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

Abstract

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.

Population Genome Sequencing of the Scab Fungal SpeciesVenturia inaequalis,Venturia pirina,Venturia aucupariaeandVenturia asperata

“…Detailed analysis of the V. inaequalis and V. pirina predicted secretome identified specific SSPs putatively involved in host specificity and recognition by apple resistant genes (Deng et al 2017). Similarly, in our report, differences in gene complement might explain the host range of the different species, especially the host specialization of V. inaequalis formae specialis and the low virulence of V. asperata on apple (Caffier et al 2012). In the PacBio assembly, genes tend to be located within GC-equilibrated regions (Figure 2B and 2C).…”

“…Certain populations have experienced a secondary contact with subsequent gene flow, resulting in the introduction from a wild population into the agricultural compartment of strains that are virulent on resistant cultivars carrying the Rvi6 resistance gene (Guérin et al 2007; Lemaire et al 2016). On apple, scab can also be caused by another Venturia sp., V. asperata initially described as a saprophyte (Samuels and Sivanesan, 1975), and recently reported as a pathogen on apple trees (Caffier et al 2012). Other Venturia species cause scab damages on their respective Rosaceae hosts, V. pirina (Aderh.)…”

Population genome sequencing of the scab fungal speciesVenturia inaequalis,Venturia pirina,Venturia aucupariaeandVenturia asperata

“…Detailed analysis of the V. inaequalis and V. pirina predicted secretome identified putative race-cultivar and host-species specific SSPs (Deng et al, 2017). Similarly, in our report, differences in gene complement might explain the host range of the different species, especially the host specialization of V. inaequalis formae specialis and the low virulence of V. asperata on apple (Caffier et al, 2012). In the PacBio assembly, genes tend to be located within GC-equilibrated regions ( Figure 2B and 2C).…”

Evaluation of Venturia pyrina virulence on European pear (Pyrus communis) cultivars by an in vitro methodology

“…Pathogenicity of new apple scab inducing pathogen V . asperata in France was demonstrated using detached apple fruits (Caffier et al., 2012). In our study, the main challenge of the method was the duration of the incubation period needed for the development of lesions with abundant sporulation of the pathogen on leaves in case of compatible interaction.…”