Tegniese Nuus • Technical News
Series on Volunteer Potatoes
IV: POSSIBILITIES FOR THE CHEMICAL
CONTROL OF VOLUNTEER POTATOES
Dr James and Anette Allemann (University of the Free State)
Volunteer plants are a major problem as they are extremely
aggressive weeds that compete with crop plants for water,
nutrients and light. This can lead to a reduction in yield that
is directly proportional to the numbers of volunteer plants
found on the land – the greater the numbers, the greater
the yield reduction. Even in maize, a plant considered
to be the best in dealing with competition from volunteer
potatoes, yield losses can amount to between 23 and 62%.
The longer volunteer plants are on the land, the worse
the competition effect, and the larger the yield reduction.
Control of these plants is critical within the first 2 to 6
weeks after planting, as this is the period when most
crops are extremely sensitive to weed competition. Also,
if not controlled in time, these volunteer plants will initiate
tubers that will continue the problem for a number of years
following the potato crop, and this can lead to the total
failure of the crop rotation system to control potato pests
and diseases prior to planting the next potato crop.
The previous article in this series discussed the use of nonchemical methods to control volunteer potato plants, whilst
this article will address the possibilities of chemical control.
There are a number of possibilities in this regard, such as
the use of sprout inhibitors, soil fumigants and herbicides,
both pre- and post-emergence.
Sprout inhibitors
These products are usually applied to prevent tubers from
sprouting during storage, but can also be applied to plants
at the end of the growing season to prevent the formation
of the small undesirable tubers that are often the source of
volunteer plants. Sprout inhibitors should only be used by
table producers and never applied close enough to seed
potato plantings so that spray drift could contaminate the
seed potato fields.
A number of products are available to suppress tuber
sprouting such as chlorpropham and maleic hydrazide.
Chlorpropham is registered in South Africa (Act 36 of
1947) for use as a sprout inhibitor during storage, even
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CHIPS • Januarie/Februarie 2015
though it also has herbicidal activity against potatoes in the
field. Malic hydrazide is possibly the best known sprout
inhibitor and is used successfully in commercial potato
plantings throughout the world. In South Africa, however,
it is only registered for the prevention of sprouting in stored
onions.
In order to prevent sprouting of volunteer tubers the
product needs to be applied as a full cover spray as it is
absorbed by the foliage and translocated to the tubers
where it remains and prevents the buds from sprouting,
so preventing development during the following season.
If applied early after tuber initiation it not only prevents
sprouting, but also tuber growth. The product has been
shown to reduce the numbers of volunteer plants by 70 to
80% if used correctly, but the results depend on the cultivar
used as well as the size of the tubers. Although sprouting
and shoot emergence from all size tubers are suppressed,
the effect is unfortunately the lowest in the smaller tubers,
i.e. those that cause the greatest problem.
Soil fumigation
This involves the use of a volatile chemical, mainly to
suppress nematodes and other soil pathogens. However,
certain of these products have been shown to markedly
reduce tuber viability and suppress the growth of volunteer
potato plants. It is difficult to obtain uniform applications
of these fumigants, and together with uneven distribution
through the soil, caused by soil type and conditions
coupled with the use of sub-lethal applications rates, can
result in poor performance and variable results.
Some well-known soil fumigants are ethyl dibromide
(EDB), metam sodium (MS) and metam potassium (MP),
all of which are registered for use on seedbeds in South
Africa. EDB is more effective when applied during
cooler winter months, but requires specialised application
techniques, while MS and MP can be applied through an
irrigation system. At high enough dosages more than 90%
suppression of volunteer plants can be obtained using MS
and MP (Figure 1). An added advantage of using soil
Tegniese Nuus • Technical News
Figure 1Effect of the soil fumigant metham potassium at a rate of 1 035 kg ai ha-1 on the emergence of potato plants
Figure 2An example of the variation in control of potatoes, from total death of plants, through severe growth inhibition to
growth suppression by a pre-emergence herbicide
fumigants is that they also reduce the numbers of annual
weeds by more than 90%, and are effective on both
dormant as well as non-dormant tubers.
Although very effective, fumigants are fairly costly, thereby
making them viable only on small areas, or preceding
high value crops. However, factors such as soil moisture
and temperature during application, as well as proper
placement and dispersal of the product have an effect on
the dosage of the fumigant required, and this is an aspect
requiring further research.
