South African
Avocado Growers’ Association Yearbook 1987. 10:69-71.
Proceedings of
the First World Avocado Congress
University of California, Riverside,
California 92521, USA
Herbicides have been developed
for use in avocado culture to either replace or supplement other weed control
methods such as hoeing, tillage and mowing. Residual, soil-active, preemergence
herbicides provide long-term control of weeds arising from seeds. Postemergence
contact herbicides control annual weeds that escape soil residual herbicide
treatment. Foliar-active, translocated herbicides may be used to control
perennial weeds. The best combination of herbicides varies with orchard
conditions. Proper co-ordination of herbicide and other weed control methods is
required to obtain maximum benefits from, and to reduce problems associated
with, each method of weed control. Benefits derived from proper use of
herbicides are varied. Herbicides can be used where cultivation is not possible
or desirable. Injury to trees by equipment is reduced. Destructive effects of
tillage on soil structure is decreased and soil erosion is less than for tilled
soil. Orchard management and pest control are easier in non-tilled orchards
without cover crops. Problems from misuse of herbicides may also occur. These
include poor weed control, tree damage, injury to non-target organisms and
contamination of the environment.
Research has been done on the use of herbicides for weed control in
avocado culture in California. The project has continued for about 30 years and
has included over 100 herbicides. The research has involved all aspects of
herbicide development, ranging from greenhouse screening to field efficacy
trials, residue analysis and developing recommendations for avocado growers.
The basic research procedures have remained the same with minor changes, made
as needed to fit different field or laboratory situations.
Small avocado plants, about 30 cm tall, are placed in containers with a
standard soil mix. Soil residual herbicides are mixed directly into the soil,
while various concentrations of foliar-active herbicides are sprayed directly
onto the leaves. The plants are rated for herbicide symptoms and harvested for
weight and size measurements.
Three-tree replicated plots are established in 3- to 10-year-old
orchards. The herbicides are applied at different rates, either directly to the
soil or to the weeds up to 5 m on each side of the base of trees. Trees are
visually rated for injury symptoms and fruit are collected as needed at various
intervals for herbicide residue analysis.
Useful results from the numerous herbicide trials will be summarised in
this manuscript, instead of providing many tables concerned with individual
experiments. The summaries will cover soil residual, contact and translocated
herbicides.
Those that may be used in avocado culture, include diuron, napropamide,
norflurazon, oryzalin and simazine.
Each herbicide has a spectrum of weeds that it controls and other weeds
that are either partially controlled, or not at all. They are applied directly
to the soil surface and moved into the soil, where they kill weed seedlings.
Diuron
(N'(3,4-dichlorophenyl)-N,N-dimethylurea) - this may safely be used as a
pre-emergence application to the soil surface for the control of many annual
weeds. Registration of diuron use in avocados is being considered by the United
States of America regulatory agencies. The herbicide must be moved into the
soil either by rainfall, or overhead irrigation within one month after
application to achieve maximum weed control. Avocado foliage sprayed with the
herbicide may be damaged under high temperature conditions; also when a
surfactant is added to the spray mixture to increase contact activity to small
annual weeds. Diuron should not be used near avocado trees grown in very sandy
or gravelly soils with low organic matter content, since severe injury may
occur to the trees. While diuron controls many annual broadleaved and grassy
weeds, it does not control some of the Medicago
and Euphorbia species.
Napropamide
(N,N-diethyl-2-(1-naphthalenyloxy)propanamide) - this is a relatively unstable
pre-emergence herbicide. Controls weeds best if incorporated into the soil by
either rainfall or sprinkler irrigation within three days after application.
Fall applications followed by rainfall, are more effective than spring applications.
About 2 cm of water after application, provides adequate movement of the
herbicide into the soil. Too much water immediately after application, may
reduce weed control. Herbicide injury has not been noted, even on newly-planted
trees. Napropamide does not control some cool-season, winter-annual weeds such
as those in the Cruciferae family and it is weak on some summer annuals such as
those in the Solanaceae family. The herbicide has desirable soil-residual
properties and controls most annual grasses very well. Repeated annual
applications suppress Cyperus esculentus.
Norflurazon
(4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3-(2H)-pyridazinone) -
this is being considered for registration for use in avocado culture in the
United States of America. It is a pre-emergence herbicide that requires
incorporation into the soil, using over 2 cm of water either by overhead
irrigation or rainfall. Young trees may be damaged in very sandy soils with low
organic matter content. The herbicide effectively controls annual grassy and
several broadleaved weeds and suppresses some important perennials. Several
common broadleaved weeds, however, are not controlled by this herbicide.
Oryzalin
(4-(dipropylamino)-3,5-dinitrobenzenesulfonamide) - this is a relatively broad-spectrum
pre-emergence dinitroaniline herbicide. It does not require mechanical
incorporation, if either rainfall or overhead irrigation occurs within 30 days
after application. The amount of water required for best weed control varies
from 1,5 to 5 cm, depending upon soil conditions, mainly texture. The herbicide
leaches only a short distance into soils. Trees are not injured by the
herbicide, even in sandy soils with low organic matter content. Some weeds in
the Cruciferae and Compositae. families are only partially controlled by
oryzalin.
Simazine
(6-chloro-N,N'-diethyl-l ,3,-5 triazine-2,4-diamine) - a symmetrical triazine,
it is widely used in avocado culture. It is relatively non-volatile, has low
water solubility and low toxicity to organisms without photosynthetic
mechanisms, The herbicide is applied to the soil surface and must be moved into
the soil by either overhead irrigation or rainfall within one month after
application. Simazine is not toxic to avocado foliage and is safe to use around
established trees in sandy loam or heavier soils with more than one per cent
organic matter. Simazine does not completely control some summer annual
grasses. Other weeds in the Amaranthaceae, Chenopodiaceae and Compositae have
developed resistance to simazine after several years of repeated use.
