INVESTIGATION OF ERYSIPHACEOUS FUNGI, PODOSPHAERA PRUNICOLA AND ERYSIPHE NECATOR, AS HOSTS OF FUNGAL VIRUSES AND EFFECT OF INOCULUM CONCENTRATION AND FRUIT DEVELOPMENT STAGE ON FRUIT INFECTION OF SWEET CHERRY BY PODOSPHAERA PRUNICOLA
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This study aimed to explore mycoviruses associated with the fungus Podosphaera prunicola and the fungus Erysiphe necator, causal agents of powdery mildew of sweet cherry and grapevines respectively. This study also aimed to understand the effect of inoculum concentration and fruit growth stage on the powdery mildew incidence and severity of cherry fruit. To explore mycovirus associated with P. prunicola and E. necator, double-stranded RNAs extracted from the fungal tissues were analyzed by next-generation sequencing and bioinformatics. The results revealed the presence of eight distinct mycovirus-like sequences in P. prunicola. Five of these sequences constitute the genome of three putative members of the genus Alphapartitivirus and two additional sequences constitute the genome of a putative member of the genus Betapartitivirus in the family Partitiviridae. These novel mycoviruses were provisionally named as Podosphaera prunicola partitivirus 1, 2, 3 and 4. The remaining eighth sequence, which shared similarity to members of the family Virgaviridae, was identified as a genomic component of a ‘new’ virus species and provisionally named as Podosphaera prunicola tobamo-like virus. Similarly, the results showed the presence of eight mycovirus-like sequences in E. necator. Five of these sequences represent three new mycoviruses related to the genus Alphapartitivirus of the family Partitiviridae and remaining three sequences represent three new mycoviruses related to the genus Mitovirus of the family Narnaviridae. These new mycoviruses were tentatively named as Erysiphe necator partitivirus 1, 2, and 3 and Erysiphe necator mitovirus 1, 2 and 3 reflecting their E. necator origin and their respective phylogenetic affiliation. To understand the effect of inoculum concentration and fruit growth stage on the powdery mildew incidence and severity on sweet cherry fruit, fruits of cv. ‘Sweetheart’ were inoculated with different conidia concentrations in three different fruit development stages. The results showed that conidial concentration significantly influenced the disease development and fruits became increasingly susceptible to powdery mildew infection with maturity. Additionally, real-time quantitative PCR assay of P. prunicola conidia collected throughout the fruiting season showed variation in the airborne conidial concentrations.