Rice blast disease: Pathogen diversity, breeding for resistance and variation in an avirulence gene (AVR-Pita)

Rice blast, caused by Pyricularia grisea, is a constraint in the southern rice-producing states of the U.S. Molecular and genetic examination of over 1000 isolates recovered from rice indicated that the blast pathogen in Arkansas in 2000-2003 was predominated by isolates that belong to a single genetic group (group A, >95% of the isolates). Isolates in groups B and D were also recovered in 2000-2003 (<3% for each group); however, no group C isolates were recovered during this period. Virulence reactions in greenhouse pathogenicity tests on a differential set of cultivars could be used to distinguish isolates from groups A, B and D. In addition, seven virulence phenotypes were identified within a subset of 40 rice blast isolates that belonged to group A based on disease reaction on a modified differential set of cultivars. A group of isolates from crabgrass were non-pathogenic on rice. The AVR-Pita avirulence gene was also examined from pathogenic rice isolates from several different countries. PCR was used to amplify a 1086 by product from 59 U.S. rice isolates that were avirulent on cultivars containing Pi-ta; 25 U.S. rice isolates which lacked AVR-Pita were virulent on Pi-ta containing cultivars. AVR-Pita also was amplified from 32 rice isolates from Egypt, Colombia, China, Philippines and India that were avirulent on Pi-ta containing cultivars; AVR-Pita was not amplified from 2 isolates from Colombia which were virulent to Pi-ta containing cultivars. Examination of sequence diversity of AVR-Pita indicated that there were geographic subgroups. Segregation analysis of blast resistance on breeding material and parental lines was evaluated under greenhouse and field conditions. Statistical comparisons of blast reactions among the progeny of a given cross and its reciprocal cross indicate that there may be a cytoplasmic or maternal inheritance factor affecting the disease reactions. About 1200 entries in the rice breeder germplasm collection also were screened for resistance the rice blast pathogen.