Diseases of melon (
Reported viruses
Several viruses have historically been severe problems of melons in Arizona although their incidence and severity fluctuate. These are listed with their host range, symptoms, vector and available control strategies:
Papaya ringspot virus (Potyviridae, Potyvirus, ssRNA) (Watermelon mosaic virus 1) infects cucurbits only. It causes mottled and mosaic leaves, blisters and distortion. It is sap transmissible and is carried non-persistently in aphids; it is not seed borne. There is tolerance in some varieties.
Watermelon mosaic virus (Potyviridae, Potyvirus, ssRNA) (Watermelon mosaic virus 2) infects cucurbits, many weed species and especially legumes; it causes variable symptoms including mottled and mosaic leaves and stunting (photos 1 and 2). It is sap transmissible and is carried non-persistently in aphids; it is not seed borne. There is tolerance in some varieties. It occurs throughout Arizona wherever cucurbits are grown.
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Zucchini yellows mosaic virus (Potyviridae, Potyvirus, ssRNA) infects cucurbits but little is known of other off-season hosts. It causes severe mosaic, shoe-stringing, stunting, and blisters (photo 1). It is sap transmissible and is carried non-persistently by aphids; it is not seed borne. There is tolerance in some varieties.
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Lettuce infectious yellows virus (Closteroviridae, Crinivirus, ssRNA) infects cucurbits and many other hosts; it has been uncommon in Arizona for the past several years. It causes leaf yellowing, usually on older leaves and stunting (photos 1 and 2). It is carried semi-persistently by the sweet potato whitefly, Bemesia tabaci biotype 1 and has disappeared as biotype B, not a vector, has become dominant. It is not seed borne. Sanitation and weed control may reduce incidence.
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Beet curly top virus (Geminiviridae, Curtovirus, ssDNA) infects cucurbits and many other hosts; it causes leaf cupping and rolling, plant dwarfing and chlorosis. It is transmitted by the beet leafhopper, Circulifer tenellus. It is rarely problematic in melons in Arizona. There are no tolerant varieties, and no control measures are practiced.
Squash mosaic virus (Comoviridae, Comovirus, ssRNA) infects cucurbits only; it causes a mild mosaic to a severe leaf mottle, yellowing and stunting (photo 1). Squash mosaic virus is seed borne, sap transmissible and carried persistently by cucumber beetles. The best prevention is the use of clean seed.
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Cucumber mosaic virus (Bromoviridae, Cucumovirus, ssRNA) infects cucurbits and many other hosts; it causes mild to severe leaf mosaic, stunting and deformed fruit. It is sap transmissible, is carried non-persistently in many species of aphids. It may be seed borne in other hosts but reportedly is not seed borne in cucurbits. In Arizona, it is often more of a problem on peppers than on cucurbits.
Cucurbit yellow stunt disorder virus (CYSDV) (Closteroviridae, Crinivirus, ssRNA) is a whitefly transmitted virus that was first observed in southwestern Arizona in fall 2006. Although the virus has been observed in Texas in the lower Rio Grande Valley and the Presidio area of West Texas since 1999 as well as in Sonora, Mexico, it was not detected in the Southwestern deserts until fall 2006. In fall 2007, it was widespread throughout all the melon growing areas in Arizona. The virus is most severe in cantaloupe in which it causes yellowing of leaves over much of the plant and low fruit sugar.
A diagnostic feature in melon leaves is the presence of green dots within the yellowing leaf (photo 1). Watermelon is susceptible, but is not as severely affected as cantaloupe. The known natural host range is restricted to cucurbits. Preventive measures for cantaloupe may include whitefly control and the eradication of volunteer cucurbits in nearby areas such as citrus orchards and backyard gardens. However, the origin of the rapid widespread infections in southwestern Arizona in fall 2006 is unknown, and local control measures may be of little help if they were the result of sudden infestations of whiteflies carrying the virus from other areas as a result of an active "monsoon" .
For more information, see the following web sites: http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Cucurbit/CucViruses/CucVirPhotoList.htm
http://www.ipm.ucdavis.edu/PMG/r116300611.html
http://www.ipm.ucdavis.edu/PMG/r116101811.html
December 7, 2007