Importance and Pathogenic Variation of Wheat Tan Spot in Ethiopia

Abstract

Tan spot or yellow leaf spot is one o f the major diseases of wheat worldwide; however, its significance has not been well explored so far in Ethiopia. This study was initiated to examine its distribution, importance and pathogenic variability in major wheat producing
regions of Ethiopia. A survey was conducted mainly in Arsi and some wheat fields at Debre- Zeit and Holetta in 1998. Fifty-seven leaf samples were collected and analyzed in the laboratory at Kulumsa. Out o f these, 70% were infected by the tan spot fungus. The disease severity varied from low to high levels according to locations. Based on the greenhouse preliminary pathogenicity tests, five isolates were selected for further characterization in comparison with four isolates of different origin. Nine tan spot isolates were used to inoculate 24 bread wheat cultivars under controlled environments. There were significant differences among isolates, cultivars and isolate x cultivar interactions. The Ethiopian isolate (ET57/98) was the most aggressive followed by the Indian isolate (Ind-119). Applying a hierarchical agglomerative cluster analysis and centroid method, the isolates were grouped into two large clusters while the cultivars formed more than five clusters. Despite the presence of significant isolate cultivar interactions, their differences were not distinct enough to categorize them into physiologic races. Twenty-one commercial bread wheat cultivars
from Ethiopia were inoculated with a mixture of three aggressive isolates at two-leaf stage under controlled conditions and only three cultivars had shown adequate level of resistance. Therefore, the wheat improvement program in Ethiopia should focus on developing
genotypes with resistance to tan spot.