A new scab-like disease on apple caused by the formerly saprotrophic fungus Venturia asperata

Atypical scab-like symptoms were reported for the first time in 2007 in the south of France on fruits of apple cultivars carrying the Rvi6 (=Vf) major resistance gene to Venturia inaequalis. With microscopic observations, nucleotide sequence data and pathological tests, it was shown that the causal agent was Venturia asperata. Scanning electron microscopy was used to compare its infection process and conidiogenesis to those of Venturia inaequalis on apple and Venturia pirina on pear. Venturia asperata produced fewer hyphae and fewer spores than the two other Venturia species, and resulted in weaker symptoms. This fungal species was previously described as a saprotroph on apple leaf litter. This is the first report of damage on apple fruits caused by V. asperata. Changes in host and cultural practices may have created a new context favourable for the emergence of this pathogen. It was also detected on symptomless leaves and on overwintered leaves on the ground. Pseudothecia developed on overwintered leaves and released ascospores over a 2-month period from the end of March until the end of May, suggesting that the fungus is able to survive from season to season. However, it is not yet known if this new disease will establish over coming years and become an emergent disease.