Department of Plant Pathology, Washington State University (WSU), Pullman 99164-6430
WSU-Mount Vernon Research & Extension Unit, Mount Vernon 98273-4768
University of Idaho, Aberdeen Research and Extension Center, Aberdeen 83210
Phosphorous acid for control of tuber rots caused by Phytophthora infestans, P. erythroseptica, and Pythium ultimum was applied to foliage of potato cultivars at various application timings and rates under growing conditions in the Pacific Northwest at Othello and Mount Vernon, WA, and Bonners Ferry and Aberdeen, ID in 2001 to 2003. Efficacy was assessed by artificially inoculating harvested tubers. Mean incidence and severity of late blight tuber rot in tubers inoculated with US-8 and US-11 isolates of Phytophthora infestans usually were significantly less when the foliage from which the tubers were obtained was treated with phosphorous acid than when it was not treated at all locations. With two applications of phosphorous acid, late blight tuber rot in the tuber-resistant cv. Umatilla Russet was significantly less than for Ranger Russet. For phosphorous acid at a rate of 9.37 kg a.i./ha, late blight tuber rot control achieved with two applications at 2-week intervals was not consistently improved across locations by making an additional application 2 weeks later. In 2003, incidence and severity of late blight tuber rot did not differ significantly between the rates of 7.49 and 9.37 kg a.i./ha at both Othello and Mount Vernon. Late blight tuber rot incidence and severity were significantly less at a rate of 7.49 kg a.i./ha when the application schedule began at initial tuber bulking rather than when the first application was made 4 weeks after initial tuber bulking at Othello, but not Mount Vernon. Incidence of pink rot was significantly less in inoculated tubers from plots treated with three applications of phosphorous acid than in tubers from nontreated control plots at Mount Vernon in 2002 and 2003, Bonners Ferry in 2002, and Aberdeen in 2003. Pink rot severity was reduced significantly by both two and three phosphorous acid applications at Mount Vernon in 2002. Pink rot incidence, but not severity, was reduced significantly at all timings when either 7.49 or 9.37 kg a.i./ha was applied at Mount Vernon in 2003. Control of Pythium spp. by phosphorous acid was not evident in this study. Total tuber yield at harvest did not differ significantly among the phosphorous acid treatments and the nontreated control at Othello and Mount Vernon in 2001 and 2002.