Drainage and irrigation
Excess irrigation and rainfall are considered to be the
most important factors that increase the severity and
spread of Phytophthora-incited diseases. In turn, the
duration of free water, in soil or on foliage or fruit is
the most important environmental factor in the
development of disease caused by Phytophthora
because it is during this time that propagules
proliferate and infect (Erwin and Ribeiro 1996). In
addition, zoospores, cysts and chlamydospores
travel in the soil in irrigation water, rainfall run-off
and movement of soil. Orchards should be
established on land that is well-drained and not
subject to flooding. Therefore, sloping ground is
preferable. Ideally, the soil should be drained to a
depth of 1.5 metres. Mounding of the soil around the
tree promotes good drainage (Broadley 1992). Row
crops should be planted on raised beds to prevent
free water from contacting the plants (Erwin and
Ribeiro 1996). To reduce the rate and extent of buildup
of inoculum, plants should be irrigated less
frequently so that free water drains away (Lutz et al.
1989). In areas where rainfall is the main source of
water, optimal horizontal and vertical drainage are
necessary to prevent water-logging. Spraying water
on the trunks of trees should be avoided as
constantly wet bark may encourage the
development of cankers.
Organic amendments and mulching
Mulching stimulates plant root growth, increases
nutrient uptake, decreases evaporation from the soil,
increases soil-water holding capacity, reduces
surface water run-off, facilitates drainage, regulates
soil temperature, and provides a high level of
nutrients for soil microbes (Aryantha et al. 2000).
Amendments can either enhance or suppress
disease, depending on their nature. Phytophthora is
inhibited by alfalfa meal, cotton waste, soybean
meal, wheat straw, chicken manure and urea.
Ammonia and volatile organic acids released by
decomposing organic matter kill Phytophthora, and
the residual organic matter stimulates competitive
and antagonistic microorganisms in the soil
(Lazarovits et al. 2001). While these mechanisms
suppress the growth of Phytophthora, they may also
create phytotoxicity to the plant roots, making them
less attractive to colonisation by the pathogen
(Erwin and Ribeiro 1996). Aryantha et al. 2000
showed that the addition of fresh or composted
chicken manure to potting mix significantly reduced
the survival of P. cinnamomi and the development of
disease in lupin seedlings. Chicken manure more
effectively suppressed P. cinnamomi and plantdisease
symptoms than did cow or sheep manure.
All composts increased soil organic matter, total
biological activity, and populations of antagonistic
actinomycetes, fluorescent pseudomonads, and
fungi. However, chicken manure also stimulated the
production of endospore-forming bacteria, which
was positively correlated with lupin seedling
survival. The addition of composted manures is
necessary for disease development but it is not
sufficient for biological control. Mulches may also
reduce the impact of phytophthora root rot if used
from the time of orchard establishment or if the
disease is not too far advanced. The ‘Ashburner
system’, based on improved drainage and mulches,
has been successfully employed to manage
phytophthora root rot of avocados (Broadley 1992).
Chapter 7.3 reports that mulches are also effective in
managing phytophthora root rot of papaya.
Companion and cover cropping
Companion cropping can reduce the impact of
phytophthora diseases. For example, in the
subtropics of Australia, banana and avocado are
planted together. The bananas provide mulch and
reduce soil water after heavy rain. This system
reduces the impact of root rot caused by P. cinnamom
Care must be taken to choose a companion crop that
does not compete too heavily with the orchard crop.
Cover crops, when incorporated into the soil, increase
the amount of organic material, which encourages the
growth of microbes that suppress Phytophthora
(Broadley 1992).
Fertilisers
Some forms of nitrogen have been shown to favour
an increase in disease, while other forms suppress
disease (Schmitthenner and Canaday 1983).
Generally, the role of fertilisers or nutrients in
controlling or suppressing phytophthora diseases is
unclear. Some reports indicate that fertilising
improves plant vigour and hence resistance to
disease, while others indicate that pathogen
infection is favoured because of improved plant
Fungicides
Protectant
Bordeaux mixture
This is perhaps one of the oldest known fungicides,
formulated in 1885 by Millardet to control the
Oomycete Plasmopara viticola, which causes downy
mildew on grapevine (Millardet 1885). Bordeaux
mixture has been used to successfully control many
diseases caused by different species of Phytophthora.
The fungicide adheres well to foliage, but has a
disadvantage in that its active ingredient, copper,
can have a significant toxic affect in some plants and
non-target organisms (Brown et al. 1998). In
addition, Bordeaux mixture is a combination of
copper sulphate and calcium hydroxide, and thus is
somewhat labour-intensive to prepare and apply
(Erwin and Ribeiro 1996). Also, in tropical areas with
high rainfall, the fungicide may be washed off.
Systemic
Phenylamides (acylanilides)
This group of chemicals includes furalaxyl
(Fongarid), metalaxyl (Ridomil) and benalaxyl
(Galben). All three chemicals are active against the
Peronosporales, but metalaxyl is the most widely
used (Erwin and Ribeiro 1996). This fungicide is a
xylem-translocated compound with an upward
movement in plants in the transpiration stream
(Edgington and Peterson 1977). Thus, metalaxyl and
related acylanilide compounds have no effect on root
diseases if applied as a foliar spray because they are
not transported to the roots. Metalaxyl is usually
applied as a soil drench and it is very effective (Guest
et al. 1995). Due to its systemic nature, metalaxyl is
transferred from seed, roots and leaves to new growth
(Cohen and Coffey 1986) and is therefore effective at
controlling infection beyond the roots. Metalaxyl is
water soluble, and is effective against all species of
Phytophthora in vitro at much lower doses than
protectant fungicides. The biochemical mode of
action of metalaxyl involves inhibition of RNA
synthesis. It is highly inhibitory to sporangium
formation, and also reduces chlamydospore and
oospore formation (Cohen and Coffey 1986). It also
has a high level of persistence within the plant. The
presence of metalaxyl within the plant can prevent
colonisation of leaf tissue by mycelium, because it
inhibits the growth of hyphae (Erwin and Ribeiro
1996).
