Evaluation of potato late blight management utilizing host plant resistance and reduced rates and frequencies of fungicide applications

Field experiments were carried out (2001-2003) to evaluate the efficacy of combining host resistance with reduced rates and frequencies of the residual contact fungicide fluazinam to control foliar potato late blight (Phytophthora infestans). Potato cultivars (cvs.) and advanced breeding lines (ABL) developed in four states were used in combination with fluazinam applications at 0, 33%, 50%, 66% and 100% of the manufacturer recommended application rate (MRAR) applied at 5, 7, 10 or 14 day application intervals. Values of relative area under disease progress curve (RAUDPC) in the untreated plots demonstrated that cvs. and ABL were significantly different in susceptibility to late blight. The cv. Jacqueline Lee was used as a resistant reference and the cvs. Snowden and FL1879 as susceptible. Accordingly, the ABL MSJ317-1 and MSJ461-1 were classified as resistant and the ABL W1386, W1201, MN19515, MN15620, MN19157, MN98650-8, ND2470-27, ND5822C-7 as susceptible. The cvs./ABL Dakota Pearl, Dakota Rose, MN19350 and W1355-1 were classified as susceptible in 2001 and as moderately susceptible in 2002. Application of the full rate of fluazinam at 5 or 7 day intervals resulted in effective control in all cvs./ABL except the cv. Dakota Pearl at 7 day application interval in 2001; at 10 or 14 day intervals it provided effective control in some cv./ABL and either partial or ineffective control in others. The 50% or 66% MRAR of fluazinam applied at 5 or 7 days provided effective blight control in resistant and moderately susceptible cvs./ABL and partial control in susceptible cvs./ABL. Fluazinam applied at 33% MRAR (in 2001 trials), at any application interval, provided effective blight control in the resistant cvs. and was either partially effective or ineffective in the susceptible cvs.. The study demonstrates that potato cvs./ABL with reduced susceptibility to late blight can be managed with reduced fungicide rates and longer application intervals, thus offering a less expensive option for potato late blight control.