March 10, 2005
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of a leaf spot and stem canker caused by Myrothecium roridum on watermelon in the United States
K. W. Seebold, Jr., D. B. Langston, Jr., and R. C. Kemerait, Jr., Department of Plant Pathology, University of Georgia, Coastal Plain Experiment Station, Tifton 31793; and J. E. Hudgins, University of Georgia Cooperative Extension Service, Bainbridge 31717. Plant Dis. 89:342, 2005; published on-line as DOI: 10.1094/PD-89-0342A. Accepted for publication 7 Dec 2004.

_Myrothecium roridum_ Tode:Fr, pathogenic to a number of cucurbit species, causes fruit rots, cankers on crowns and stems, and leaf spots. Hosts include cantaloupe and honeydew (_Cucurbita melo_) and cucumber (_Cucumis
sativus_) (1,3). In June 2004, following a period of heavy rainfall, numerous round-to-oblong, brown lesions with concentric rings were observed on leaves of watermelon (_Citrullus lanatus_) cv. Desert King at the Blackshank Farm in Tifton, GA. Disease was localized in the field and severity was low (<5 percent of leaf area affected). No symptoms were observed on fruit.

Sections of tissue were removed from the margin between healthy and diseased tissue and plated on acidified, 25 percent potato dextrose agar (aPDA). A small plug of agar and mycelium were removed from colonies that
emerged from lesions and were transferred to aPDA. Isolated colonies were characterized by a white, floccose mycelium with concentric, dark green-to-black rings of sporodochia bearing viscid masses of conidia.

Conidia were cylindrical with rounded ends and measured 6 to 8 x 1.5 to 2.5 micrometers. The features of the fungus were consistent with the description of _Myrothecium roridum_ (1,2). Pathogenicity tests were conducted in a temperature-controlled greenhouse. 25 watermelon plants (cv. Desert King) were inoculated with a conidial suspension of _M. roridum_ (500 000 conidia per ml) plus 0.1 percent vol/vol Tween 20. Inoculum was applied on leaves and stems until runoff with a hand-held mister, and plants were placed in a dew chamber for 72 h. 10 plants were sprayed with sterile, distilled water to serve as controls. Inoculated and noninoculated control plants were removed from the dew chamber and maintained at 25 to 28 degrees C.

Symptoms appeared 8 days after inoculation and were characterized by round, dark lesions with concentric rings; noninoculated plants were symptomless. Sections of symptomatic tissue were plated, and _M. roridum_ was reisolated.

Although _M. roridum_ is a common pathogen of melons and cucumber, to our knowledge, this is the 1st field report of a leaf spot caused by _M. roridum_ on watermelon in the United States. No further occurrences of the
disease on watermelon have been observed in Georgia since the initial discovery of _M. roridum_ in 2004; however, losses could be potentially severe if widespread infection of fruit were to occur.

References:

(1) B. D. Bruton. Crater Rot. Pages 49-50 in: Compendium of Cucurbit Diseases. T. A. Zitter et al., eds. The American Phytopathological Society, St. Paul, MN, 1996.

(2) M. B. Ellis. Page 552 in: Dematiaceous Hyphomycetes. CAB International, Wallingford, UK, 1971.

(3) D. F. Farr et al. Page 809 in: Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989.

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[_Myrothecium roridum_ (Mr) is pathogenic on melon, causing significant economic losses up to 30 percent. The disease is favored by wet weather conditions that also favor development of other foliar pathogens and can be
controlled by the use of protectant fungicides. The disease occurs infrequently and has been observed mostly in the Lower Rio Grande Valley. Mr produces macrocyclic trichothecenes that are antibiotic/phytotoxic compounds produced by both fungi and plants. Some Mr-induced macrocyclic trichothecenes are identical to trichothecenes found in 2 species of the plant genus _Baccharis_. Production of macrocyclic trichothecenes by Mr has been implicated as a possible virulence factor in plant diseases caused by this fungus. However, the role of these compounds in the biology of_Baccharis_ species is unclear. - Mod.DH]

Links:
<http://dallas.tamu.edu/People/wmackay/cucumis.pdf>
<http://plantpathology.tamu.edu/texlabn/vegetables/Watermelon/wmelon.htm>
<http://www.fgsc.net/asil95/poster13.html>