These are used to control weeds that escape soil residual herbicide
treatments. They are effective against small annuals, but do not control
perennial weeds. Weed oil was once the most widely used contact weed control
agent, but since the oil price has increased, use of weed oil declined and
other contact herbicides, such as paraquat, have replaced it.
Paraquat
(1,1'-dimethyl-4,4'-bipyridinium ion) - a post-emergence contact herbicide, it
gives best control of annual weeds when they are in the two- to four-leaf
stage. Older weeds require higher herbicide rates and the addition of a
surfactant for adequate control. Best results are also obtained under low-light
conditions and during warm weather. Weeds that are stressed and those with waxy
and hairy surfaces, are sometimes tolerant to paraquat. It is compatible with
many soil residual herbicides and can be used as a tank mix to control growing
weeds. The herbicide damages avocado trees if it comes into contact with the
foliage or green bark. Paraquat is toxic to humans and must be used with
caution.
These herbicides are used to control annual and perennial weeds that
escape soil-residual and contact herbicides. They can move from the point of
application throughout the entire plant and into the underground portions and,
thus, kill persistent perennials. Although many translocated herbicides have
been tested for use in avocado culture, only a few are both effective and safe
enough for general use.
Fluazifop (+)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]-propanoic
acid) - this is one of the herbicides being developed to be used specifically
for grass control without damage to broadleaved plants. It can be applied
directly to the foliage of avocados without injury to the trees. Grasses under
environmental stress are difficult to control, therefore they should be
vigorously growing when the herbicide is sprayed to the foliage. After
tillering and seedhead formation control is substantially decreased, addition
of either a crop oil concentrate or non-ionic surfactant is often required for
best herbicidal activity.
Glyphosate
(N-(phosphonomethyl)glycine) - it can control virtually any weed if directed
treatment onto weed foliage is possible. Contact of the herbicide with the
foliage or green bark of most plants can result in serious damage. Avocados are
somewhat tolerant to glyphosate and minor contact through drift or minimum
spray to the lower foliage does not cause severe damage. Glyphosate injury
symptoms appear slowly and may not occur for several days, either on the
avocados or weeds. Plants that are either under moisture stress, or in a
dormant state, are difficult to control with glyphosate. Single glyphosate
treatments will kill most simple annuals, but repeated treatments are required
for control of perennial weeds. Even though glyphosate is a translocated
herbicide, virtually none of the parent chemical and its primary metabolite
have been found in avocados treated under field conditions.
These are used when individual herbicides do not control the weeds at
rates safe for the trees. When annual weeds escape a soil-residual herbicide,
different choices are available:
(1) Combination with another soil residual herbicide to broaden the spectrum of weeds controlled. Combinations of simazine with either diuron, flurazon or oryzalin, may be used to take advantage of the weed control spectrum of each herbicide, while the dosage levels of all are retained at the minimum required for best results.
(2) Addition of a contact herbicide such as
paraquat, to control annual weeds that escape the residual herbicides.
(3) Post-emergence treatment with a translocated
herbicide to control difficult annual and perennial weeds that are established
in the orchard. Glyphosate mixes well in sprays with several of the
soil-residual herbicides and thus it is possible to control germinating weeds
by the soil-residual herbicide and growing weeds by the translocated herbicide.
This is best practised for taking advantage of the best weed control
properties of each herbicide, to avoid increased populations of weeds that are
tolerant to the herbicide and to prevent the build-up of weeds that become
resistant to specific herbicides. An example of a typical herbicide rotation
includes the following sequence:
(1) Early in spring, glyphosate is applied to control established weeds.
(2) Late in spring, diuron is applied to control
early season germinating grasses and some broadleaved weeds.
(3) During early to mid-summer, paraquat is applied
to control annual weeds that escaped the diuron treatment.
(4) Early fall applications of simazine are made to
control germinating broadleaved weeds. At this time, the surviving perennial
weeds are treated a second time.
Variations in the use of chemicals and the time of their application
will be used in different orchards depending upon weeds and environmental
conditions.
Weed control programmes involving herbicides in avocado culture, require
careful advance planning and continual monitoring to achieve maximum
effectiveness. A first consideration involves the type of weeds present and
their susceptibility to the herbicides available for use in the orchard. The
nature of the soil should also be considered, since herbicides leach more in
sandy soils than in heavy clay soils. Their persistence is shorter in soils
with high organic matter and in environmental conditions that favour microbial
decomposition of the herbicide. Crop damage from soil-residual herbicides is
more likely in sandy, low organic matter soils.
The physiological status of the plants will also affect herbicide
toxicity. Time of application of post-emergence herbicides is often critical
for best weed control. Young, vigorous, rapidly growing plants are generally
easier to control than old, mature and stressed plants. Translocated growth
regulator herbicides, such as glyphosate, should be applied to perennial weeds
when photosynthates are moving into the underground portions of the plants.
Soil-residual herbicides that are transported in the xylem, will move more
rapidly to the leaves when conditions are favourable for water movement
throughout the plant.
When the same herbicides are applied to the same field for several years, the tolerant and resistant weeds become more prevalent. Avocado orchards should be monitored periodically to determine the nature of the vegetative shift. Herbicides used should be changed as necessary to maintain control of the shifting weed population. Perennial weeds will become the dominant vegetation in orchards repeatedly treated with soil-residual herbicides alone. Some grasses will increase in simazine-treated orchards. Herbicide rotation and combination may be used to overcome many of the problems associated with weed population shifts.
CONCLUSION
Herbicide programmes for complete weed control in avocado orchards are
feasible, especially if used in conjunction with other available weed control
methods that are adapted to the locality.