Yellowing of leaves, stunting and death of plants, and distortion of leaves and fruit were common in many squash and pumpkin fields this summer. These disease symptoms were caused by insect spread viruses. There is no cure once a virus invades a plant. Viruses are systemic and spread throughout the plant.

Squash samples were collected in the Delta area and screened for virus by AgDia (Elkhart, Indiana). Two different types of virus were identified; Potyvirus and Geminivirus. Potyvirus are spread by aphids; Geminivirus (i.e. Beet Curly Top Virus) are spread by leafhoppers.

The spread of aphid- and leafhopper-transmitted viruses are more severe in years when dry windy weather conditions are common. Native vegetation dries out forcing insects into irrigated fields. Winds also carry these insects into our area from neighboring counties and states. Reports indicate aphids have been carried 500 kilometers (310 miles) by wind. Differences from year to year and region to region in aphid migration have been shown to have a great influence on the spread of several aphid-borne plant viruses.

Virus can be picked up from infected weeds or vegetables by aphids after only 5 to 10 seconds of probing. Once the virus is acquired, the aphid can immediately transmit the virus to the next plant probed. Most of the aphid borne viruses are carried on or near the mouthparts of the insect and do not multiply in the insect.

Leafhoppers transmit viruses in a different manner. Leafhoppers require a longer feeding period when compared to aphids, but still may acquire the virus in as brief a period as one minute. Once acquired, leafhoppers may be able to transmit viruses throughout their lives. Some viruses multiply within leafhoppers.

The indiscriminate probing of aphids and leafhoppers in search for a suitable feeding host can result in the rapid and wide-spread transmission of viruses.

Prevention of insect-vectored viruses requires control of the insect and elimination of virus-infected plants.

Leafhopper-spread virus problems can be reduced in small gardens if the the plants are shaded. Leafhoppers avoid feeding on shaded plants. This may be impractical in large commercial fields.

Spray applications need to begin when the plants are still young. Research in Sweden has shown a 60-70% reduction in virus incidence if local virus sources (i.e. weeds and infected plants) are eliminated and control treatments target aphids beginning in early June.

Satapathy, M.K. (Hadidi, et.al, 1998, page 192) reports applying the improper insecticide or too low a dosage can increase insect movement within the field leading to further disease spread. He further reports the use of persistent insecticides that paralyze or kill almost immediately are very effective in managing insect-vectored diseases.

The Colorado Pesticide Guide for Vegetable Crops (Colorado State University Extension) provides some recommendations for insecticides for pumpkins and winter squash. Chemical dealers will have additional recommendations. Read and follow label instructions before using any pesticide.

Weed control in and around the field or garden is necessary to prevent the transfer of viruses. An infected perennial weed will likely retain the virus in its roots during the winter. This virus-infected weed is a source of infection upon regrowth the following year. Annual weeds infected early in the season will be a continual source of virus infection unless destroyed.

Insecticide sprays applied to areas where weeds can not be destroyed help eliminate aphid and leafhopper populations before they have an opportunity to move into neighboring crops. Contact insecticides are recommended for this purpose. Several sprays will need to be made during the growing season unless a longer lasting systemic insecticide is applied.

Many viruses are also spread mechanically through sap. Cultivating or working around infected plants can result in the transmission of virus-contaminated sap from an infected to uninfected plant.

Eliminating infected vegetable plants as soon as noted will help reduce spread by insects, cultivation equipment and animals (to include humans) moving through the field.

References Used:
Hadidi, A., R.K. Khetarpal, & H. Koganezawa (eds). 1998. Plant Virus Disease Control. APS Press.
Jones, J.B., J.P. Jones, R.E. Stall, T.A. Zitter. 1991. Compendium of Tomato Diseases. APS Press.
Maloy, O.C. 1993. Plant Disease Control: Principles and Practice. John Wiley & Sons, Inc.
Zitter, T.A., D.L. Hopkins, & C. E. Thomas. 1996. Compendium of Cucurbit Diseases. APS Press.