High Efficacy and Low Risk of Phytotoxicity of Sulfur in the Suppression of Strawberry Powdery Mildew

Abstract

Sulfur is a multisite inhibitor of strawberry powdery mildew (Podosphaera aphanis) recommended for fungicide resistance management, yet growers may refrain from its use due to purported phytotoxicity under conditions often qualitatively defined as “hot”, or “during periods of high temperature”. Our objectives were to (i) determine phytotoxicity of sulfur on three strawberry cultivars treated in controlled environments at temperatures of 30, 35, and 40°C for periods of 1, 2, 4, and 8 h after the sulfur application; and (ii) evaluate phytotoxicity and efficacy of sulfur on strawberry grown in open fields and high tunnels in Florida. Under controlled conditions, cultivars Florida127 and Florida Beauty exhibited minor phytotoxicity when exposed to sulfur at 30°C for 2 to 8 h. Cultivar Florida Radiance, however, was more sensitive, with greater phytotoxicity as exposure time increased. We found that weekly applications of sulfur on cultivar Florida127 significantly reduced P. aphanis foliar and fruit disease as well as or better than the standard fungicide treatment, and resulted in 80 to 300% higher yields (compared with nontreated controls) in open field and high tunnel trials, respectively. Ambient temperature did not exceed 35°C in open field trials, where no phytotoxicity was observed. In the high tunnel, where only mild leaf phytotoxicity was observed, temperatures exceeded 40°C for a total of 20 h across 9 days. Our results suggest that sulfur has benefits for disease suppression and fungicide resistance management that greatly surpass the minimal phytotoxicity observed only at temperatures above 35°C for a period of at least 2 h.