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First Report of Monosporascus cannonballus on Watermelon in Northern Mexico

In 2010 and 2011, diseased watermelon plants (Citrillus lanatus [Thunb.] Matsun and Nakai) had chlorotic and wilted leaves and vines prior to harvest in three out of four sampled commercial fields in the Municipality of Matamoros, State of Coahuila, in the north-central region of Mexico known as La Comarca Lagunera. Disease incidence across the fields was 30%. Diseased plants also showed necrotic lesions and loss of secondary and tertiary roots, which can render roots unable to obtain an adequate supply of water and nutrients supporting the aboveground part of the plant before fruit maturity. Roots of affected plants contained perithecia with asci and ascospores typical of Monosporascus cannonballus Pollack & Uecker (4). This fungus has been found in hot semi-arid climates with saline and alkaline soils. Daytime temperatures above 40°C are frequent in north-central Mexico during the watermelon growing season. Small root pieces from 30 plants with disease symptoms (10 plants per field) were taken and surface-sterilized with 1.5% sodium hypochlorite, placed on potato dextrose agar (PDA) medium with 0.5 g/L of streptomycin sulfate at two petri dishes per plant and five root pieces per petri dish, and incubated for 7 days at 25°C in the dark. The fungus was isolated with a frequency of 60%. Mycelia were identified from root tissue based on morphological characteristics. DNA was also extracted in CTAB buffer followed by a phenol/chloroform purification and precipitation in isopropanol and ethanol (2). The internal transcribed spacer region was then amplified from isolate 1 using PCR, sequenced, and submitted to GenBank (Accession Number JQ599552). Pathogenicity of isolates was confirmed on watermelon plants (cv. Sweet summer 800) under greenhouse conditions at 25 to 32°C. Inoculum was produced in a sand-oat hull (Avena sativa) medium (0.5 l of sand, 45 g of oat hulls, and 100 ml of distilled water) and incubated for 50 days (1). Watermelon seeds were sown in sterile sand in 20-cm diameter and 12-cm deep polyurethane containers, where inoculum was added to reach a soil concentration of 20 CFU/g. Four seeds were sown in each of five inoculated containers; plants were thinned to two per container after emergence (each container representing a replication). Similarly, plants were also grown in four noninoculated containers and used as controls. After 50 days, all watermelon plants inoculated with M. cannonballus showed root necrosis in contrast with roots from noninoculated plants. M. cannonballus was reisolated from 80% of inoculated plants, confirming Koch's postulates. M. cannonballus causes severe damage on watermelon and other cucurbits such as cantaloupe (Cucumis melo). This fungus has been reported in the United States, Spain, Tunisia, Libya, Israel, Italy, the Netherlands (plants from Russia), Saudi Arabia, India, Japan, Taiwan, Brazil, Guatemala, and Honduras. To date, M. cannonballus has been reported on watermelon in 1996 in the State of Colima in southeastern Mexico (3). However, to the best of our knowledge, this is the first report of M. cannonballus on watermelon in northern Mexico.

References: (1) B. D. Bruton et al. Plant Dis. 84:907, 2000. (2) B. R. Lovic et al. Phytopathology 85:655, 1995. (3) R. D. Martyn et al. Plant Dis. 80:1430, 1996. (4) F. G. Pollack and F. A. Uecker. Mycol. 66:346, 1974.