Late blight resistance: comparing direct versus border row inoculations in field trials.

Mollov, Dimitre S., and James M. Bradeen, University of Minnesota, Department of Plant Pathology, 1991 Upper Buford Circle, St. Paul, MN 55108

Late blight (LB) (Phytophthora infestans) is among the costliest of potato diseases worldwide.  Resistance to LB is a common objective in potato breeding programs, requiring field screening.  Two methods may be used for germplasm resistance screening: “direct inoculation” (experimental genotypes inoculated directly) or “border row inoculation” (experimental genotypes interleaved with rows of a LB susceptible cultivar which receives the inoculum). While direct inoculation reduces land usage and border seed costs, border row inoculation might reduce microclimate variation and provide more even inoculum distribution.  Our research objective was to compare the efficacy of direct vs. border row inoculations for determining LB resistance.  In 2005, 248 genotypes from various breeding programs were planted as two replications of four hills each in each of two paired blocks (one with and one without borders) at Rosemount, MN.  At 65 days after planning, P. infestans US8 inoculum was introduced at 6000 sporangia per linear meter using CO₂ sprayers. In the borderless block, experimental genotypes were directly inoculated. In the border row block, only the border rows were inoculated. Frequent overhead irrigation maintained high humidity.  Symptoms were scored two times per week for three weeks beginning 14 days post inoculation using the CIP scale (1=0% to 9=100% infection).  AUDPC scores were calculated to evaluate disease progression.  Resistant, moderately resistant, moderately susceptible, and susceptible categories were defined for both methods.  Preliminary data reveal that, as expected, border row inoculation resulted in slower disease development in experimental genotypes compared to direct inoculation, but not significantly so.  AUDPCs were highly similar with the majority of genotypes classified in the same resistance categories under both methods.  The experiment will be repeated in 2006.  It appears that for breeding purposes, direct inoculation results in faster disease development in experimental genotypes and provides the same phenotypic results as the border row method.