Austin K. Hagan and J. Randy Adridge

Within the last decade, Phytophthora shoot blight and root rot has become a common and devastating disease of annual vinca (Catharanthus roseus) in both production greenhouses as well as the landscape. Once introduced into landscape beds on diseased annual vinca, the causal fungus Phytophthora parasitica is impossible to eradicate. Available data indicate that all commercial lines of annual vinca are susceptible to Phytophthora shoot blight and root rot. Previous trials have demonstrated that Aliette WDG alone or tank-mixed with Fore 80W provided effective control of this disease on pot- and bed-grown annual vinca. AAES trials were conduced in a simulated landscape planting to confirm the efficacy of registered and experimental fungicides for the control of Phytophthora shoot blight and root rot on annual vinca.

METHODS
On May 29, 1998, annual vinca ‘Peppermint Cooler’ were planted on established raised beds in a Benndale sandy loam heavily infested with P. parasitica. They were in a square on 1-foot centers at the Brewton Experiment Field. Just prior to planting, approximately 400 pounds per acre of 13-13-13 fertilizer was tilled into the beds. Throughout the growing season, calcium nitrate was applied at 2-week intervals through the drip irrigation system at a rate of 10 pounds per acre. With the exception of the monthly drenches of Subdue 2E, all treatments were applied to run-off as directed foliar sprays at 2-week intervals from June 15 through August 24, 1998 at the rates specified in the table. Plant survival was determined on June 23, July 17, and August 5, 1998.

RESULTS
Populations of P. parasitica in the beds were so high that some of the plants in some test plots succumbed to the root rot phase of this disease no more than a week after the first fungicide application. By mid-July, substantial stand losses were noted in all plots except those treated with the 5 pounds of Aliette WDG or a combination of 5 pounds of Aliette WDG and 2 pounds of Fore 80W per 100 gallons of spray volume. Among all treatments, the lowest survival rate was the 45% recorded in the untreated controls.
By early August, none of the fungicide treatments proved effective in controlling Phytophthora shoot and root rot on annual vinca in beds heavily infested with P. parasitica. Among all fungicide treatments, the highest survival rate of only 50% was noted in the Heritage 50W-treated plots. Otherwise, the level of plant survival for most of the remaining treatments did not substantially differ from the 0% survival rate recorded in the untreated control. No phytotoxicity was observed at anytime during this fungicide trial.

In summary, fungicides are not necessarily the answer to controlling Phytophthora shoot blight and root rot on annual vinca. In beds where the causal fungus has not been introduced, previous fungicide trials indicate that Aliette WDG applied at 2-week intervals at 5 pounds per 100 gallons of spray volume gave effective season-long protection from this disease as compared with a 25% survival rate for the same treatment where the causal fungus was already well established in the beds. The combination of favorable weather patterns and high inoculum pressure was overwhelming and all fungicide treatment regimes failed to protect the annual vinca from attack by P. parasitica. Right now, the best defense against Phytophthora shoot blight and root rot in a pathogen-infested bed is planting a disease-resistant summer annual.