Predicting durability of a disease resistance gene based on an assessment of the fitness loss and epidemiological consequences of avirulence gene mutation

Durability of plant disease resistance (R) genes may be predicted if the co st of pathogen adaptation to overcome resistance is understood. Adaptation of the bacterial blight pathogen, Xanthomonas oryzae pv. oryzae (Xoo), to v irulence in rice is the result of the loss of pathogen avirulence gene func tion, but little is known about its effect on aggressiveness under field co nditions. We evaluated the cost in pathogenic fitness (aggressiveness and p ersistence) associated with adaptation of Xoo to virulence on near-isogenic rice lines with single R genes (Xa7, Xa10, and Xa4) at two field sites end emic for bacterial blight. Disease severity was high in all 3 years on all lines except the fine with Xa7. Of two Xoo lineages (groups of strains infe rred to be clonally related based on DNA fingerprinting) detected, one, lin eage C, dominated the pathogen population at both sites. All Xoo strains we re virulent to Xa4, whereas only lineage C strains were virulent to Xa10. O nly a few strains of lineage C were virulent to Xa7. Adaptation to virulenc e on Xa7 occurred through at least four different pathways and was associat ed with a reduction in aggressiveness. Loss of avirulence and reduced aggre ssiveness were associated with mutations at the 3' terminus of the avrXa7 a llele. Strains most aggressive to Xa7 were not detected after the second ye ar, suggesting they were less persistent than less aggressive strains. Thes e experiments support the prediction that Xa7 would be a durable R gene bec ause of a fitness penalty in Xoo associated with adaptation to Xa7.