Insect and Pumpkins Don't Mix
The Insect Link to Pumpkin Diseases
Thomas A. Zitter
Professor, Department of Plant Pathology
Cornell University, Ithaca, NY 14853
The striped cucumber beetle (Acalymma vittatum) (Fig. 1) is by far the most important insect pest of cucurbits, particularly in New York, the northeastern states, and reaching into eastern Canada. In addition to cause direct crop injury to cucurbits by adult feeding, and occasional damage to roots by larval feeding, the striped cucumber beetle is most important as a vector of plant diseases, and in particular bacterial wilt that affects most cucurbits (Erwinia tracheiphila) (Fig. 2). Cucumber beetles, and the closely related Diabrotica species (Diabrotica undecimpunctata or the spotted cucumber beetle) (Fig. 3), are attracted to cucurbits by naturally occurring bitter cucurbitacins (oxygenated tetracyclic terpenes), which, as an aside, has also be used to enhance insecticidal control. Cucumber cultivars with the non-bitter allele (bi), such as found in "Country Fair" and "Marketmore 80", are less preferred by cucumber beetles. Spotted beetles are also attracted to cucurbit flowers by blossom volatiles (indoles). Beetles cause direct damage to cotyledons, developing young leaves, blossoms, pumpkin fruit and handles (peduncles), which can make the fruit unmarketable (Fig. 4). Bacterial transmission from overwintered beetles in the spring to young cucurbits occurs when contaminated fecal material comes in contact with wounds caused by beetle feeding. E. tracheiphila only multiplies in the xylem (Fig. 5) of susceptible cucurbits (pumpkins, summer squash, winter squash, muskmelons, cucumbers, but not watermelon) so the massing of beetles during the feeding process near veins will enhance transmission of bacteria from fecal matter. Besides vectoring bacterial wilt, striped cucumber beetle larvae have been shown to transmit Fusarium wilt of muskmelon, and adults (western striped cucumber beetle and spotted cucumber beetle) also transmit squash mosaic virus (Fig. 6), which is primarily a concern for muskmelons and cantaloupes. A single dominant allele in cucumber is known to confer resistance, but in a homozygous state, undesirable associated problems occur. Growers generally rely on insecticides for control (imidacloprid, carbaryl, etc.) when the crop is threatened, especially if bacterial wilt is known to be a problem in the growing area.
|Fig. 1 Striped Cucumber Beetle||Fig. 2 Bacterial Wilt||Fig.3 Spotted Cucumber Beetle|
|Fig. 4 Pumpkin Handle Damage||Fig. 5 Bacterial Wilt infected xylem on right||Fig. 6 Squash Mosaic Virus|
The squash bug, Anasa tristis (Fig. 7&8), occurs throughout the United States wherever cucurbits are grown. The squash bug has been reported to attack nearly all cucurbits, but prefers plants of Cucurbita species. For example, C. maxima [buttercup squash] is very susceptible while C. moschata [butternut] is more resistant. Foliage is the primary site of feeding, resulting in yellowing, blackening, and eventually wilting of the foliage and plant. This"anasa wilt" is caused by the insect secreting highly toxic saliva into the plant. This damage can mimic that caused by bacterial wilt. Damage can also be seen on maturing fruit, and the amount of damage is proportional to the density of squash bugs. For individual plants, localized injury results from the gregarious behavior of the bugs attacking a weakened plant. In the case of plant disease involvement, a phloem-limited bacterium is responsible for yellow vine decline of cucurbits (watermelon, cantaloupe, squash, and pumpkin, but apparently not in cucumber) that is vectored by squash bugs (Fig. 9&10). So far the disease has been found primarily in southern and western states (Texas, Oklahoma, and Tennessee), with one confirmation from Massachusetts. Insecticides when applied to the base of plants (carbaryl, endosulfan, esfenvalerate, and permethrin) are especially effective, because bugs tend to cluster there. Also the use of plastic mulches provides good harborage for squash bugs, with bugs accumulating at the plug opening. An old home gardening practice of placing large cabbage leaves near the crop to provide a place for bugs to congregate during the day, assures an easy place to find them, and then an easy place to crush them.
|Fig. 7 Squash Bug-Nymphs & eggs||Fig. 8 Squash Bug-Adult & eggs||Fig. 9-Yellow Vine Decline||Fig. 10 Yellow vine decline phloem discoloration|
The six-spotted leafhopper (Macrosteles fascifrons) (Fig. 11), along with several other phloem-feeding leafhoppers, is responsible for the transmission of the aster yellows phytoplasma. Aster yellows infection in New York has been noted on summer squash, pumpkins, and winter squash (Fig. 12) (Zitter, unpublished data), but is generally considered to be more of a curiosity. Infected plants are easy to diagnosis because of conspicuous yellowing of young leaves, proliferation of secondary shoots, and rigid and erect habit of the plants. Flowers are usually malformed and may have prominent leafy bracts. Yellow summer squash fruit turn green, (Fig. 13) winter squash are malformed and off-colored, and pumpkins are misshapened. Separate applications of carbaryl and esfenvalerate are not warranted, given the sporadic occurrence of the disease.
