Evaluation of Gambian and Malian sorghum germplasm against downy mildew pathogen, Peronosclerospora sorghi, in Mexico and the USA

The recent outbreak of sorghum downy mildew (SDM) in Texas, USA caused by pathotype P3 of Peronosclerospora sorghi, which is resistant to standard fungicide seed treatment, and the subsequent development of a new pathotype, P6, that overcame resistance in some hybrids, has emphasized the importance of continuing to develop new sources with genetic resistance. Eighty-two exotic Gambian and Malian germplasms and 10 sorghum lines commonly used as SDM pathotype differentials were field-evaluated in a randomized complete block design replicated three times at one Mexican location (Ocotlan, Jalisco) in 2004 and 2005, and two USA locations (Louise and New Taiton, TX, USA) in 2005 to identify new sources of SDM resistance. Accessions PI609151 and PI609442 from Mali had high levels of SDM resistance at all locations. Malian accession PI612815 also had a moderate to resistant reaction to SDM in two of the three locations. Accession PI522108 from Gambia was resistant in Mexico but susceptible in Louise, TX, USA. The reaction of the 10 lines used as differentials suggested the presence of a pathotype in Mexico that differed from those in the USA.