Jul 26, 2011

Watermelon mosaic virus detected in winter squash

Watermelon mosaic virus 2 (WMV2) was confirmed by Jan Byrne, plant pathology diagnostician from MSU Diagnostic Services, on winter squash. Several viruses infect and damage squash, cucumbers and pumpkins in Michigan, but WMV2 is the most common. Other troublesome viruses that can occur in Michigan include cucumber mosaic virus (CMV) and zucchini yellows mosaic virus (ZYMV). Each of these viruses may infect squash, cucumber, zucchini and pumpkin alone or in combination. Infected plants may be stunted and distorted (Photo 1).

Zucchini yellows mosaic virus, CMV and WMV2 all produce mosaic on foliage and fruit of infected plants. Plants are especially damaged when infected as young plants because the yields are reduced and fruit quality is often affected by bumps and discoloration (Photo 2). Symptoms caused by ZYMV are often more severe than those caused by CMV and WMV2. Along with causing severe distortion and discoloration of the fruit, ZYMV significantly reduces yields. Zucchini yellows mosaic virus causes such devastating crop loss that plants with mosaic symptoms that test positive for this virus should be destroyed to limit spread of this disease within a region. Positive identification of the virus(es) infecting the crop is based primarily on serological tests, such as enzyme-linked immunosorbent assay (ELISA). Identification based on field symptoms alone can be misleading.

Photo 1. General viral symptoms include leaf mosaic and distortion.

Photo 2. Some virus-infected fruit become discolored and develop bumps.

Watermelon mosaic virus 2 infects vine crops and legumes (beans and peas) and other plants outside the cucumber plant family (Table 1), while ZYMV infects primarily vine crops (Table 2). Cucumber mosaic virus infects many vegetables and other plants unrelated to vine crops.

Table 1. Plants known to serve as hosts for WMV2.

Table 2. Plants known to serve as hosts for ZYMV.

It is unlikely that all hosts of ZYMV and WMV2 have been identified. Cucumber mosaic virus, WMV2 and ZYMV overwinter in biennial and perennial plants and are usually carried to new plantings by aphids. Within plantings, CMV is spread chiefly by aphids, but may also be spread by cucumber beetles. Watermelon mosaic virus 2 and ZYMV are also spread by aphids. While these viruses can be transmitted in plant sap through mechanical wounding (for example, harvesting knives), spread by this means is considered minor.

Only a short feeding time (as short as 15 seconds) is necessary for an aphid to pick up the virus from an infected plant. Once the aphid has picked up the virus, it can transmit the virus immediately to a healthy plant. Aphids retain the virus for a short time and may lose the ability to transmit the virus within four hours.

Applying insecticides to cucurbits to control these viruses has not been effective. However, growers should be aware of surrounding crops and weeds that may serve as aphid sources. Insecticide applications to surrounding crops that are serving as aphid sources may be helpful. Reflective mulches, plastic coverings and oil sprays, when used experimentally, have delayed and reduced infections, but have not been used commercially because of cost and disposal problems of mulches and covering materials. Weed control in and around plantings may help reduce infections, but will likely be inadequate by itself. Known weed hosts of WMV2 include (but are not limited to) lambsquarter and shepherd’s purse. Planting a border crop that is attractive to aphids, but not a host to the viruses, around a susceptible vine crop has not been evaluated in Michigan. In theory, the aphids would feed first on the border crop, greatly diluting any virus that the aphid might be carrying before feeding on the susceptible vine crop. Although this practice may be helpful, it would not eliminate occurrence of the virus. Currently, use of resistant varieties is the most effective approach in managing these virus diseases and should be coupled with weed control.

For more information, go to Mary Hausbeck’s Vine Crops website.

Mary K. Hausbeck, Michigan State University Diagnostic Services