Characterization and determination of Mode of Inheritance to late blight resistance in selected potato genotypes
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Potato is an important source of food and income of most farm family in East African highlands. Development of superior varieties with improved agronomic characteristics will enhance potato productivity and profitability. However, late blight (Phytophthora infestans) accounts for the greatest constraint to potato production and yield loss particularly in tropical highlands including those in Uganda. Consequently a study was conducted to i) characterize and identify potato cultivars to be used as parents to generate late blight resistant and high yielding progenies, ii) determine the mechanisms of inheritance to late blight resistance among new progenies and select promising progenies for further screening iii) evaluate yield stability among population B3 potato cultivars in Uganda. To achieve the first objective nineteen potato cultivars, including six local varieties and thirteen new clones were evaluated for late blight resistance, pollen production, pollen germination, tuber yield, and other desirable characteristics. Based on the ratio of relative area under disease progress curve (rrAUDPC), one of test cultivars, Uganda 11 and Victoria were the most resistant and most susceptible cultivars, respectively. The local cultivars Nakpot 5, Kachpot 1, Kachpot 2 and Sangema had low rrAUDPC ranging between 1.4 and 2.7. Among new cultivars, CIP 396026.103 and CIP 393280.82 were the most resistant with 0.9 and 1.0 respectively with late blight resistance compare to that of Cruza. The cultivars CIP 396244.12, CIP 396031.108, CIP 396038.107 and CIP 396004.255 had low rrAUDPC ranging between 2.0 and 2.7. Assessment of fresh tuber yield showed that the local cultivars had between 4 and 6 tubers per plant, while the new clones had between 10 and 16 tubers per plant. Mean total fresh tuber yields per unit area from local varieties ranged between 5.6 and 8.6 t ha-1, while new clones yielded between 28.0 t ha-1and 31.5 t ha-1. Assessment of these cultivars to be used as parents for pollen production and viability showed that the maximum pollen production and germination was obtained in flowers whose corolla had been fully open for three days (stage 2). During September-December 2009 season, pollen production ranged from 2.1 mg in Victoria to 7.7 mg in CIP 396004.255. During February-June 2010 season, pollen production ranged from 0.8 mg in Victoria to 11.6 mg in CIP 396026.103. Maximum pollen production during this season was obtained from CIP 396004.255, CIP 395111.13 and CIP 395011.2 which produced more than 6.0 mg. February-June cropping season, high yielding cultivars in pollen production were CIP 396026.103, CIP 396004.255, CIP 396031.108, CIP 396031.119 and CIP 396038.107 which produced more than 7.0 mg of pollen grains. uring September-December 2009 season, pollen germination ranged from 15.3% in Victoria to 34.3 % in CIP 396031.108. In February-June 2010 season, pollen germination ranged from 22.0% in CIP 395112.19 to 65.7% in CIP 396026.103. The cultivars with highest pollen germination were CIP 396031.108, CIP-395111.13 and CIP 396026.103 where more than 26.0% pollen grains germinated. During February-June 2010 season, the cultivars with high pollen germination were CIP 396026.103, CIP 396031.108 and CIP 393280.82 with more than 61.0%. In general, high pollen production and germination means were obtained during February-June 2010 cropping season. To determine the mechanisms of inheritance of resistance to late blight, two groups of F1 population progenies were evaluated. The first set contained progenies obtained from single crosses with Uganda 11. The second set contained progenies obtained in a half diallel cross using six parents selected from the population B3 potato cultivars. The F1 progenies were evaluated in the field for late blight severity under natural conditions in addition to artificial inoculation with a local late blight isolate. The segregation ratios observed in F1s suggests the occurrence of major (R) genes controlling late blight resistance. However, resistance to late blight severity displayed epistatic effects, partial dominance and additive effects in the crosses with Uganda 11. From the above observations, the existence of dominant R- genes action was evident. From this group, 190 F1 progenies were selected for further field screening. Results from F1 progenies generated from a half diallel mating involving six parents indicated that there was notable variation for late blight resistance among the progenies. The general combining ability (GCA) and specific combining ability (SCA) effects were significant (P≤ 0.05) for late blight resistance implying the importance of both additive and non-additive effects, respectively. The relationship between GCA and SCA effects (2σ2gca/(2σ2gca + σ2sca) =10% ) in F1 progenies suggests that non-additive effects were more important than additive effects. Broad sense coefficient of genetic determination was 92%, while the narrow-sense coefficient of genetic determination was 9% of gain to late blight resistance. These results suggest that backcrossing would be a better strategy for progress in breeding procedures for improving quantitative resistance to potato late blight. From this group, 57 F1 progenies were promising for LB resistance and were selected for further screening. The stability of fresh tuber yield for population B3 cultivars was assessed in four environments in two seasons using Additive Main effects and Multiplicative Interaction (AMMI) analysis. The analysis of variance revealed significant environmental effects on fresh tuber yield. The AMMI statistical model showed that the most stable clones for fresh tuber yield (t ha-1) were CIP 391046.14, CIP 39511.2, CIP 395111.13, CIP 396031.119, CIP 96038.107 and CIP 396031.108. However, other test cultivars had desirable attributes and could be tested in more locations at altitudes below the one used in this study to identify where they could be best adapted. From this study, future potato breeding endeavours should consider pollen from flowers that are not more than three days old after the corolla had opened. The F1 progenies generated from Uganda 11 as male or female parent had promising progenies with appreciable resistance to LB, high yield and desirable tuber characteristics, and should be further evaluated in diverse environments. The F1 progenies from the population B3 diallel cross should also be further screened for quantitative resistance to LB, high yield and other desirable tuber characteristics. The thirteen advanced clones tested at Kalengyere Research station (2400m a.s.l) should be further screened in diverse environments to identify those that could be best adapted to mid and low altitude areas.