VIRULENCE AND POPULATION STRUCTURE OF Phytophthora infestans (Mont) de Bary AND MANAGEMENT OF POTATO (Solanum tuberosum L.) LATE BLIGHT IN NORTHWESTERN ETHIOPIA

Abstract

Potato (Solanum tuberosum L.) is an important food security crop in Ethiopia. However, production of the crop constrained mainly by plant pathogenic oomycete, Phytophthora infestans. Effective management practices of the disease require identification of virulent races, distributions and population structure of Phytophthora infestans. Thus, this study was conducted to identify virulent races and distribution of Phytophthora infestans in northwestern Ethiopia, determine population structure and origin of lineages of the pathogen in Ethiopia, evaluate the potential of inorganic salts for the management of late blight of potato, determine the effect of integrating inorganic salts and host resistance to manage the disease, determine the rate and magnitude of genetic progress made in resistance breeding, assess the contributions of late blight resistance to tuber yield variation and the recycling potential of varieties for late blight disease management. The study of virulence spectrum was conducted on 74 isolates collected from Awi Zone, South Gondar and West Gojjam zones on Black’s potato differentials based on their reactions. The population structure study was conducted on 138 isolates collected from Awi, East Hararghe, South Gondar, West Arsi, West Gojjam, and West Shewa zones of Ethiopia using 12 plex-microsatellite markers. Eight inorganic salts were evaluated on the detached leaf assay of potato leaves in the laboratory. For integrating the study, three potato varieties (Belete, Gudene and Jalene) with different levels of resistance and four treatments (two inorganic salts viz. potassium phosphite and potassium phosphate, Ridomil fungicide and untreated or control) were arrangement in factorial combination in randomized complete block design (RCBD) with three replications at Adet and Debre Tabor in 2018 and 2019. For genetic progress, a total of 26 potato varieties were evaluated under natural infection at Adet and Debre Tabor in 2018 and 2019 in RCBD with three replications. The virulence study of the 74 isolates reaction to differentials revealed 6, 11 and 16 race types at Awi, South Gondar and West Gojjam Zones, respectively. Shannon diversity index was 0.80 and 27 physiological races were detected for the entire isolates, but the diversity index was 0.75, 0.84 and 0.92 for isolates collected from Awi, South Gondar and West Gojam Zones, respectively. The results of population structure study revealed the presence of one mitochondrial haplotypes (Ia) and 51 different multilocus genotypes (MLGs), including two clonal lineages (EU2_A1 and a new (ET-1) lineage. A European origin clonal lineage EU2_A1 was a dominant clonal lineage in all locations except West Arsi which was dominated with ET-1 lineage. The old US-1 lineage was completely displaced by the EU2_A1 and ET-1 lineages in Ethiopia. The treatment of inorganic salts showed highly significant (P<0.01) inhibitory effect on infection and lesion growth on the detached leaf assay. Treating leaves with Ridomil fungicide, Potassium phosphite and Potassium phosphate at the rate of 20 g l-1 reduced leaf lesion area by 99.4, 95.6 and 95.5%, respectively, as compared to untreated leaves. In the integrated study; disease severity, AUDPC, disease progress rate, marketable and total tuber yields were highly significantly (P<0.01) influenced by the interaction of inorganic salts spray and varieties. The highest AUDPC was calculated for Jalene followed by Gudene and Belete varieties at unsprayed plots over two seasons and locations, while it was lowest at fungicide sprayed plots. The AUDPC calculated for Jalene was reduced by xv 1278.2 Unit?% and 1787%-days in 2018 and 2019, respectively, at fungicide sprayed compared to unsprayed plots. The AUDPC reduced for Gudene and Belete by 988 and 601 %-days in 2018, respectively, at fungicide sprayed compared to unsprayed plots, while it reduced by 670.34 and 379%-days in 2019 for Gudene and Belete, respectively. But the reductions of AUDPC due to fungicide spray for Gudene and Belete over locations at both years had non-significant difference with the reductions of AUDPC due to spray of plot with potassium phosphate. The three varieties produced higher marketable and total tuber yields with lower relative yield loss at plots sprayed with fungicide compared to unsprayed plots over locations and years, and Jalene variety exhibited the highest marketable tuber yield losses of 43.21 and 59.74% in 2018 and 2019, respectively, at unsprayed plots compared to plots sprayed with fungicide. The annual rate of susceptibility reduction to late blight was 0.13 and 0.09 over locations in 2018 and 2019, respectively, while the annual rate of marketable tuber yield gain was 0.45 and 0.37 over locations in 2018 and 2019, respectively. Jarso farmers’ cultivar was identified as most susceptibile to late blight disease, while four improved varieties (Wechecha, Belete, Bubu and Gera) showed relatively high level of resistance and had higher tuber yields. The research findings showed the populations of Phytophthora infestans were complex with high virulence race diversity and the information could be used as a baseline for additional verdicts of the pathogen populations for surveillance programs to design proper disease management strategies and for sustainable production practices. It also showed the importance of integrating host resistance with inorganic salts application, particularly, the use of Potassium phosphite to better management of late blight disease in northwestern Ethiopia. The development of resistant varieties and genetic gain in the reduction the severity of late blight disease over years of potato breeding activities was encouraging, and the resistant potato varieties could be recommended for potato production during the rainy season as one management option against late blight disease in northwestern Ethiopia. Thus, the research findings allow making recommendation on the importance of continuous monitoring the pathogen populations, the development of resistant varieties to quantitative resistance, and the integration of host resistance with inorganic salts or fungicide to better management of the late blight disease.