Patterns of virulence variation in the interaction between Lactuca spp. and lettuce powdery mildew (Golovinomyces cichoracearum)

Golovinomyces cichoracearum sensu stricto (lettuce powdery mildew) is a common pathogen of the family Asteraceae (Compositae), mainly of species of the tribe Lactuceae. However, information on the natural distribution and virulence variation of G. cichoracearum on wild Lactuca spp. is rather limited. In the Czech Republic, samples of G. cichoracearum were collected in wild populations of prickly lettuce (Lactuca serriola) during 2005–2008. These isolates of G. cichoracearum were screened on Lactuca species (L. sativa, L. serriola, L. sativa × L. serriola, L. saligna and L. virosa) by using a leaf-disc assay. The main goals were to assess virulence variation in natural populations of this pathogen, determine host-pathogen reaction patterns and search for the race-specificity in interactions between Lactuca species and isolates of G. cichoracearum. On the basis of results from the inoculation studies, a Lactuca spp. differential set was developed for characterization of virulence variation of this pathogen. The experimental work was divided into four sequential steps. In 2005, mainly L. serriola accessions were used for the virulence study. In 2006, the set was enlarged with genotypes of L. sativa, and L. sativa × L. serriola hybrids. In 2007, genotypes of L. sativa and L. serriola previously showing clear differential responses to G. cichoracearum isolates were selected and the set was supplemented with some accessions of L. saligna and L. virosa. Finally, in 2008, the number of differentials was settled on 13 accessions of Lactuca spp. (six cultivars of L. sativa; one L. sativa × L. serriola hybrid; two accessions of L. serriola, L. saligna and L. virosa) that showed broad variation in host-pathogen reaction patterns. Overall, the inoculation studies confirmed the existence of many pathogen races, and demonstrated that the interaction between Lactuca spp. and G. cichoracearum is race-specific. In the future, more experimental studies on the pathogenic variability of G. cichoracearum s. str., as well as application of the genetic and molecular genetic approaches, is essential to understand this host-pathogen interaction in detail. These data may be important for applications in crop breeding and improvement.

► G. cichoracearum was collected in wild populations of prickly lettuce.
► Fungus isolates were screened on Lactuca spp.
►  Host-pathogen reaction patterns were determined.
► Differential set was developed for characterization of virulence variation.
► Study confirmed the existence of many pathogen races.