A severe outbreak of melon yellow mosaic disease caused by Zucchini yellow mosaic virus in the Punjab province of Pakistan

In 2003 a serious disease of muskmelon (Cucumis melo) appeared in commercial fields in the Punjab, Pakistan. Symptoms of the disease were characteristically chlorosis, leaf curling and deformation, vein clearing and the formation of leaf-like outgrowths on the upper leaf surface of some leaves (Fig. 1). The disease led to almost 100% loss of the crop in some areas and was present again in 2004, with a similar phenotype and losses.

To identify the virus(es) associated with the disease, infected plant samples were collected from several locations in the Punjab Province. On the basis of symptoms and host cucurbit-infecting viruses were suspected. Samples were screened for the presence of Zucchini yellow mosaic virus (ZYMV; Desbiez & Lecoq, 1997) and Cucumber mosaic virus (CMV) by DAS-ELISA using virus specific polyclonal antisera (Loewe, Germany). DAS-ELISA showed the presence of ZYMV in all symptomatic samples collected from the epidemic area. For samples collected in the Punjab, CMV was detected at a low incidence in tomato plants with symptoms of yellow chlorosis growing near the melon fields, but not in melon. Healthy indicator plants (melon, cucumber and Nicotiana benthamiana) were mechanically inoculated with plant sap extracted from symptomatic melon leaves by the leaf abrasion method. The virus was mechanically transmitted from the infected samples to healthy melon seedlings and induced symptoms, including the leaf-like outgrowths on the upper leaf surface; typical of ZYMV infection. No infection was observed in N. benthamiana, which is a diagnostic host for the other common cucurbit potyvirus Watermelon mosaic virus. This confirms that ZYMV is the causative agent of the disease of melon.

Although ZYMV has been reported from Pakistan on some cucurbits, with an incidence of about 15% on samples collected from the North West Frontier Province (Ali et al., 2004), we could not find a report of the virus infecting melons. Moreover, the enation-like phenotype was observed for the first time and on a large scale in Pakistan, although this phenotype is known to be caused by ZYMV in other parts of the world. We are currently investigating ZYMV-encoded genes causing this phenotype.