the infection of a large proportion of the crop. Most cultivars currently grown on the annual hill system in the southeastern U.S. are susceptible to anthracnose fruit rot. Anthracnose resistant cultivars offer the best long term hope for reliable disease control. This information developed in this paper on conditions favoring anthracnose infection in the field will be used directly by strawberry growers to manage the disease in their field, and by extension and research scientists studying this disease and its control.
Technical Abstract: Colletotrichum acutatum and C. fragariae both cause anthracnose flower blight and fruit rot of strawberry. Anthracnose lesions on ripe fruit are circular and tan to light brown initially, later becoming dark brown and sunken with pink masses of conidia. Lesions on green fruit often originate on individual achenes and remain small and black until the fruit ripens. On infected flowers, lesions quickly engulf the entire flower and part of the pedicel which becomes dark brown, dry and brittle. The influence of temperature and developmental stage of flowers and fruit on their susceptibility to anthracnose was determined using two strawberry clones. Plants with one to three inflorescences were inoculated with a conidial suspension of isolates of C. acutatum or C. fragariae, incubated for 48 hours at 100% relative humidity at 10, 15, 20, or 25 oC, and then held in the greenhouse at 20 oC. The percentage of infected flowers and fruit was determined 7, 14, and 21 days after inoculation. Open flowers and pink or red fruit were very susceptible to infections (43 to 100% infected seven days after inoculation), while green fruit and closed buds were more resistant (only 5 to 8% infected seven days after inoculation). The optimum incubation temperature was 20 oC with an incidence of 31% infected flowers and fruit seven days after inoculation. When plants were incubated at 15 or 25 oC, approximately 25% of their flowers and fruit were infected seven days after inoculation. Only 5% of flowers and fruit were infected seven days after inoculation following incubation at 10 oC. More flowers and fruit were symptomatic seven days after inoculation when inoculated with C. fragariae (25%) than when inoculated with C. acutatum (19%); however, 14 and 21 days after inoculation there were no differences in disease due to fungal species. Seven days after inoculation fewer flowers and fruit had anthracnose symptoms on the anthracnose crown rot resistant ‘US70’ (16%) compared to the anthracnose susceptible ‘Tangi’ (26%).