Herbicides
Herbicides are manufactured and used to deal with
specific types of weeds in specific cropping situations.
Products can be either selective, controlling certain plants
without damaging others, or non-selective, killing all
plants. Application times for the products also vary, being
classified as pre-plant, pre-emergence or post-emergence
depending on the timing of application during the cropping
cycle. Herbicides can also be classified as systemic or
contact products, with the former being absorbed by
CHIPS • January/February 2015
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Tegniese Nuus • Technical News
plants and killing them over time, while the latter usually kill
foliage on contact.
tuber production is through complete shoot removal prior to
the shoots initiating tubers.
Ongoing research shows that volunteer potato plants
are very difficult to eradicate using herbicides, with most
products tested proving to be either ineffective or only
partially effective (Figure 2). The greatest problem is
caused by the biology of the potato tuber, as large food
reserves available in the parent tuber, coupled with a
number of adventitious buds that can sprout after the death
of the apical sprout, enable recovery from damage that
would be lethal to most other weeds.
Thus far the use of single conventional herbicides has
proven to be unsuccessful in the control of volunteer
potatoes. Due to the devastating effect these plants
have on succeeding crops, various regimes of herbicide
combinations have been researched with variable results.
The problem is further compounded by the variation in
the time of emergence of volunteer potato plants, which
makes application timing of many post-emergence (foliage
applied or contact) products very difficult in order to obtain
good control. Most contact herbicides do nothing more
than kill the shoots that have emerged from the soil at the
time of application, and the tuber is able to produce new
sprouts which then emerge long after the primary plants
have been killed. If these shoots are not treated, it will
result in the production of tubers that will create further
problems the following season. The only way to prevent
Moontlikhede van chemiese beheer van
opslagaartappels
Hierdie artikel bespreek verskillende chemiese
metodes (spruitinhibeerders, grondberoking en
onkruiddoders) wat aangewend kan word om
opslagplante te beheer.
• Alhoewel spruitinhibeerders soos
malïenhidrasied die voorkoms van opslag plante tot 80% kan verlaag wanneer dit reg
aangewend word, kan dit nie in die omgewing
van saadaartappellande gebruik word nie.
Ongelukkig is die effek van hierdie middels op
klein knolle, die grootste bron van opslag plante, beperk.
• Grondberokingsmiddels soos metam natrium
kan knol-kiemkragtigheid merkwaardig verlaag
en ook die groei van opslagplante onderdruk.
Dit kan die getalle van beide opslagaartappel plante en eenjarige onkruid met tot 90%
verlaag indien dit reg aangewend word en
teen hoog genoeg toedieningshoeveelhede.
Hierdie behandelings is egter duur, en die
resultate kan deur verskeie faktore soos
grondvog, temperatuur en kultivar beïnvloed
Page 52 •
CHIPS • Januarie/Februarie 2015
Conclusion
The chemical control of volunteer potato plants is very
difficult due to the numerous factors which play a role,
including climatic and soil factors, the biology of the tuber
and time of tuber initiation. The use of various chemicals in
isolation from other methods of control reduces the chances
of successful control of these troublesome weeds. The best
results in volunteer potato control are normally obtained
when all methods of control are combined in an integrated
management program.
The next article in the series will discuss the use of preemergence herbicides for the control of volunteer potato
plants. C
word.
• Die biologie van die aartappelknol veroorsaak
beheerprobleme deur onkruiddoders,
want hierdie plante kan skade oorleef wat die
meeste ander onkruide sal dood. Die variasie
ten tye van verskyning van opslagplante maak tydsberekening van na-opkoms
onkruiddodertoediening moeilik, en enige
oorblywende plante kan knolle produseer wat
tot toekomstige probleme kan lei.
• Enkel-onkruiddoders is tot nou sonder
sukses aangewend in die beheer van
opslagaartappelplante, en kombinasies
van onkruiddoders het wisselvallige resultate
gelewer.
• Dit is dus duidelik dat chemiese beheer
van opslagplante moeilik is as gevolg van die
talryke faktore wat ’n rol speel.
• Die gebruik van verskeie chemikalieë in
afsondering verlaag die kanse op suksesvolle
beheer.
• Die beste resultate in opslagbeheer word
verkry wanneer alle beheermetodes
gekombineer word in ’n geïntegreerde
bestuursprogram vir opslagplante.