Melon represents the most widespread cucurbit in Italy. In recent years melon has been subjected to significant losses in yield and quality due to an increasing number of soil‐borne fungal diseases. The collapse of melon, caused by a complex of fungal pathogens, including Monosporascus cannonballus, Acremonium cucurbitacearum, Plectosporium tabacinum and Rhizopycnis vagum, represents one of most destructive diseases worldwide. The purpose of this study was to determine the occurrence of collapse throughout melon‐producing areas in Italy in recent years, to verify the identification of isolates collected, and to test their pathogenicity on melon and other cucurbits. Several fungi were isolated from symptomatic roots of melons in the Italian production areas. The identification was supported by PCR with a species‐specific primer and DNA sequence data. RFLP and sequence analyses showed the existence of a substantial homogeneity among Italian M. cannonballus isolates. Given the self‐incompatibility of these isolates it is impossible to ascertain vegetative compatibility groups (VGC) and consequently genetic relatedness cannot be studied. The frequency of isolation of fungal species varied with geographic locations, M. cannonballus being present mainly in Central Italy, while A. cucurbitacearum and P. tabacinum were most common in Apulia. In pathogenicity tests under greenhouse conditions M. cannonballus, A. cucurbitacearum and P. tabacinum caused collapse symptoms and root rots, whereas R. vagum was found to be a weak pathogen.

This study was undertaken to isolate indigenous plant growth-promoting (PGP) bacteria from solarized soil effective in the biocontrol of Monosporascus cannonballus, the cause of root rot and vine decline of melon, which is one of the most destructive soilborne diseases of this crop worldwide. The screening strategy resulted in the selection of two interesting PGP bacteria as biocontrol candidates against M. cannonballus belonging to the same microbial community. The two bacterial species, identified according to phenotypic, physiological tests and analysis of the 16S rDNA sequence as Bacillus subtilis/amyloliquefaciens (BsCR) and Pseudomonas putida (PpF4), showed PGP traits and in vitro antagonistic activity towards M. cannonballus. Antagonism by BsCR was characterized by a consistent inhibition of the pathogen in vitro growth; PpF4 strongly inhibited the development of perithecia of the pathogen. Under greenhouse conditions, the selected bacteria were tested for their biocontrol activity in the pathosystem melon-M. cannonballus. BsCR alone and in combination with PpF4 determined a consistent decrease in the disease symptoms. BsCR and the combination of the bacterial strains significantly increased root biomass in both inoculated and un-inoculated plant. Upon seed treatment with BsCR, the accumulation and isoenzyme induction of peroxidase in roots as biochemical marker for induction of resistance were found, thus indicating that BsCR may reduce the disease severity also by the activation of the plant defence responses. The study highlights the synergistic biocontrol potential of B. subtilis BsCR and P. putida PpF4 in the integrated management of root rot and vine decline of melon caused by M. cannonballus.

The use of inoculum of arbuscular mycorrhizal fungi (AMF) in nursery represents a promising field in horticulture because of its known benefits in terms of plant growth and bioprotection. The present work was undertaken to determine the effect of mycorrhizal inoculation with Rhizophagus irregularis in a nursery medium on the containment of melon root rot and vine decline (MRRVD) caused by the soil-borne pathogen Monosporascus cannonballus. The percentage of mycorrhization, biomass and yield following mycorrhizal inoculation were also evaluated. Biocontrol activity was assessed in greenhouse pot experiments upon artificial inoculation of M. cannonballus and in a two-season field experiment under production conditions in an unheated greenhouse with a history of MRRVD. On the basis of the mycorrhization parameters, the interaction appeared to be established within 30 days after inoculation. The total shoot growth in the mycorrhized plants was significantly higher when compared to the control, while the root growth was unaffected. Upon artificial inoculation of M. cannonballus, mycorrhization provided complete protection against the pathogen. Greenhouse experiments under production conditions during spring cropping season showed that pretransplanting inoculation with R. irregularis significantly decreased the severity of the disease. Also, the average fruit weight of mycorrhized plants was significantly higher than the untreated control. Nevertheless, in summer crop, the bioprotection activity of AMF failed. Present results indicate that the use of AMF in a nursery setting can contribute to the prevention of the onset of this problematic soil-borne disease within a sustainable and integrated soil-borne disease management.

A survey for the presence of Olpidium spp. on melon (Cucumis melo L.) was conducted during the beginning of 2013 in central Italy in an unheated greenhouse, located in the melon-producing coastal area of north Latium (central Italy, Viterbo Province) (42°23′09.31″N, 11°30′46.10″E) with a history of monosporascus root rot and vine decline (MRRVD). For this aim, 10 soil samples were collected adjacent to the roots of plants symptomatic of MRRVD, represented by root lesions and rots and loss of smaller feeder roots. Olpidium was baited from collected infested soil by growing melon (cv. Dinero) plants for 45 days. Bait plants grown in sterilized soil were used as negative controls. All the baited melon roots were analyzed by morphological and molecular methods. For the morphological analysis, feeder roots were clarified in a 1.5% KOH solution for 24 h (2) and observed under a light microscope to record the presence or absence of sporangia and resting spores of Olpidium spp., which were observed in baited melon plants grown in infested soil and not in control roots. In particular, stellate resting spores were referred to as O. virulentus because this species cannot be distinguished from O. brassicae, which does not colonize melon. O. bornovanus had smooth-walled resting spores with a honeycomb-like pattern (2). For molecular analysis, DNA was extracted from 21 melon roots and tested by multiplex PCR to confirm Olpidium spp. identification (2). Based on molecular identification, O. virulentus was identified in 40% of samples, and O. bornovanus was identified in 10%. There were no mixed infections in the same sample. Two amplified PCR products, corresponding to O. bornovanus and O. virulentus expected fragment sizes of 977 and 579 bp respectively, were sequenced (GenBank Accession Nos. KF661295 and KF661296). BLAST analysis of the sequences showed 99% nucleotide identity with O. bornovanus isolate CH from Japan collected in melon roots (AB205215) and O. virulentus isolate HY-1 from Japan collected in lettuce roots as reported by Sasaya and Koganezawa (3) (AB205204, formerly O. brassicae). At the end of the experiment, the root systems of all inoculated plants appeared brown, whereas neither symptoms nor sporangia and resting spores were observed in roots of control plants. Olpidium spp. are root-infecting plant pathogens of melon (4), acting as vectors of Melon necrotic spot virus (MNSV) and other destructive plant viruses (1). Moreover, they are directly involved in the induction of germination of ascospores of Monosporascus cannonballus, the causal agent of MRRVD of cucurbits (4). To our knowledge, this is the first report of O. virulentus and O. bornovanus on melon in Italy. References: (1) A. Alfaro-Fernández et al. J. Phytopathol. 91:1250, 2009. (2) J. A. Herrera-Vásquez et al. Mycol. Res. 113:602, 2009. (3) T. Sasaya and H. Koganezawa. J. Gen. Plant Pathol. 72:20, 2006. (4) M. E. Stanghellini and I. J. Misaghi. Phytopathology 101:794, 2011.