Effect of Phytophthora cinnamomi isolate, inoculum delivery method, and flood and drought conditions on vigor, disease severity scores, and survival of blueberry plants

Four studies evaluated the effects of Phytophthora cinnamomi isolate, inoculum delivery method, and flood and drought conditions on vigor, disease severity scores, and survival of blueberry plants grown in pots in the greenhouse. Phytophthora cinnamomi isolates were obtained from blueberry plants or the root zone of plants with symptoms of Phytophthora root rot. Rabbiteye 'Tifblue' and southern highbush 'Biloxi' and 'Star' blueberry plants were grown in a potting medium infested with one of five isolates of P. cinnamomi using four inoculum preparation and delivery methods. Inoculum was prepared as a mycelial slurry, zoospore suspension, or grown on rice grains or vermiculite. The effect of flood treatments on root rot symptom development was investigated by immersing the pots containing plants grown in P. cinnamomi infested potting medium for 24 or 48 h at 3 or 6-week intervals. The effect of drought was investigated by watering plants 4, 5 or 7 days each week. Plants were assessed for plant vigor and disease severity at regular intervals. Based on the results of the four studies the following protocol was developed to achieve rapid, consistent root rot symptom development. Blueberry potting medium should be infested with a virulent isolate of P. cinnamomi using the rice grain inoculum delivery method. Rooted cuttings of germplasm to be tested should then be transplanted into the infested medium, and the pots containing the plants should be flooded for 48 h every 6 weeks to allow rapid infection of the root systems. Disease evaluations based on above-ground symptoms should be made beginning six months after transplanting the cuttings into the infested potting medium. Rooted cuttings of a susceptible southern highbush cultivar should be used as the assay plant when evaluating chemical or biological control products or when studying environmental effects. We will use this protocol to study environmental effects on the development of Phytophthora root rot of blueberries and to evaluate blueberry germplasm for tolerance to root rot, and to determine the efficacy of chemical and biological products for root rot control.