Downy mildew (DM) caused by Sclerospora graminicola is a dreaded disease of pearl millet in the semi-arid tropics. In India, the disease is quite severe on single-crops F1 hybrids and causes substantial yield losses. The on-farm DM surveys during 1994-2006 in the hybrid-intensive states of Maharashtra, Rajasthan, Gujarat, and Haryana revealed increased DM susceptibility of a hybrid when grown in the same field for more than three consecutive crop seasons, indicating the emergence of new virulences at different locations/environments over time. Greenhouse studies carried out at ICRISAT further confirmed the evolution of new virulent populations of DM fungus in India.
To manage the disease through host plant resistance it is imperative that resistance against new virulent populations is identified. Scientists took 129 pearl millet germplasm accessions from 16 countries that were identified as highly resistant (=10% incidence) in the downy mildew nursery at ICRISAT, India during 1990-93 and re-evaluated them against the new population of S. graminicola in the downy mildew nursery as well as in the greenhouse during 2006. Of the 129 accessions, 60 were found resistant in the field and 25 in the greenhouse screens. Twenty-one lines that showed resistance both in field and greenhouse screens were further evaluated in the greenhouse against 11 diverse virulent pathotypes of S. graminicola collected from different pearl millet growing areas in India and maintained at ICRISAT. Differential reactions to downy mildew resistance were observed among the accessions against the pathotypes. None of the 21 accessions showed resistance to all the 11 pathotypes. However, 3 accessions (IP 18295, P 1449-2 and YL-18) were resistant (=10% incidence) to 10 pathotypes. Two accessions, IP 18298 and IP8289 showed resistance to 9 pathotypes, whereas, IP 22396, YG-2 and YG-8 were resistant to 8 pathotypes. The germplasm lines with resistance against multiple pathotypes looked the most promising for use in the resistance breeding program.
Virulence changes over time in S. graminicola populations signifies the genetic potential of the pathogen to evolve virulent populations to match the resistance of the new host cultivars. ICRISAT is closely monitoring the virulence change in the pathogen populations and the resistance against new pathotypes available in the pearl millet germplasm.
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