Aggressiveness of Mycosphaerella graminicola Isolates from Susceptible and Partially Resistant Wheat Cultivars

Abstract

The selective effect of quantitative host resistance on pathogen aggressiveness is poorly understood. Because two previous experiments with a small number of bread wheat cultivars and isolates of Mycosphaerella graminicola had indicated that more susceptible hosts selected for more aggressive isolates, we conducted a larger experiment to test that hypothesis. In each of 2 years, six cultivars differing in their levels of partial resistance were planted in field plots, and isolates were collected from each cultivar early and late in the growing season. The isolates were inoculated as populations bulked by cultivar of origin, field replicate, and collection date on seedlings of the same six cultivars in the greenhouse. The selective impact of a cultivar on aggressiveness was measured as the difference in aggressiveness between early and late isolates from that cultivar. Regression of those differences on disease severity in the field yielded significance values of 0.0531 and 0.0037 for the 2 years, with moderately resistant cultivars selecting for more aggressive isolates. In a related experiment, the protectant fungicide chlorothalonil was applied to plots of two susceptible cultivars to retard epidemic development. When tested in the greenhouse, isolates of M. graminicola from those plots were significantly more aggressive than isolates from the same cultivars unprotected by fungicide.

Seasonal Changes Drive Short-Term Selection for Fitness Traits in the Wheat Pathogen Zymoseptoria tritici

“…"Mark-release-recapture" experiments based on artificial inoculations were performed with fungal pathogens of wheat, including Z. tritici (15,35,40,41; see also the reviews by Zhan and McDonald [16,17]). Under natural conditions, given the high diversity of Z. tritici populations (20), the earliest cohort of lesions is caused by different genotypes, and the probability of recapturing one of them on the upper leaves at the end of the epidemic is almost zero.…”

“…Fungal development is temperature dependent (13,14), so that seasonal fluctuations, which are suspected to constitute shortterm selection pressure for fitness traits, might modify the dynamic of annual epidemics even within a year. Moreover, several infection cycles can amplify short-term selection pressure due to competition between pathogen strains (15)(16)(17).…”

Genetic Structure and Aggressiveness of Erysiphe necator Populations during Grapevine Powdery Mildew Epidemics

“…Villareal and Lannou (38) and Andrivon et al (2) showed that directional selection for increased aggressiveness takes place in Erysiphe graminis and Phytophthora infestans populations, respectively, during the epidemic stage. Some fungal plant pathogens alternate between highly aggressive life stages and life stages in which highly aggressive strains are selected against, whereas the aggressiveness of other fungal plant pathogens, such as Mycosphaerella graminicola (9) and E. necator, remains stable.…”

Analyzing the Influence of Qualitative Resistance Selection Pressure on Variation of Aggressiveness in Plasmopara Halstedii

“…Similarly, Gandon and Michalakis (2000) predicted that increased levels of quantitative host resistance may select for increased aggressiveness of parasites, leading to increased crop losses. Cowger and Mundt (2002) showed that wheat cultivars with good quantitative resistance selected more aggressive isolates of Mycosphaerella graminicola. However, this is not always true, Sullivan et al (2005) reported that tobacco cultivars with high levels of quantitative resistance did not select for more aggressive isolates of Phytophthora parasitica var.…”