Rosy Outlook for
To keep ahead of mite
populations in the greenhouse,
entomologist Michael Parrella
and graduate research assistant
Christine Casey recommend that
growers examine 38 rose plants
for every 10,000 square feet
|Spider mites and a disease called
powdery mildew can make life miserable for roses growing in greenhouses. But
ARS is helping these splendid plants
fight back. The agency is among the sponsors of research at the University of
California at Davis that will yield effective, environmen tally friendly ways
to control mites and mildew.
Roses produced in greenhouses, instead of outdoors, are the ones you find for
sale as cut flowers at your local florist shop. ARS plant pathologist Edwin L.
Civerolo, the agency's representative for the greenhouse-roses research, says
the investigations may help rose growers reduce the number of times they have
to spray chemicals. Sometimes that's as many as 25 to 45 times a year.
Fewer sprayings may actually boost the number of blooms that growers can
harvest. Right now, because of safety concerns, workers must wait 24 to 48
hours after spraying before they can reenter the greenhouse. That means they
may miss the opportunity to harvest blooms that could otherwise be sold.
Vigorously blooming rose plants can be harvested twice a day, according to
Civerolo. He leads the ARS Crops Pathology and Genetics Research Unit at Davis.
A single red rose.
|Spider Mite Sampling
The spider mite research will give growers new, science-based techniques for
monitoring and sampling mites that infest rose plants. About the size of the
period at the end of this sentence, spider mites use their mouthparts to suck
out the contents of leaf cells. By removing cell components called
chloroplasts, for example, mites interfere with the plant's ability to convert
sunlight into food.
"With a statistically valid sampling procedure," says Civerolo,
"growers could correctly estimate the size of a greenhouse's mite
population and from that determine whether the population poses a threat to
their plants. Low-level populations may not harm the roses, so they may not
need to be treated. If so, that would reduce growers' costs."
Preliminary results from spider mite research by Christine A. Casey and Michael
P. Parrella of the University of California at Davis suggest that growers need
to inspect, or sample, 38 rose plants for every 10,000 square feet of
greenhouse. Most commercial rose greenhouses are anywhere from 50,000 to
100,000 square feet, and most operations have from 5 to 10 greenhouses, says
Casey, a graduate research assistant.
Scanning electron micrograph of a
two-spotted spider mite feeding on
a rose leaf.
| "Roses bloom on shoots, or
canes, that sprout from the crown of the mother plant," Casey notes.
"Growers allow only the strongest canes to grow upright and produce
flowers. The others are deliberately bent. The correct place to look for mites
is the first leaf above the bend in the cane, or main stem." This
recommendation is based on studying samples collected weekly, for more than a
year, from commercial greenhouses in central and southern California, Casey
Next, Casey and Parrella want to determine the population density that triggers
overcrowding and causes mites to crawl away from the bend and up the stem.
"You want to catch the population before it becomes so large that mites
are traveling higher than the third leafthe point where the stems are
typically cut and sold," Casey explains.
Plant pathologists James MacDonald
(left) and Edwin Civerolo examine
rose leaves for the presence of
| Right now, the sampling
scheme won't work for growers who use a popular biological control agent,
a beneficial mite known as Phytoseiulus persimilis. "Because
it's time-consuming to distribute the helpful mite," says Casey,
"growers tend to put it only on plants that they know are infested.
That, of course, skews our sampling design."
The solution may be a handy dispenser, being developed elsewhere, that will
make it fast and easy to distribute mites evenly throughout a greenhouse.
All Mildews Are Not Alike
Meanwhile, investigations of powdery mildew will determine whether an
early-warning system for outbreaks of this pathogen in grapevines can be
successfully adapted to rose production. The monitoring system tracks
conditions favoring proliferation of the fungus. It relies on readings from a
package of instruments that record and report greenhouse conditions, such as
temperature, according to Linda L. Bolkan, staff research associate at the
university. Bolkan and James D. MacDonald of the university's Department of
Plant Pathology are conducting the powdery mildew studies.
"Each species of powdery mildew," Bolkan says, "is specific to a
particular crop. Grape powdery mildew, for example, is different from rose
powdery mildew. We can't expect rose powdery mildew to behave exactly as the
grape species. That means we have to fine-tune the computerized, mathematical
model that drives the grape mildew forecasting system so that it will be useful
"For example, we started our rose work with a system that assesses
conditions when temperatures range between 18 °C and 30 °C. Research
has shown that temperatures between 18 °C and 27 °C for several hours
are better for powdery mildew to reproduce and spread in roses," says
Bolkan. "So by feeding this new temperature information into the model, we
can make more precise and accurate forecasts of rose powdery mildew."
"The grape model, for outdoor production, is temperature driven. It
doesn't rely heavily on relative humidity as a variable. However, we're
investigating relative humidity because we think it might be important in
greenhouse rose production," Bolkan says. "We're also looking at leaf
wetness, because powdery mildew can't grow on totally saturated leaves.
"Through a telemetry system that connects us to the commercial greenhouses
participating in our project," says Bolkan, "we receive readings
every 15 minutes. Scouts hired by cooperative extension advisers helping with
this study visit these greenhouses every week and inspect plants for powdery
mildew incidence. We can compare their on-site observations with the off-site
forecasts we've made here in the laboratory and can see what needs to be done
to improve the accuracy of the model."
In addition to ARS, other sponsors of the research include the State of
California Department of Pesticide Regulation; Roses, Inc.; the American Floral
Endowment; and the California Cut Flower Commission.By
Wood, Agricultural Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
Edwin L. Civerolo is in the
USDA-ARS Crops Pathology and Genetics Research Unit, 378 Hutchison Hall,
University of California, Davis,
CA 95616; phone (530) 754-8694, fax (530) 754-7195.
"Rosy Outlook for Greenhouse Roses" was published in the
2001 issue of Agricultural Research magazine.