This “CAUTION” post is similar to one I made a few weeks ago for cole crops; a quick way to highlight a potential problem, in hopes that consultants, gardeners, etc., will do some scouting to investigate if/to what extent they may be affected.
We saw, for the first time this season, some western striped cucumber beetle (Acalymma trivittatum) activity on both both weedy volunteer and cropped cucurbits.
This pest is of particular concern because it vectors bacterial wilt, a plant pathogen caused by Erwinia tracheiphila bacteria. Researchers now suspect that, rather than overwintering in the intestinal tract of adult beetles, the bacterium overwinters in the sap of alternate host plants (i.e. volunteer and weedy cucurbit species). The alternate host plants may not show symptoms of being infected, which can make management difficult.
As adult cucumber beetles feed, the beetles become infected with bacterial wilt, and then transmit it to crops. This infection can be direct (feeding on one host then another), or secondary (fecal contamination of already wounded tissue). Once the disease is established, it cannot be managed with pesticides, so ‘awareness’ of cucumber beetle activity levels, and subsequent control if necessary, is considered the best preventative tactic.
A few tips for scouting bacterial wilt in cucurbits:
- Melons, squash, and cucumber are considered more susceptible than zucchini and watermelon, but all related plants (Cucurbitaceae) are at riska
- Damage can occur quickly – scout 2-3X/week for beetle pressure and wilt symptoms
- Symptoms can be immediate on some plants, and not occur until after fruiting on others
- Leaves may look dull green, yellowing at leaf margins
- Vines wilt during the day, but seem to recover at night
- Quick diagnostic test (photos below): stems/vines are cut close to the crown, and a ‘stringy’ sticky substance appears when the two halves are pressed then pulled apart from each other. b
PHOTO CREDITS: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
There is a great new eXtension article about biology and management of cucumber beetles in organic farming systems available at: https://beav.es/ZYJ (it’s ok – we’ll use a Beavs shortlink to promote WSU just this once…there’s some great people/research going on up there!)
aDISCLAIMER: regional differences in pathogen expression are likely, do not rely on literature from other areas
bDISCLAIMER: may not work for all species or all cases