Aphid-borne viruses and silverleaf
whitefly can completely devastate cantaloupe crops, causing serious
economic losses to growers. At the UC Kearney Research and Extension
Center (KREC), we studied the use of reflective plastic mulch and
straw mulch to manage aphid-borne viruses and silverleaf whitefly
in cantaloupes without the use of pesticides.
Late-season cantaloupes are grown on approximately 5,000 acres on
the West Side of the San Joaquin Valley. While susceptible to a number
of diseases and insect pests, by far the most important problem over
the past several years has been the complex of aphid-borne viruses,
primarily cucumber mosaic virus (CMV), watermelon mosaic virus (WMV)
and zucchini yellow mosaic virus (ZYMV), which vary in severity by
season. Disease severity increases as the growing season progresses,
and fall is the worst time of the year for all of the viruses. The
viruses kill and stunt plants, reducing yield.
These diseases are transmitted by several aphid species, and they
are characterized by leaf symptoms including severe distortion (crinkled,
puckered and misshapen), yellowing, and light and dark-green mottling
(mosaic symptoms). These viruses may occur singly or in combination
and can only be distinguished from each other serologically.
Whiteflies damage cantaloupe by sucking the juices out of the plants,
which can kill young plants and severely stunt older ones, resulting
in almost no fruit set. Silverleaf whitefly transmits a number of
viruses (Gemini viruses).
Currently there are no cantaloupe varieties resistant to the aphid-borne
viruses. In addition, insecticides offer little relief because the
viruses are acquired by the aphids and transmitted to the plants within
seconds, long before the aphid vector acquires a lethal dose of insecticide.
Insecticides may actually enhance the spread of aphid-borne viruses
by stimulating vector activity. Many insecticides stimulate the aphid’s
nervous system, causing it to move from plant to plant very rapidly.
This results in the infection of more plants than would occur in those
visited by a nonintoxicated aphid, which settles down and feeds on
one plant before moving to another one. This occurs before the insecticide
has a lethal impact on the aphid. In contrast, whitefly infestations
can be somewhat relieved by imidacloprid, a systemic insecticide.
However, the development of resistance to imidacloprid among whiteflies
is a major concern.
Reflective, metalized plastic mulch, formed by adhering a thin coat
of aluminum ion to a sheet of polyethylene, has been shown to help
control other types of aphid-borne viruses as well as whiteflies.
This control is due to the fact that reflective plastic mulch reflects
ultraviolet (UV) wavelengths, unlike black or clear plastic mulches.
Flying aphids and whiteflies are repelled by these UV wavelengths.
The outcome is to delay and reduce the incidence of aphid-borne viruses.
The onset of virus disease symptoms can be delayed by 3 to 6 weeks
in plants grown over this mulch, which was critical for normal flowering
and fruiting. In addition, reflective plastic mulch delayed and reduced
the severity of silverleaf whitefly infestations in zucchini squash,
pumpkins and cucumber. This mulch was as effective as a preplant application
of imidacloprid in managing whiteflies.
Reflective plastic mulch can be applied with ordinary mulch-laying
equipment. In addition, a version of this mulch has a lattice network
of openings down the center and can be applied following machine planting;
the plants then emerge through openings. (Refelective plastic mulch
is slightly more expensive than black plastic mulch, but the latter
does not work to repel aphids or whiteflies.)
As part of a conservation tillage study, we also found that wheat
straw mulch can help manage apid-borne viruses and whitefly in cucurbits.
In zucchini squash grown over straw mulch , yields were as high and
the incidence of aphid-borne virus diseases was no greater than in
plants grown over reflective plastic mulch. Plants grown over straw
mulch produced higher yields than those grown over plots that had
received a preplant application of imidacloprid. Straw mulch also
deterred colonization by silverleaf whitefly and reduced the incidence
of squash silverleaf.
Field study, sampling protocols
In studies at KREC, the effectiveness of reflective plastic and wheat
straw mulches for the management of aphid-borne viruses and silverleaf
whitefly in cantaloupe were compared to conventional bare soil production.
The study field was prepared for planting using conventional procedures:
disking, preirrigation, fertilizer application (500 pounds per acre
of 15-15-15 [nitrogen-phosphorus-potasium]), herbicide application
(soil incorporated bensulide [Prefar] at 6.5 quarts per acre) and
bed shaping (60 inch beds). The reflective plastic mulch was applied
using standard mulch laying equipment. Surface drip-tape was laid
down the center of each bed under the plastic. Drip tape was also
placed down the center of all remaining beds. The wheat straw was
spread by hand. To accomodate seeding, holes were cut every 30 inches
into the plastic mulch and the straw was “scratched” aside
every 30 inces down the center of the beds.
In summary, the occurrence of aphid-borne virus diseases was significantly
reduced with both mulches as opposed to bare soil, and reflective
plastic performed better than wheat straw. Silverleaf whitefly numbers,
both adults and nymphs, were reduced equally by plastic mulch and
wheat straw, and were significantly lower than with bare soil. Reflective
plastic produced mature melons sooner and more cartons per acre than
the other production systems. The reflective plastic system also produced
a greater number of large-sized melons, which are favored in the late-season
market. Plants grown over straw mulch produced higher overall yields,
including large-size melons, than those grown over bare soil.
