Thursday, June 2, 2011

Cucumber Beetles and Bacterial Wilt

These pictures show bacterial wilt.  It can start rather innocuously.  The first picture shows normal leaves and then, at the bottom, a few that are wilted but which don't look awful.  As soon as a day later the leaves and then the whole plant will wilt, then turn dry and crisp. This may start on just a part of a leaf, as in the second picture.

With the wet weather the gardens are getting in later than usual, but the pests have hatched on time.  So this year they're voraciously waiting for our seeds to hatch.  I think that seedlings are even more susceptible than seeds, and so I always grow my squash family plants from seed in the garden, rather than transplanting.

Striped Cucumber Beetles are about 1/4 inch long, with a rectangular back striped yellow and black.
They will feed on all cucurbits except watermelon, plus tomatillos and a few other plants.  (I've found them on tomatoes and milkweed).  They like the undersides of leaves, especially lower down on the plant, and the interior of blossoms.  (There is another species called Spotted cucumber beetle, which we don't have).

At least in the community gardens the beetles aren't here in large enough numbers to damage the cucumbers themselves, but they carry a devastating disease called Bacterial Wilt.

About 30 days after the infected beetles start to feed on a plant, the water-carrying pores in the plant stems become clogged with bacteria and the plant wilts dramatically and dies.  It's very important to get rid of the cucumber beetles, their eggs and larvae.  We may get as many as four generations during the summer, and if the last generation in the fall has fed from plants carrying disease, the disease will carry over the winter in the bodies of the cucumber beetles.  So pulling and destroying any plants with the wilt will also reduce the amount of disease next year.

The plants can be protected by floating row cover, or by spraying with a kaolin clay.  The floating row cover (the best known is Reemay, but you can get a lighter-weight insect control fabric, which is a spun polyester) keeps the beetles from getting to the plants, and is a useful solution for root maggots, potato beetles, and bean beetles as well.  The cover has to be removed at blossom time for pollination.  Otherwise you can get varieties that are parthenocarpic, meaning they don't require pollination.  They'll also be seedless if protected from pollination.  Johnny's seeds shows varieties that are parthenocarpic in their descriptions.

Surround is one brand of kaolin clay, available from Gardens Alive and perhaps other places, which will keep the cucumber beetles from eating.  These are the two best solutions to prevent feeding. I've found that the Surround even lasts through a rain or two.  The most difficult thing about it is application.  It tends to clog sprayers, but the fine mist of a sprayer covers better than applying by hand.  (You might experiment with Kaopectate, but I've really no idea if it would work, or if there's anything in it that would harm the cukes.  It's obviously edible, so that's not a problem.  Try it on just a few plants at first.)

The beetles can be killed with rotenone and/or pyrethins (sometimes called pyrethrum), acceptable on organic programs.   But even killing the beetles that have landed on the plants will potentially let them feed initially and transmit the disease, so prevention is the best strategy. (The disease is transmitted through their feces which contaminate the open wounds in the plant from their feeding).

The beetles are even more attracted to tomatillos than to cucumbers, so tomatillos can be grown as a "trap crop" to collect the beetles away from the plants you want to grow.  Then it's important to collect and destroy the beetles on the tomatillos, so they don't build up a big population that way. The tomatillos don't particularly suffer from the beetles, and you can use them in salsa.  I wonder whether having a large number of beetles that feed exclusively on tomatillos will reduce the number of infected beetles (since the tomatillos don't have bacterial wilt), but don't know if that's so. 

During cucumber beetle season turn over the lower leaves of plants in the early morning when the beetles are sluggish, and either catch the beetles and throw them into soapy water, or crush them.  They fly readily, so catching them in mid-day is difficult.  Also rub off the yellow eggs they've laid on the undersides of the leaves.

Some varieties of cucumbers and summer squash are more attractive to the beetles, and some are more susceptible to the disease.  If you pay attention to the varieties you grow you might be able to determine this.  County Fair is a variety of cucumber (available by catalog) reputed to be somewhat resistant to the bacterial wilt.

Another strategy is to plant successive crops of cucumbers.  Often we'll get some cucumbers before the wilt destroys the plant, so if you've planted every 2 weeks you may get an adequate harvest in spite of the disease.

This is the most challenging pest we face in our Community Gardens, since the damage from the disease is irrevocable.


  1. How lucky we community gardeners are to have such a knowledgable coordinator like you to provide us with gardening information. Thank you Kathy!

  2. I'm not an experienced gardener, so I didn't know what to make of my own cucumber-growing experiences this year (in the North East Kingdom of Vermont). Starting with seedlings, half the plants died, the output has been puny, and I've had one 'mutant' cuke. Thing is, almost everyone else I talk to is having some sort of cucumber-growing problem in this area. Heard anything on it? Thanks . .

  3. Michael - I don't know what your specific problem is, since my guess is you have generally cooler weather than we do in the Burlington area.

    In our sandy soil lots of things failed to thrive early on because we had such a wet late spring that the little fertility we had leached out, so we needed to sidedress a lot.

    Then it got hot and dry, which is the perfect growing condition for powdery mildew (downy mildew grows in wet damp conditions, but we don't usually get it).

    The powdery mildew is always such a surprise, because it's so counterintuitive that you'll get a mildew when it's hot and dry.

    I've got a new crop of cukes started, and I'm combatting the potential for mildew with a tablespoon of baking soda per gallon of water (which makes an inhospitable condition for the spores) and I relented and got some Neem oil, which is effective against powdery mildew and insects.

    Any "mutant" is likely to be a seed problem, but could be a disease or even aphids.

    Send me a picture if you'd like at and I'll take a look and tell you what I think. You can also always send plant material to Ann Hazelrigg at UVM who will take a look at it and diagnose any disease. She's also quite helpful by phone. I'd call her soon, as school is about to start up and it will probably take her more time to get to anything.

    201 Jeffords Building
    63 Carrigan Drive
    University of Vermont
    Burlington, VT 05405

    Tel: (802) 656-0493
    Fax: (802) 656-4656

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  5. I see from my comment above that I'm recommending baking soda for powdery mildew - but I'm reading more and more that milk will help keep the mildew from ever taking hold, and may have some curative properties as well. The recommended dilution is 1 part milk to 10 parts water, and perhaps it should be fresh milk. I'm going to try fresh milk this year on a few plants, and dried milk powder diluted on a few others, as being less expensive than fresh milk.