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.

An experiment was conducted to determine the host resistance of potato against Phytophthora infestans for twenty-five potato genotypes in 2010 and 2011 at Khumaltar, Lalitpur, Nepal using four assays: three for foliage resistance (field, whole-plant and detached leaf) and one for tuber resistance (tuber slice). An isolate of P. infestans collected from Lalitpur (LPR-1) was used for inoculation at a concentration of 3 x 10 3 sporangia ml -1 in all assays. Infected foliage area in the field and whole-plant assays, lesion size on detached leaves, and colony growth on tuber slice were all individually converted to 0-9 interval scale for susceptibility. Field assessment, considering the most robust measure of resistance, was used as benchmark for comparing the other assays. Sixteen of the genotypes had very little disease in the field (scale value <1) indicating they were probably expressing race-specific resistance, which has historically been short lived. Susceptibility levels measured in the whole-pant assay were highly correlated (r = 0.90) with converted field scale values, although the correlation was lower for the detached leaf assay (r = 0.63) and least for tuber-slice assay (r = 0.46). Low correlation in the detached leaf assay was assumed to represent lower resolution of the single-cycle assay. Low correlation in the tuber assay may have also reflected genetic differences as foliage and tuber blight resistance are not always correlated. Genotypes with extreme resistance in the field were frequently identified as having partial resistance in the other assays, which could mistakenly be interpreted as more durable field resistance. The consequences for selecting durable resistance are discussed.

A farmer's participatory field experiment was accomplished during three consecutive years  (2003-2005) for the sustainable management of black scurf disease of potato at Mainapokhar, Bardiya, Nepal. A heavily infested field with Rhizoctonia solani was selected prior to experimentation. Experiment was laid out in randomized complete block design with three replications. The efficacy of two concentrations (2% and 3%) of boric acid and 0.15% of Thiophanate Methyl were tested for disease control through seed treatment. An antagonistic fungus Trichoderma harzianum was used as seed and soil treatment. As per treatment plan some plots were sterilized with formaldehyde 1% prior to planting. The plots having healthy seed (pre-basic seed) planted in sterilized plots showed minimum disease(6.8%) followed by 3% boric acid treated seed planted in sterilized plots (10.8%) and 2%  boric acid without soil sterilization (13.1%) and seed and soil treated plot with T. harzianum (17.1%). Farmers did not accept treatments connected with soil sterilization by Formalin. After analyzing the overall results of three years experiment farmers choose seed treatment with 2% boric acid prior to planting and identified application of Trichoderma harzianum for black scurf management as second option.Nepal Agric. Res. J. Vol. 8, 2007, pp. 56-62DOI: http://dx.doi.org/10.3126/narj.v8i0.11581

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