Identifying Sources and Mechanisms for Resistance to Colletotrichum gloeosporioides in Strawberry

Colletotrichum crown rot (CCR; caused by Colletotrichum gloeosporioides) can cause severe plant losses in strawberry production in Florida. Increasing resistance in genotypes developed through the strawberry breeding program at the University of Florida is important, as resistance to the disease among commercial cultivars is variable. In this study 29 genotypes from the Fragaria supercore collection were evaluated for resistance to CCR during the 2012-13 and 2013-14 seasons at the UF Gulf Coast Research and Education Center. Plots consisting of five plug plants were randomized across four replicate raised beds and allowed to establish for five weeks before inoculation. Each plant was inoculated by spraying 2 ml of a suspension containing 1×10⁵ spores/ml directly over the crown. Plants were rated as susceptible when 75% of the canopy had collapsed and were recorded weekly from November through April in both seasons. The area under the disease progress curve (AUDPC) for plant collapse was calculated for each plot and analyzed to distinguish the most resistant genotypes. The accessions of F. virginiana PI612320, PI612323, and PI612325; F. chiloensis PI551736; and UF selections FL 10-128 and FL 10-29 had AUDPC values of zero at the end of the first season. The most susceptible genotypes, which had plant mortality between 75% and 100%, had AUDPC values ranging from 79.9 to 109.9. Similar results were observed in the second season. In a separate evaluation, seven genotypes, including four commercial cultivars, were utilized to characterize resistance mechanisms. At least 15 plants of each genotype were inoculated by either spraying the spore suspension on top of the crown or injecting the spore suspension into the crown with a syringe. Plant collapse was recorded every other day and the AUDPC for each cultivar calculated six weeks after inoculation. A comparison between inoculation methods showed that plant collapse was higher when the inoculum was injected. However, ranking the genotypes according to AUDPC values showed that those considered resistant had the lowest values for each inoculation method, suggesting that genetic resistance is expressed directly in the crown tissues. Information on resistance of strawberry crowns to CCR in important germplasm is informative for breeding efforts against this disease.