FRUITAND
VEGETABLES
FRUIT
The information and recommendations in this newsletter are applicable to
North Carolina
and may not apply in other areas.
Over the past two weeks the Plant Disease and Insect Clinic at North Carolina State University has received several samples of squash and pumpkin with symptoms of mosaic virus infection. There are over 30 viral diseases recorded on cucurbits, but relatively few of them occur in North Carolina on a regular basis. For cucurbits in North Carolina, we are usually dealing with one, or a mixture, of four viruses: Watermelon Mosaic Virus (WMV; formerly called WMV-2); Zucchini Yellow Mosaic Virus (ZYMV) and/or Papaya Ringspot Virus type W (PRSV-W; formerly called WMV-1) and Cucumber Mosaic Virus (CMV).
These viruses are all vectored by aphids and can also be mechanically transmitted. At the Plant Disease and Insect Clinic, the exact identity of these viruses is not routinely checked due to time and financial constraints. However, the symptoms are characteristic enough to be 99 percent confident in the diagnosis of mosaic virus. Symptoms can be quite variable, but generally consist of light and dark green patches on leaves that are frequently raised to give the appearance of blisters. Symptoms generally appear first on new growth. Fruit will also develop unusual coloring and blisters depending on when infection occurs. Leaves can also be severely malformed (e.g., turned to ribbons, become thick and leathery, etc.)
What can one do about mosaic virus diseases in cucurbits? Many things can be done, but few provide a consistently high level of economical disease control. Mosaic viruses are one of the main limiting factors to fall cucurbit production in North Carolina. However, pumpkin growers have few options given their market window. Asgrow and Harris Moran seed companies have released several transgenic squash varieties that are resistant to several of the most common viruses. Some transgenic pumpkin varieties will soon be available, but their level of resistance and commercial acceptability has not been determined. The problem with virus-resistant transgenic plants is that they are only resistant to a certain virus(es). It is not uncommon to have viruses present for which the plant does not have resistance. This makes its resistance of little value.
The precocious yellow gene in squash can masque mild virus symptoms on fruit by producing a yellow stem rather than a green one. This provides about two weeks of protection during harvest. The down side is that the yellow stem is less desirable for the market. Producers in some states (e.g., New Jersey) routinely use reflective mulches to protect cucurbits early in their development. The reflective mulches serve to disorient aphids, preventing them from landing and feeding on foliage, thus preventing virus transmission. Reputable sources say that virus infection can be reduced by 85 percent using reflective mulches. Physical separation of cucurbit fields will also reduce the likelihood of virus-laden insects moving from infected to healthy fields.
As a general rule, remember that controlling virus vectors (e.g.,
aphids) does not control viruses. The reason is that insects can
transmit viruses quicker than it takes an insecticide to kill them.
It also does not take very many vectors to transmit enough virus to
do damage.
Web page last updated on August 17, 1998 by Stephen J. Toth, Jr..