The aims of the present study were to assess the reaction of melon accessions to crater rot, caused by Myrothecium roridum, and study the inheritance of resistance. In the first experiment, 30 melon accessions from northeastern Brazil were assessed in a randomized block design with two replicates. Each plot was formed by a row of 12 plants, from which three fruits were sampled and used for inoculation. Each fruit was inoculated at four equidistant sites on the surface. Ten wounds were made at each site to an approximate depth of 3.0 mm, to which 0.05 mL of conidial suspension was placed at a concentration of 10 6 spores mL -1 of sterilized distilled water with the aid of a pipette. The fruits were placed on Petri dishes and enveloped in polyethylene bags containing hydrophilic cotton, both of which were moistened with sterilized distilled water. Five days after inoculation, the injured area (in mm 2 ) was assessed. Accessions A-9, A-14 and A-30 were resistant to the CMM-609 isolate of M. roridum. In the second experiment, the parents AF-682 and UFERSA-05 were used as standards of susceptibility and resistance, respectively. From these plants, generations F 1 , F 2 , RC 11 and RC 12 were obtained. The six populations were grown under field conditions in a randomized block design. Resistance to crater rot on the melon fruits exhibited complex inheritance, with six loci and the involvement of additive and dominance effects.
Approximately 15,000 ha of melon (Cucumis melo L.) are grown in the northeastern section of Brazil, mostly for export to Europe during the winter months. Surveys for melon vine decline diseases were carried out in farms in the municipalities of Mossoró (Rio Grande do Norte) and Quixeré (Ceará) during 2002 and 2003. Symptoms typical of vine decline were observed in several fields and included yellowing of crown leaves just prior to harvest and collapse of many of the vines. Affected plants exhibited necrotic root systems and lacked most of the secondary and tertiary feeder roots. Numerous perithecia were observed on roots which, when examined with a microscope, showed characteristic asci and ascospores of the fungus Monosporascus cannonballus Pollack & Uecker (2). Isolations were made from the crown region and primary and secondary roots of affected plants by excising 4- to 6-mm pieces that were surface sterilized for 30 to 60 s with 1.5% active chlorine solution. Seven tissue pieces from each plant part were placed on potato dextrose agar (PDA) containing 0.5 g liter-1 of streptomycin sulfate. Plates were examined daily for fungal growth for 7 days, and hyphal tips from all colonies were transferred to PDA for subsequent growth and sporulation. M. cannonballus was isolated from 50% of the root sections. All isolates produced only one ascospore per ascus. Pathogenicity of four isolates was confirmed in the greenhouse on the muskmelon cv. Temprano Rochet. Inoculum was produced in a sand-oat hulls (Avena sativa L.) medium (0.5 liter of sand, 46 g of ground oat hulls, and 37.5 ml of distilled water) and incubated at 25°C for 1 month. Colony forming units (CFU) were quantified by serial dilution using 1% hydroxyethyl cellulose. A sterilized mixture of equal portions (vol/vol) of sand and peat moss was used to fill plastic pots (17 cm in diameter), and inoculum was added to produce an inoculum concentration of 20 CFU g-1. Five melon seeds were planted in each pot and after germination, were thinned to one seedling per pot. There were five replicated pots for each treatment with an equal number of uninfested pots. Plants were evaluated for disease 45 days after sowing. Roots were exposed by carefully washing the potting mix away. All isolates of M. cannonballus tested were highly aggressive and caused severe root necrosis compared with the noninoculated control plants. M. cannonballus was reisolated from symptomatic plants, confirming Koch's postulates. Double cropping in the same fields for several years has created serious problems in Brazil, which are related to this soilborne pathogen that also causes root rot and vine decline of watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai) worldwide (1). To our knowledge this is the first report of M. cannonballus in Brazil and South America. References: (1) R. D. Martyn and M. E. Miller. Plant Dis. 80:716, 1996. (2) F. G. Pollack and F. A. Uecker. Mycologia 66:346, 1974.
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