Strategies for the Management of Strawberry Anthracnose

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Title: Strategies for the Management of Strawberry Anthracnose
Author: Schwegel, Rosemary
Advisors: Dr. Frank Louws, Committee Chair
Dr. Marc Cubeta, Committee Member
Dr. Fred Fuller, Committee Member
Dr. Jean Ristaino, Committee Member
Abstract: Strawberry anthracnose, which can be caused by Colletotrichum acutatum, C. dematium, C. fragariae, or C. gloeosporioides, is an economically important disease in North Carolina. As a response to the Fall 2003 anthracnose epidemic in NC resulting from the use of latently infected Canadian planting stock, three related studies were conducted to present management options for this disease, which is difficult to control chemically and is often introduced through planting stock. First, to secure a source of uninfected plants, three strawberry transplant sources (NC Registered, NC Certified, and noncertified Canadian) were evaluated in the field for the presence of anthracnose and horticultural characteristics. None of the plants showed symptoms of anthracnose. The Canadian plants showed significantly greater leaf dry weights, root dry weights, and leaf areas before planting, but this difference decreased over time. Crown dry weights of the Canadian plants remained greater on average than those of the NC plants over the course of the growing season. Root health ratings for the Canadian plants were significantly greater than those of the NC plants at the peak bloom and harvest samplings. The Canadian plants produced a significantly greater total berry number (69% greater) and weight (53% greater) but a slightly lower average berry size than the North Carolina-produced plants. This difference in yield may be due to the difference in transplant size, the difference in root health, or the possibility that NC-grown plants require more frequent overhead watering after planting for good establishment. In a second study designed to evaluate diagnosis and control measures for the disease, petioles were sampled at two sites in NC, each containing 30,000 plug plants, 105 petioles from the first site and 100 from the second. The level of infection was evaluated visually in the field and by a bioassay of the collected petioles in which the petioles were treated with herbicide to induce senescence of the plant tissue and encourage fungal sporulation. Spore suspensions of isolates collected from the sampled petioles were plated on media containing the strobilurin fungicide pyraclostrobin, and the percent germination was compared to the incidence of germination for isolates which had never been exposed to strobilurin fungicides. The collected isolates were not found to be resistant to pyraclostrobin, indicating that this fungicide may prove to be a viable management option for strawberry anthracnose in fruiting fields. In the third study, a real-time PCR protocol for detection of Colletotrichum spp. in strawberry petiole tissue was developed and compared to the previously described bioassay method. Petioles were sampled from strawberry plants grown in a controlled environment and artificially inoculated with C. acutatum, and from field-grown strawberry plants naturally infected with C. gloeosporioides; the sampled petioles were screened using both assays. The real-time PCR protocol was more rapid and showed significantly greater sensitivity than the bioassay when evaluating the artificially inoculated petioles. A lack of sensitivity in the real-time PCR assay was observed when evaluating the naturally infected petioles; this problem was likely due to poor tissue disruption of the tougher field-grown petioles during the DNA extraction protocol. Once issues of contamination and poor tissue disruption are addressed, real-time PCR may become an important tool for sensitive and specific detection of Colletotrichum spp. in strawberry tissue.
Date: 2006-04-27
Degree: MS
Discipline: Plant Pathology

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