RESUMO
Twenty phosphonate products found in the agrochemical market in Ecuador and Peru were evaluated in bioassays for the control of foliar potato late blight, caused by Phytophthora infestans. Eight phosphonate products were evaluated in 16 field experiments done in Peru, Ecuador, Kenya, and Nepal. A meta-analysis across locations involving 71 combinations of potato genotype by site and year demonstrated a significant relationship between phosphonate application rate and efficacy for controlling late blight on potato foliage. The meta-analysis revealed that phosphonate rates of approximately 2.5 g a.i./liter provided efficacy similar to that of the conventional contact fungicides mancozeb and chlorothalonil used at similar rates. At rates higher than 2.5 g a.i./liter, the efficacy of phosphonate was superior to the contact fungicides. Overall, late blight control by phosphonate appeared relatively stable in field experiments across locations. An analysis of field experiments and 64 combinations of potato genotype by site and year showed no correlation between the susceptibility level of potato genotypes and efficacy of phosphonates. The cost of both phosphonate compounds and contact fungicides varied greatly among the countries of the field study; however, in Kenya, control with phosphonate was clearly less expensive than with mancozeb.
RESUMO
Accumulated rainfall thresholds were studied in seven field experiments conducted in Ecuador and Peru for their value in timing applications of fungicide to control potato late blight, caused by Phytophthora infestans. Fungicide regimes based on accumulated rainfall thresholds ranging from 10 to 70 mm were compared with calendar spray schemes of every 5 or 7 days. Very low thresholds of 10 to 20 mm gave similar levels of control and resulted in similar number of sprays as did calendar spraying. However, neither low thresholds nor calendar sprays were effective in protecting susceptible potato genotypes in over half of the experiments. Thresholds of 25 to 50 mm of rainfall led to reduction in the number of sprays needed to protect resistant cultivars but also resulted in high levels of disease on susceptible cultivars. We conclude that timing fungicide sprays based on accumulated rainfall thresholds could be a successful component of integrated management strategies that include cultivars with moderate or high levels of resistance. The simplicity of measuring accumulated rainfall means that the technology can potentially be used by resource-poor farmers in developing countries.