Occurrence of chasmothecia and mating type distribution of Podosphaera xanthii, a causal agent of cucurbit powdery mildew in Northern Italy

Powdery mildew is a widespread disease that causes important losses to cucurbit production. The main agents of the disease are Podosphaera xanthii and Golovinomyces orontii. To determine the occurrence of chasmothecia as overwintering forms of both fungal species in northern Italy, powdery mildew-infected samples from cultivated cucurbits were collected in different locations of the provinces of Bologna and Mantova during 2010, 2011 and 2012. Only the sexual stage of P. xanthii was found, indicating that in northern Italy, contrary to what reported from other areas of the Mediterranean basin, the pathogen overwinters as chasmothecia. In parallel, to determine the frequency and distribution of both MAT 1-1-1 and MAT 1-2-1 P. xanthii idiomorphs, a multiplex-PCR with MAT idiomorph-specific primers was carried out on 147 monoconidial isolates obtained from infected leaf samples. The obtained frequencies were tested for random mating. Results showed a MAT ratio that tended to 1:1, supporting the finding of the sexual stage thus suggesting the occurrence of actively mating populations and that sexual reproduction plays a significant role in the life cycle of P. xanthii in this area. The lack of G. orontii chasmothecia suggests that this species might have alternative overwintering strategies. ascospore viability, multiplex-PCR, powdery mildew, sexual recombination.