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Disease Note.

First Report of Monosporascus Root Rot/Vine Decline of Watermelon in Mexico. R. D. Martyn, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843 and Weslaco 78596 . J. S. Batten, Y.-J. Park, and M. E. Miller, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843 and Weslaco 78596. Plant Dis. 80:1430. Accepted for publication 11 October 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1430C.

In April 1996, several commercial watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai cvs. Tri-X-313 and Sangria) and melon (Cucumis melo L. cvs. Durango and Honey Brew) fields near the city of Tecoman, Colima, Mexico were surveyed for vine decline diseases. Various vine decline symptoms were observed on plants in most fields; however, in one watermelon field, symptoms were particularly severe. The plants were approximately 2 weeks from maturity with numerous fruits. Disease symptoms included yellowing and necrosis of the foliage, and many of the vines had collapsed, exposing the fruit to intense solar radiation. The root systems of affected plants were lacking most of the secondary and tertiary roots, and there were numerous necrotic lesions on the lateral roots. Root samples brought back to College Station, TX, were examined with a dissecting microscope, and fruiting structures presumed to be perithecia of the fungus Monosporascus cannonballus Pollack & Uecker (3) were observed on roots from the field with severe symptoms. Isolations from surface-disinfected roots of both the triploid and diploid plants from this field yielded cultures tentatively identified as M. cannonballus. Six isolates were confirmed to be M. cannonballus by polymerase chain reaction-amplification of the internal transcribed spacer region of the rDNA with Monosporascus-specific primers (1) and by observation of perithecia and characteristic ascospores in pure culture several weeks later (2,3). None of the Mexican isolates contained any detectable double-stranded RNA. Pathogenicity of each isolate was confirmed in the greenhouse on the muskmelon cultivar Magnum 45. Inoculum was grown on sterile, red milo sorghum (Sorghum bicolor (L.) Moench) seed for 2 weeks and then 20 g of inoculum was mixed with a pasteurized potting mix (sand/perlite/vermiculite; 1:1:1) in 15-cm-diameter plastic pots. Five seeds of cv. Magnum 45 were planted in each pot and, after germination, were thinned to two seedlings per pot. There were five replicated pots for each treatment. Plants growing in uninfested soil served as a negative disease control while inoculum of the Monosporascus isolate AZ90-33 served as a positive disease control. Each Mexican isolate was highly aggressive and caused severe stunting and root necrosis, relative to the noninoculated control plants, that was indistinguishable from that caused by the known pathogenic isolate of M. cannonballus. In some cases, infection resulted in death of the seedlings within 7 to 10 days. Monosporascus was reisolated from symptomatic plants from each treatment. Although other muskmelon and watermelon fields showed various vine decline symptoms, M. cannonballus was not confirmed from these other fields. In North America, M. cannonballus has been reported from only three states in the U.S. (Texas, Arizona, and California). This report extends the known range of M. cannonballus in the Western Hemisphere to now include southwestern Mexico. Other reports of this fungus are from southern Europe (Spain), the Middle East (Israel), North Africa (Tunisia and Libya), India, and the Far East (Japan and Taiwan) (2).

References: (1) B. R. Lovic et al. Plant Dis. 79:1169, 1995. (2) R. D Martyn and M. E. Miller. Plant Dis. 80:716, 1996. (3) F. G. Pollack and F. A. Uecker. Mycologia 66:346, 1974.