Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
I am seeing and getting reports of virus infected squash, pumpkin and cucumber fields, so I thought I’d go over some information about viruses. Several aphid species are responsible for transmission of the most common viruses in cucurbits. Although some cucumber beetles have been shown to vector some viruses (such as Cucumber mosaic virus) their success rate under field conditions makes them a minor contributor to most virus infection problems.
However, Squash mosaic virus I and II (SqMV) is vectored most commonly by spotted cucumber beetles and possibly by a few other species of cucumber beetles. The spotted cucumber beetle can carry the virus for 10-20 days and transmit the virus when it regurgitates fluid into their feeding site. SqMV-I infection usually results in mild plant symptoms while SqMV-II infection results in severe plant symptoms. SqMV is usually first introduced into a field via seedborne infection and is not very common in cucurbit fields in the mid-Atlantic compared with the viruses vectored by aphids.
Aphid vectored viruses belong to two main virus families: potyviruses: papaya ringspot virus-W (PRSV), watermelon mosaic virus (WMV), and zucchini yellow mosaic virus (ZYMV); and cucumoviruses: cucumber mosaic virus (CMV).
Aphid Virus Infection and Symptoms
WMV is capable of infecting all commercially grown cucurbits. The most common symptoms caused by this virus is leaf mosaic (variegated patterns of dark and light green to yellow that form a mosaic) and leaf distortion (Fig. 1A). Symptoms may vary from plant to plant according to the species or varieties, virus concentration in the plant, timing of infection, single or mixed infections, or temperature making symptoms mild or more severe (Fig 1). External symptoms may develop within four or five days after young plants become infected but may take up to 14 days to develop when the foliage is older and more mature. Symptoms develop more rapidly at 79° to 89°F than at 61° to 75°F. Cucurbit plants rarely become infected in the seedling stage.
Typically, viruses affect most cucurbit fruit by causing lumps, bumps and rings to appear on the skin of the fruit (Fig. 2). However, at times there is little loss if the fruit has been pollinated and begins to grow before virus infection occurs. Infection just at pollination may cause the fruit to have blotches or stripes of green or yellow color (Fig. 3). If the plant is infected before pollination there usually is no fruit production, but if some are produced then symptoms on the fruit include surface discoloration, bumps and other fruit deformity, early browning, shrinking or death, small fruit size and poor yields. Secondary infection by other microorganisms may occur on the virus infected fruits and cause soft rot.
On plants, viruses can either infect the plant alone or together. If a plant is infected by only one virus, the symptoms generally are milder than if by two or more. Infection by two viruses initially causes strong mosaic and distortion on leaves. Infected plants have smaller and smaller new leaves. Late stage infections include leaves that turn yellow or become scorched along the edge.
Aphid species
Out of a possible 50 species of aphids that can be found in cucurbit fields, only a few have been shown to carry and effectively transmit the mosaic viruses. The melon (Aphis gossypii) and green peach (Myzus persicae) aphids were strong vectors while the potato (Macrosiphum euphorbia) and bean (Aphis fabae) aphids were poor vectors. The corn leaf aphid is one of the most numerous in pumpkin and squash fields but does not carry the virus.
Management
Aphids transmit the virus to plants through their sucking mouthparts. Viruses that are non-persistently transmitted are difficult to manage because the aphids acquire and transmit the virus so quickly. The non-persistent (NP) acquisition or transmission of the virus is completed in a matter of seconds to 1 minute. NP viruses cannot spread very far from where they were originally acquired. Pesticides sprayed on the plant will eventually kill the aphids, but too late to stop them from transmitting the virus. Therefore, insecticides have little effect on NP virus transmission by transient, non-colonizing aphids, though insecticides can control direct damage (foliar deformation and honeydew deposits) and secondary transmission of the virus in a field. Insecticides include Fulfill, Beleaf, the neonicotinoids and some of the pyrethroids.
Resistant varieties: A limited number of resistant varieties are available for certain viruses on squash and pumpkin while cucumber has many more resistant cultivars available – see the Mid-Atlantic Commercial Vegetable Production Recommendations guide. Most of the squash cultivars with virus resistance have just intermediate resistance, which means that the plant can restrict the growth and development of the virus and show less severe symptoms compared with a susceptible plant. Some cultivars of yellow summer squash carry a “precocious yellow gene,” which mask the color-breaking that is common with most cucurbit viruses.
Reflective mulch: This mulch is highly reflective (Fig. 4) and the light reflecting off the mulch confuses the aphids when they fly over the mulch and therefore do not land on the plants. The mulch works until the cucurbit plants cover the plastic mulch. This control method can increase the time with no infection occurring in the field by 2-4 weeks. For crops such as squash or cucumbers this can be the difference between just a few harvests and many harvests.
Planting dates: Virus infection is less severe when cucurbits are planted earlier in the season. The fruit is not affected as much in earlier plantings because the fruit was set before the virus arrived. Planting several successions of cucumber or squash will help to mitigate all but late season virus infections.
Weeds as Alternative Hosts
Many weeds can act as reservoirs for viruses even though they show no symptoms. Aphids will often land on these weedy plants and probe the plants at which time they acquire the virus in a matter of seconds. Some of these weeds include: Shepherd’s purse, Virginia pepperweed, Chicory, Canada thistle, Jerusalem artichoke, Prickly lettuce, Dandelion, Cocklebur, Endive, Escarole, Sunflower, Yellow rocket, many Wild mustards and radish, Marsh yellowcress, Pennycress, several Chickweed species, Common lambsquarter, and most Morning glory and Speedwell weeds.
Figure 1. Virus symptoms on plants can be more severe (A) or milder (B)
Figure 2. Fruit with virus infection causing lumps and bumps
Figure 3. Fruit with mild virus infection
Figure 4. Reflective mulch and cucurbit planting