Evaluation of advanced R-gene free potato genotypes for late blight resistance, yield, cooking and processing qualities
Late blight (Phytophthora infestans) is a major constraint to potato production in the tropical highlands of Kenya causing significant yield losses of 30 - 75% in highland tropics of Kenya. Farmers rely on fungicide application to manage the disease. However, control is often inadequate due to limited fungicide applications and use of cultivars with low to moderate resistance to late blight. Use of resistant cultivars is viewed to be more sustainable both economically and environmentally. However, resistance alone does not guarantee adoption by farmers of any cultivar as farmers have other preferences like earliness, good storability and/or good cooking characteristics. Moreover, resistance breaks down owing to changes in pathogen population necessitating the need for evaluation of new germplasm to replace old varieties whose resistance has broken down. The objectives of the study were to assess early and late harvest performance; stability of R free late blight resistant genotypes; and to determine the effect of harvesting date on the storage, cooking and processing qualities of R free late blight resistant potato tubers in Kenya. Ten advanced late blight resistant potato genotypes free of R genes from population B3 and two checks Tigoni, moderately resistant to late blight and Kerr's Pink, susceptible to late blight were used. Field experiments were conducted at two sites; Tigoni, Limuru and Marimba, Meru. Field and storage experiments were laid out in a randomised complete block design (RCBD) with three replications while the cooking and processing quality experiments was laid out in a completely randomized design replicated eight times. Field experiments were harvested at 90 (early) and 120 (late harvests) days after emergence (DAE). Significant differences (P ~ 0.05) were observed for tuber yields, AUDPC, foliage maturity, specific gravity, tuber weight loss (%), ~routing (%) and tuber rots (%) among the potato genotypes at early and late harvests. The AUDPC ranged from 35 to 3803 and was significantly higher for Kerr's Pink than all population B3 genotypes and Tigoni. Population B3 genotypes performed better at late than early harvests but the increase in tuber yields due to delayed harvest varied with genotype and was greater at Tigoni, Limuru than at Marimba, Meru. Significant negative correlations between AUDPC and tuber yield were observed on the local checks but no correlations were observed on population B3 genotypes. Rankings of genotypes with respect to reactions to disease severity and tuber yield for early and late harvests varied across seasons and locations. AMMI analysis showed that the proportion of genotypic variance was larger than that due to the environmental variance and the G X E interaction. Genotypes (G), environments (E) and the G X E interactions accounted for 43.0% and 53.4%, 39.6% and 29.7%, 17.5% and 16.9% for tuber yield while for AUDPC it accounted for 80.2% and 82.3%,5.0% and 4.6%,14.8% and 13.1% of the treatment sum of squares at early and late harvests respectively. Five genotypes at early harvests and all population B3 genotypes except two genotypes at late harvests were stable while for late blight resistance, four and six genotypes at early harvests and late harvests were stable respectively. Except for genotype 393280.57, population B3 genotypes had acceptable specific gravity (above 1.07), high acceptability scores (scores of over 5) for use as boiled potatoes, chips and crisps of good quality and acceptable low weight losses (below 10%) at early and late harvests. Population B3 genotypes commenced sprouting by the 4th week except four genotypes that sprouted by 6th week at early harvest while sprouting was reduced to the 2nd week and 4th week at late harvest respectively. Most of population B3 genotypes from early and late harvests can be kept for 10-12 and 6-8 weeks except four genotypes that can store well for over 12 and 10-12 keks respectively. Kerr's Pink and Tigoni sprouted by the second week of storage with Kerr's Pink having its % sprouting levelling off regardless of the harvesting date. Levels of resistance in population B3 potato genotypes varied from moderate resistance to high resistance and population B3 entries 385524.9, 389746.2, 392617.54, 393371.58, 393385.39 and 393385.47 were better performers and could be considered for on farm trials. Most of the population B3 genotypes were suitable and acceptable for storage, cooking and processing qualities.