Abstract

Drenches and directed sprays of two rates of azoxystrobin (Heritage 50W) were evaluated at 2- and 4-week intervals for the preventative control of Phytophthora shoot blight (Phytophthora parasitica) in a simulated landscape planting of annual vinca (Catharanthus roseus). ‘Tropicana Rose’ and ‘Pacifica Punch’ annual vinca were planted in May 1998 and April 1999, respectively, adjacent to beds known to be heavily infested with the causal fungus P. parasitica. Regardless of application rate and fungicide placement, the survival rate of plants was often higher when treated with azoxystrobin on a 2-week rather than on a 4-week schedule. Fungicide placement appeared to have little impact on the efficacy of azoxystrobin for the control of Phytophthora shoot blight. In 1998, survival rates of 85 and 90% for ‘Tropicana Rose’ were obtained with directed sprays of 0.35 (9.6 oz/100 gal) and 0.7 gm a.i./liter (19.2 oz/100 gal), respectively, of azoxystrobin applied on a 2-wk schedule as compared with a 10% survival rate for the unsprayed control. Overall, directed sprays of 0.7 gm a.i./liter (19.2 oz/100 gal) of azoxystrobin applied at 2-week intervals gave the best protection from Phytophthora shoot blight. Among the azoxystrobin drench treatments, 10.7 gm a.i./100 m² (0.7 oz/1000 ft²) applied at 2-week intervals had the highest survival rate in both years.