GENETIC DIVERSITY AND TEMPORAL DYNAMICS OF VENTURIA INAEQUALIS POPULATIONS FOLLOWING TWO APPLE SCAB EPIDEMICS IN PENNSYLVANIA

Open Access
Author:
Zhang, Lili
Graduate Program:
Plant Pathology
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 01, 2010
Committee Members:
  • Maria Del Mar Jimenez Gasco, Thesis Advisor
Keywords:
  • Venturia inaequalis
  • genetic diversity
  • temporal dynamics
  • microsatellite
Abstract:
Apple scab, caused by the ascomycete Venturia inaequalis, is one of the most important diseases in apple production worldwide, mainly due to increasing challenges in disease management. Populations of the pathogen continue to develop resistance to nearly every fungicide used, and there is a limited availability of resistant cultivars with desirable agronomic characteristics. V. inaequalis possesses a characteristic mixed reproduction system, which provides an ideal model to study temporal dynamics of population structure. Understanding temporal changes of population diversity and composition of local V. inaequalis populations may shed light on the evolutionary potential and key evolutionary forces shaping V. inaequalis populations, and it is also an important step in the improvement of apple scab management. In this study we intensively investigated population diversity and temporal dynamics of local Venturia inaequalis populations in a research orchard of the Fruit Research and Extension Center in Biglerville, PA. Sampling was conducted on two cultivars differing in their resistance to apple scab: ‘Golden Delicious’ (susceptible but carrying resistance gene Vg) and ‘Rome Beauty’ (highly susceptible without no known resistance genes), at the beginning (May) and near-end (September) of two epidemic years, 2008 and 2009. Seven hundred and sixty-five single-spore V. inaequalis isolates were obtained and genotyped using seven microsatellite markers. The results showed that the overall genetic diversity of the local V. inaequalis populations was very high with Shannon-Wiener’s Index values ranging from 3.67 to 4.02, suggesting V. inaequalis is a pathogen of high risk with high evolutionary potential considering the fact that V. inaequalis undergoes annual sexual recombination. Overall, we observed a significant reduction of genotypic diversity and a dramatic shift in genotypic composition in V. inaequalis populations from ‘Rome Beauty’, whereas population structure of V. inaequalis from ‘Golden Delicious’ maintained stable throughout the epidemics. The different pathogen dynamic trends observed from the two cultivars might be due to their differential susceptibility levels to apple scab, and more intensive competition among pathogen individuals on highly susceptible cultivars than on cultivars carrying some resistance genes.