Minimising fungicides by alternating formulations and intervals to improve potato blight management and farm returns

Abstract

Late blight (Phytophthora infestans) is one of the most devastating and economic disease impacting both ware and the seed potato industry. The disease causes huge crop losses, and its management attracts heavy expenses. Efficacy successes in chemical management, as opposed to biofungicides, has resulted in the development of a number of fungicide formulations to counter emergence of chemically insensitive P. infestans strains. The objective of this study was to evaluate the efficacy and cost-benefits of applying different fungicides in alternate; while varying the spray intervals to manage potato blight and improve on net farm returns. A survey was conducted in Nyandarua County in Kenya, using a structured questionnaire, administered to potato farmers. Milraz® (Propineb 700 g kg^-1 + Cymoxanil 60 g kg^-1), Ridomil® (Metalaxyl 4% + mancozeb 64%) and Mistress 72® (Cynamoxil 4% + Mancozeb 64%) were used in vitro and field experiments. Plated pea agar and detached leaflets were amended with the fungicides at concentrations of 0, 35, 70 and 100% of the manufacturers recommended rate before inoculating with P. infestans. In the field experiment, the fungicides were applied at intervals of 7, 14 and 21 days spray as single, two or three fungicides in alternations. Results showed that, mycelial and blight lesion growth was curtailed by concentrations of 70 and 100%; while 35% concentration of the fungicides reduced mycelial growth and lesion size by 53 and 2%, respectively. In the field experiment, there were no yield and AUDPC differences (P<0.05) among the fungicides and their combinations. However, spraying the fungicides at weekly and bi-weekly intervals gave the highest yields of 17.65 and 16.4 t ha^-1, compared to tri-weekly and unprotected plots that recorded 7.93 and 0.43 t ha^-1 , respectively. In addition, application of triple fungicides in alternation reduced late blight severity by 51%; while application of two fungicides in alternation reduced severity by 39% compared to single fungicide application (21%) on average. Maximum net benefit ratio was observed on plots protected using three fungicides (31.58); followed by two fungicides applications (26.81) applied biweekly in alternate relative to single fungicide applied weekly.