Southeastern U.S. Vegetable Crop Handbook - 2021 - 39

are usually based on pest populations, field history, stage of crop's
development, variety, weather conditions, life stage of the pest, parasite,
and/or predator populations, resistance to chemicals, time of
year, and other factors. Specific thresholds are given in this handbook
for a number of pests of many crops.
Insect population sampling techniques include:
* Visual observation. Direct counts of any insect stages (eggs,
larvae, adults, etc.) are accomplished by exam ining plants or
plant parts (leaves, stems, flowers, fruits). Counts can be taken
on single plants or a pre scribed length of row, which will
vary with the crop. Usually, quick moving insects are counted
first, followed by those that are less mobile.
* Shake cloth (also known as a ground cloth). This sampling
procedure consists of using a standard 3-foot by 3-foot shake
cloth to assess insect popula tions. Randomly choose a site
without disturbing the plants and carefully unroll the shake
cloth between two rows. Bend the plants over the cloth one
row at a time and beat the plants vigorously to dislodge insects
held on stems, leaves, and branches. Count only insects
that have landed on the shake cloth. The number of sampling
sites per field will vary with the crop.
* Sweep net. This sampling procedure uses a standard 15-inch
diameter sweep net to assess insect popula tions. While walking
along one row, swing the net from side to side with a
pendulum-like motion to face the direction of movement.
The net should be rotated 180 degrees after each sweep and
swung through the foliage. Each pass of the net is counted as
one sweep. The number of sweeps per field will vary with the
crop.
Weed population sampling techniques include:
* Weed identification. This first step is frequently skipped. Perennial
weeds and certain serious annual weeds should be
controlled before they can spread. Common annual weeds
need only be controlled if they represent a threat to yield,
quality, or harvestability. It is critical to know the weed history
of a field prior to planting as many herbicides need to be
applied pre-plant. Your county Extension office is an excellent
resource for reliable weed identification.
* Growth stage determination. The ability of weeds to compete
with the crop is related to size of the weed and size of
the crop. Control of the weed using herbicides or mechanical
methods is also dependant on weed size. A decision to
control or not to control a weed must be carried out before
the crop is affected and before the weed is too large to be
controlled.
* Weed population. Weeds interfere with crop production in many
ways. They may compete for light, water, nutrients, and
space. The extent of this interference is dependant on population
and is usually expressed as weeds per foot of row or
weeds per square meter. Control measures are needed when
the weed population exceeds the maximum tolerable population
of that species.
Disease monitoring involves determining the growth stage of
the crop, observing disease symptoms on plants, and/or the daily
weather conditions in the field.
Disease control is often obtained by applying crop protectants
on a regular schedule. For many diseases, application must begin
at a certain growth stage and must be repeated every 7 to 10 days.
When environmental conditions are favorable for disease development,
delaying a spray program will result in a lack of control if
the disease has progressed too far. For certain diseases that do not
spread rapidly, fields should be scouted regularly.
Predictive systems are available for a few diseases. Temperature,
rainfall, relative humidity, and duration of leaf wetness period
are monitored, and the timing of fungicide application is determined
by predicting when disease development is most likely to
occur. One such program for Downy Mildew is available at
http://cdm.ipmpipe.org/
Weather Conditions. These are important to consider before applying
a pesticide. Spray only when wind velocity is less than 10 miles
per hour. Do not spray when sensitive plants are wilted during the
heat of the day. If possible, make applications when ideal weather
conditions prevail.
Certain pesticides, including biological insecticides (e.g. BT's)
and some herbicides, are ineffective in cool weather. Others do not
perform well or may cause crop injury when hot or humid conditions
are prevalent. Optimum results can frequently be achieved
when the air temperature is in the 70°F range during application.
Strive for Adequate Coverage of Plants.
Improved control of aphids can be achieved by adding and
arrang ing nozzles so that the application is directed toward the
plants from the sides as well as from the tops (also see Alkaline Water
and Pesticides, which follows). In some cases, nozzles should
be arranged so that the application is directed beneath the leaves.
As the season progresses, plant size increases, as does the need for
increased spray gallonage to ensure adequate coverage.
Applying insecticide and fungicide sprays with sufficient spray
volume and pressure is important. Spray volumes should increase
as the crop's surface area increases. Sprays from high-volumehigh-pressure
rigs (airblast) should be applied at rates of 40 to 200
gallons per acre at 200 psi or greater. Sprays from low-volumelow-
pressure rigs (boom type) should be applied at rates of 50 to
100 gallons per acre at 20 psi. The addition of a spreader-sticker
improves coverage and control when wettable powders are applied
to smooth-leaved plants, such as cole crops and onions.
Use a dedicated sprayer for herbicides and a different sprayer
for fungicides and insecticides. Herbicide sprays should be
applied at between 15 and 50 gallons of spray solution per acre
using low pressure (20 to 40 psi). Never apply herbicides with a
high-pressure sprayer that was designed for insecticide or fungicide
application because excessive drift can result in damage to nontarget
plants in adjacent fields and areas. Do not add oil concentrates,
surfactants, spreader-stickers, or any other additive unless specified
on the label, or crop injury is likely.
Select the Proper Pesticide. Know the pests to be controlled and
choose the recommended pesticide and rate of application. When
in doubt, consult your local Extension office. The herbicide choice
should be based on weed species or cropping systems.
2021 Vegetable Crop Handbook for Southeastern United States
39
http://cdm.ipmpipe.org/
Southeastern U.S. Vegetable Crop Handbook - 2021

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