Vine Crop Edition
by Elaine Roddy, Vegetable Crop Specialist, OMAFRA, Ridgetown

Cucumber Beetle Control
Downy Mildew and High Temperatures

Cucumber Beetle Control

The most effective cucumber beetle control is the use of Admire 240F in-furrow at the time of planting.  This treatment provides up to 4 weeks of systemic cucumber beetle control.  If admire was not used at planting, or if the residual control is starting to wear out there are a couple of foliar application options.  However, the current heat wave does pose some challenges.  

Endosulfan (Thiodan, Thionex) is less effective in hot weather.  Late evening or early morning spraying can help to improve the initial knock-down.  Residual control with this product will be limited and a subsequent spray may be necessary if a second flush of beetles moves into the field.  Malathion and Sevin are also registered for cucumber beetle control.  However, under hot weather conditions small amounts of phytotoxicity may occur with malathion.  Do not use Sevin if the plants are flowering or if there are bees in the field.

Cucumber beetles are the primary vector of bacterial wilt.  Even a fairly low population of beetles can spread this disease across the field.  Once infected, there is no cure for this disease.  Plants will rapidly wilt and die, effectively reducing the plant stand and the yield potential.  Cucumbers are the most susceptible to bacterial wilt followed by melons, pumpkins and zucchini.

 Figure 1.  Early Symptoms of Bacterial Wilt

  Figure 2. Bacterial Wilt Infected Plant

The cucumber beetle spray threshold is 1 beetle per plant.  Inspect 100 plants across the field and divide the number of beetles by 100 to determine the average number of beetles per plant.  Cucumber beetles are most active in the early morning hours.  They often hide in the soil during the heat of the day.  Population assessments done in the middle of the day may not be representative of the actual number of beetles in the field.

Downy Mildew and High Temperatures

Downy mildew is often referred to as a cooler weather disease.  However, in cucurbits it is actually active at a wide range of temperatures.  Maximum spore production occurs at temperatures between 15-20 C.  Downy mildew growth and sporulation does rapidly decline at temperatures above 30 C.  Low relative humidly and high levels of sunlight will also help to reduce spore viability.

Keep in mind that lower night time temperatures and heavy dew fall can provide downy mildew spores with an opportunity to infect the plant.  At 20 C wind borne sporangia can germinate with as little as one hour of leaf wetness.  Interestingly, the optimum temperature for zoospore germination is 25 C.

While high day-time temperatures do reduce the risk of spread; a preventative fungicide program is still the best defense against this disease.

With hotter temperatures we occasionally see amounts of phytotoxicity in the field.  Spraying in the early morning or late evening will help to prevent burn-like damage on the leaves.  At this stage the plants are growing rapidly and will quickly outgrow any damage of this type.