Austin Hagan and J. R. Akridge

Heritage 50W, a product of Zeneca Inc. is the first representative of a new class of fungicides called strobilurins to be marketed in world. Activity against a broad spectrum of disease-causing fungi, including members of the water molds, is the most unique characteristic of this systemic fungicide. Due to a very favorable toxicology package, EPA has ‘fast-tracked’ the registration of the active ingredient (azosystrobin) in this fungicide on a variety of food crops as well as turfgrasses. Heritage 50W, which is currently registered only for use on turf, has quickly become the fungicide of choice of Alabama golf course superintendents for the prevention of Pythium blight and brown patch on bentgrass tees and greens. The effectiveness of Heritage 50W for controlling foliar and soilborne diseases on woody and herbaceous landscape crops has, however, not been extensively investigated.

Phytophthora shoot blight, a devastating disease in landscape plantings of annual vinca, has proven difficult to control with fungicides. When applied at 2-week intervals, Aliette WDG alone or in combination with Fore 80W has proven effective in some trials in Alabama and Florida but not in others in controlling this disease.

In an AAES study, the impact of fungicide placement, application rate, and treatment interval on the efficacy of Heritage 50W for the preventative control of Phytophthora shoot blight on annual vinca was assessed. Best disease control was provided by bimonthly directed applications of Heritage 50W at rates of 0.6 and 1.2 pounds per 100 gallons of spray volume.

METHODS
On May 29,1998, annual vinca ‘Tropical Rose’ were planted on raised beds in a square on 1-foot centers on the Brewton Experiment Field in Escambia County, Alabama. Just before planting, 13-13-13 fertilizer at a rate of approximately 400 pounds per treated acre was broadcast over the beds and incorporated. Throughout the growing season, calcium nitrate at the rate of 10 pounds per acre was applied at 2-week intervals via the drip irrigation system. Two rates of Heritage 50W were applied as a soil drench and directed spray to the plants at 2- and 4-week intervals. Aliette SDG, which was applied at 1.1 pounds per 100 gallons of spray volume on a 4-week schedule, was included as a standard treatment. Fungicide treatments were applied from June 1 at the intervals specified in the table until August 24, 1998. Plant survival was rated on June 23, July 17, and August 5, 1998.

RESULTS
In 1998, unusually hot and dry weather patterns in June and much of July suppressed the onset and spread of Phytophthora shoot blight on annual vinca. At the first rating date, plant survival in nearly all plots was at or just below 100% (see table). When more seasonal rainfall and temperature patterns resumed in late July, shoot dieback symptoms typically associated with this disease became much more noticeable and stand thinning became noticeable. By early August, considerable differences in plant survival were noted between the fungicide treatments.

Selected directed sprays and soil drenches of Heritage 50W greatly improved the survival of ‘Tropical Rose’ annual vinca as compared with the non-treated control and Aliette WDG standard (see table). Overall, the directed sprays generally were more effective in controlling Phytophthora shoot blight than were the drench treatments. Equally effective control of Phytophthora shoot blight was obtained with directed sprays of the 0.6 and 1.2 pound per 100-gallon rates of Heritage 50W applied at 2-week intervals. On a 4-week treatment schedule, directed sprays of the 1.2 pound rate of Heritage 50W gave much better disease control as compared with the lower 0.6 pound per acre rate.

The highest level of plant survival with drenches of Heritage 50W was obtained with both rates when applied on a 2-week spray schedule. Disease control at the 4-week treatment interval as indicated by survival rates of 30 and 45% for drenches of the low and high rates, respectively, was very poor.

Overall, Heritage 50W demonstrated good activity against Phytophthora shoot blight on annual vinca. Directed sprays of Heritage 50W at both rates when applied at 2-week intervals provided the most effective preventative control of Phytophthora shoot blight. Drench treatments applied on the same 2-week schedule proved somewhat less effective than directed sprays. When applied as a directed spray or soil drench at monthly intervals, Heritage 50W generally failed to control Phytophthora shoot blight on annual vinca.