|Fig. 11- Six Spotted Leafhopper||Fig. 12-Aster Yellows||Fig. 13-Summer Squash with Aster Yellows|
Onion thrips (Thrips tabaci) (Fig 14) have a wide host range, in excess of 300 plants, but in nature they are the most important pest of onions. They are generally not a significant problem for cucurbits, but on occasion have resulted in injury especially to the pumpkin rind (Fig. 15). This has been observed in Orange Co. on several occasions. The damage results from the piercing of cells and the removal of the cell contents by larvae (also called nymphs) and adults. This superficial feeding is sufficient to cause silvering or whitening of the pumpkin rind, and may make the fruit unmarketable. Most damage occurs during hot dry seasons, beginning early to mid-August in areas where thrips have nothing better to feed on. The proximity of susceptible crops like onion as thrips sources is an important concern for adjacent crops. Thrips are the only vectors of tospoviruses including tomato spotted wilt virus (Fig. 16), which can infect several cucurbits. The insecticides esfenvalerate and permethrin may offer the needed control.
|Fig. 14-Thrips||Fig. 15-Pumpkin Rind Damage||Fig. 16-Tomato Spotted Wilt Virus|
Aphids, as illustrated by the melon aphid (Aphis gossypii) (Fig. 17), are widely distributed throughout the tropical and temperate regions of the world. Among cucurbits, cantaloupes, watermelons, cucumbers, and to a lesser extent squash and pumpkin are important hosts. Aphids can affect the crop in three ways: by direct feeding into the mesophyll cells, and eventually the phloem, by excreting honeydew onto the plant or fruit surface, and by vectoring plant viruses (Fig. 18&19). Aphids vector viruses belonging two main virus families: potyviruses, papaya ringspot virus type W, (PRSV-W, formerly WMV-1), watermelon mosaic virus (WMV, formerly called WMV-2), and zucchini yellow mosaic virus (ZYMV); and cucumoviruses, cucumber mosaic virus, (CMV). Insecticides have little effect on virus transmission by transient, non-colonizing aphids, though insecticides can control direct damage by insect feeding and honeydew production. Virus resistant varieties are already available for many cucurbits and will be available soon for pumpkins.
|Fig. 17-Melon Aphid||Fig. 18-Cucumber Mosaic or Watermelon Mosaic Virus||Fig. 19- Papaya Ringspot or Zucchini Yellow Mosaic Virus|
The greenhouse whitefly (Trialeurodes vaporariorum) (Fig. 20) occurs widely around the world but appears in temperate regions by virtue of its ability to survive in winter in greenhouses. This whitefly has a very wide host range, but it is especially suited for vegetables (tomato, eggplant, cucumber, etc) and many ornamentals. The wings of the adult form a triangle shape and they are horizontal when at rest. This distinguishes this species from the similarly appearing silverleaf whitefly (Bemisia argentifolii) and the sweetpotato whitefly (Bemisia tabaci) (Fig. 21), which hold their wings as a roof or tent-like posture when at rest. Although the greenhouse whitefly is capable of transmitting viruses, these particular viruses do not occur in our region. The silverleaf and sweetpotato whiteflies are closely related species and are not distinguished easily by appearance. The silverleaf whitefly has largely displaced the sweetpotato whitefly in southern states as the dominant species. Whitefly adults and nymphs have piercing-sucking mouthparts to feed on the phloem of host plants, resulting in localized spotting, yellowing, and leaf drop. A translocated, transitory, toxicogenic secretion by nymphs (but not by adults) of the silverleaf whitefly is responsible for damage to uninfested leaves and tissue. This gives rise to squash silverleaf disorder (Fig. 22), which is similar in appearance to Command herbicide damage (Fig. 23). Irregular ripening of tomato fruit and broccoli stems also occurs. The silverleaf and sweetpotato whiteflies are known to transmit over 60 plant viruses (none in our region unless introduced on transplants), most belonging to the geminivirus group (bipartite DNA genome). Field populations of whitefly are derived from the greenhouse (greenhouse WF) or are brought north on vegetable or ornamental transplants (silverleaf WF). Field control is usually not required, but if heavy populations occur, growers have made use of soaps and mineral or vegetable oils, insecticides (imidacloprid) and Neem products (immature insects only).
|Fig. 20-Greenhouse Whitefly||Fig. 21- Sweetpotato Whitefly||Fig. 22-Squash Silverleaf Disorder||Fig. 23-Command herbicide damage|
Thank you to the following for the use of their slides: MPH=M.P. Hoffman; TAZ= T.A. Zitter; BDB=B.D. Bruton; JVE=J.V. Edelson; TMP=T.M. Perring; and DG=D. Groff
1. Compendium of Cucurbit Diseases. 1996. Ed. T. A. Zitter, D. L. Hopkins, and C. E. Thomas. APS Press, St. Paul, MN. 87pp.
2. Pumpkin Production Guide. 2003. Ed. D. Riggs, NRAES, Ithaca, NY. 152pp.
3. Vegetable MD online web site: http://vegetablemdonline.ppath.cornell.edu for selected fact sheets, news articles, and images.