Viruses

Viruses are spread by use of infected seed, cuttings, and tubers and by insect feeding. No control measure is effective once the virus is established in the host. Methods for preventing infection include:

• planting certified virus-free material.
• using virus resistant cultivars.
• mowing weeds around the field.
• avoiding nearby sources of virus.
• not planting after susceptible earlier season crops. For example, fall cucumbers should not follow spring cucumbers in the same field.
• reducing populations of the insect vectors. Insecticides are not always effective in reducing virus spread because the virus may have been transmitted before the insecticide kills the insect.

Soaps and oils are also used to control virus-transmitting aphids and thrips, thus delaying infection. Recently, insecticide-laced 'baits', such as Adios for squash beetles, have been developed. Manufacturers claim that insects feed on the bait in preference to the actual plants and are killed by the insecticide before feeding on the crop. Thus, they will not transmit mosaic viruses to cucumbers and other cucurbits.

It is not yet clear however, whether the protection is sufficient to prevent virus transmission when high populations of virus-infected beetles are present. It is also possible that squash beetles will be lured into fields by the bait, as sometimes happens when Japanese beetle traps are placed in the middle of a crop.

• using reflective plastic mulch to delay the onset of aphid-transmitted viral diseases. The shiny surface disorients the aphids and reduces feeding. In Alabama, silver colored plastic delayed the onset of foliar mosaic virus symptoms in summer squash by 10 to 13 days compared to black plastic. In these trials, increased marketable yields compensated for the additional costs of the reflective mulch.

Mosaic viruses of cucurbits. Cucumber mosaic, watermelon mosaic, squash mosaic and several other viruses attack cucumbers, squash, watermelons and muskmelons. These viruses all produce a distinctive yellow and green mosaic pattern which is most pro-nounced on young leaves near the growing tip. The mosaic pattern often changes to an indistinguishable mottle as the leaves mature. This mottle may become less distinctive with time, and dwarfing and stunting of the vines then become more conspicuous.

Fruit infected by mosaic viruses will have mottled coloration and bumpy surfaces. Although these viruses are not easily distinguishable from each other by the novice, a distinguishing symptom of cucumber mosaic is a lack of fruit color ('white pickle'). Many cucumber cultivars are resistant to cucumber mosaic virus. Asgrow has introduced squash, muskmelon, watermelon, and zucchini cultivars with genetically engineered resistance to a number of the mosaic viruses.

Viruses survive between crops on weeds and other cucurbits. They are spread from crop to crop and from plant to plant principally by aphids, but also by cucumber beetles and sometimes by nematodes. Successive crops of cucurbits should not be planted adjacent to each other or the earlier crop can serve as a source of virus to infect the later plantings.

Mosaic viruses of solanaceous crops. Eggplants, tomato, and pepper are all susceptible to tobacco mosaic virus. Tobacco mosaic and etch viruses are the most common diseases of peppers in Louisiana and were responsible for approximately 5 percent of crop loss in North Carolina in 1988. The tomato strain of tobacco mosaic virus is seed-transmitted rather than insect-transmitted and does not infect tobacco. The primary source of tobacco etch virus is infected solanaceous weeds. Destroying infected plant material will reduce spread.

In areas of high virus infection, such as the river parishes of Louisiana, aluminum-coated plastic mulch is recommended because the reflective surface deters virus-transmitting aphids. Pepper transplants grown on aluminum coated plastic mulch may be stunted initially compared to those grown on black plastic, but growth and yield later in the season will surpass that of plants grown on black plastic mulch because of the increased virus protection.

Potato mosaic viruses are widespread in the field but losses are generally minimal. These viruses all overwinter in tubers of infected plants and are controlled primarily by planting virus-free seedstock.

Potato virus X - simple mosaic: Symptoms vary from none to a distinct interveinal mosaic pattern. This pattern is masked by bright sunny conditions. Certified potato seed is commonly grown in the north under cloudy conditions, where symptoms will be expressed and infected plants can be discarded. The X virus can reduce yields up to 10 percent and is spread by any contact between diseased and healthy plants. However, it is not known to be transmitted by any aphid species.

Potato virus A - mild mosaic: Symptoms are dwarfing and yellowish or light colored mottling of the foliage. Feeding aphids and other insects, workers and machinery all spread this virus. Potato virus Y - rugose mosaic: Symptoms include puckering of leaf surfaces, leaf mottling, stunting, and necrotic streaks along the veins on the undersides of the leaves. Leaves may drop off, eventually leaving only a small rosette of foliage at the top of the shoot. Planting certified seed and controlling aphids reduces incidence of this virus.

References

• Brown, J.E., J.M. Dangler, F.M. Woods, M.D. Henshaw, W.A. Griffey and M.S. West. 1993. Delay in mosaic virus onset and aphid vector reduction in summer squash grown on reflective mulches. HortScience 28:895-896.

• Boudreaux, J.E. 1991. Commercial vegetable production recommendations. Louisiana Cooperative Extension Service Pub. 2433.