There are several disadvantagesof using metalaxyl
and related compounds: (i) root drenching is a
wasteful method of fungicide application; (ii)
chemicals are released into soil and water systems;
(iii) soil microorganisms rapidly degrade metalaxyl,
reducing its persistence and effectiveness (Guest et
al. 1995); and (iv) resistance has developed to it
among populations of Phytophthora, particularly
P. infestans (Cohen and Coffey 1986). The issue of
metalaxyl-resistance has been partially addressed by
application of metalaxyl in combination with a
protectant fungicide, limited application of
metalaxyl during a given growing season, and not
using the fungicide for curative or eradicative
purposes (Erwin and Ribeiro 1996).
Phosphonates
This group of compounds is active against the
Peronosporales. The term ‘phosphonate’ refers to
the salts and esters of phosphoric acid that release
the phosphonate anion in solution. Phosphonates
are prepared by partially neutralising phosphorous
acid (H3PO3) with potassium hydroxide. In this text,
phosphonates will be referred to in a general context,
and mention will also be made of a specific
formulation of phosphonate, fosetyl-Al. Marketed
under the name Aliette, this compound contains an
aluminium salt of phosphonate (Cohen and Coffey
1986).
Phosphonates are xylem- and phloem-translocated
(Ouimette and Coffey 1990), with both downward
and upward movement in the host. They are nonpersistent
in the environment, as they are readily
oxidised to phosphate by soil microbes, and they
also have very low mammalian toxicity. The precise
mode of action of phosphonates is unknown, but it is
believed that they disrupt phosphorus metabolism
in the pathogen, causing fungistasis and the
consequent activation of the host defence responses
(Guest et al. 1995).
The presence of phosphonate at concentrations
below those required to inhibit mycelial growth in
vitro disrupts the virulence of the pathogen, causing
the release of stress metabolites that elicit host
defences. The consequence is that many plant
species treated with phosphonates respond to
inoculation as though they were resistant. Hence,
the effectiveness of phosphonates against plant
diseases caused by Oomycetes depends on both the
sensitivity of the pathogen to phosphonate and the
capacity of the defence responses of the host.
Therefore, there is a ‘complex mode of action’ in
response to phosphonate treatment (Guest et al.
1995).
Because of the complex mode of action of
phosphonates, results obtained from one hostcultivar
combination cannot be extrapolated from
results with analogous combinations. This is
because of the great variation in sensitivity of
different isolates of a single Phytophthora species. In
addition, phosphonate efficacy differs among host
cultivars or species, perhaps due to differences in the
type or extent of defence responses in the hosts
(Guest et al. 1995). Although the fungistatic effect of
phosphonates is not confined to the Oomycetes, it is
inexplicably variable in its effect against some
species of Phytophthora. For example, fosetyl-Al is
active against tuber rot caused by P. infestans, but is
not very effective in controlling the foliar phase of
late blight of potato (Erwin and Ribeiro 1996),
possibly indicating the activation of tissue-specific
resistance mechanisms.
Because phosphonates are phloem-translocated,
they can be applied to any part of the plant and
theoretically be transported to all other plant parts
according to source–sink relationships in the
growing plant. Phosphonates spread rapidly
throughout plant tissue; within a few minutes for
small plants such as tomato, and within days for
large trees such as avocado. Phosphonates can be
applied either as a drench, foliar spray, stem-canker
paint, or trunk injection for direct systemic control.
Fungicides applied as foliar sprays and drenches are
often limited in their effectiveness. This is because
fungicide uptake into the plant tissue is generally
poor, fungicide activity is rapidly lost due to
degradation by soil and phylloplane microbes, and
fungicides are lost to the environment through
leaching and wash-off (Guest et al. 1995).
Pressurised trunk injection forces the chemicals into
the trees, minimising wastage and environmental
contamination, and achieving maximum persistence
(Darvas et al. 1984). For each host species and each
disease, the injection rate, number of injection sites
and the timing and frequency of injection need to be
optimised. Although phosphonates persist very well
in plant tissue, sequential applications are required
to maintain concentrations essential to effective and
durable disease control, especially in perennial
crops.
Most of the hosts on which phytophthora diseases
have been controlled by phosphonates are perennial
fruit crops. Treatment is particularly effective
because the fruits are strong metabolic sinks for the
translocation of phosphonates, and because reduced
disease in one season reduces the inoculum
available in the following season. Trunk injection
can be used to treat Phytophthora infections of roots,
leaves, stems and fruits (Guest et al. 1995).
There do not seem to be many problems associated
with phosphonate usage. Unlike metalaxyl,
phosphonate-resistant isolates of Phytophthora have
not been detected after more than 20 years of use.
Principles of phytophthora disease management 159
Although some studies have shown to that soil
drenches of fosetyl-Al and phosphonates inhibit
root growth and subsequent colonisation of the roots
by mycorrhizal fungi, others have shown that
application of fosetyl-Al enhances mycorrhizal
colonisation (Guest et al. 1995). It is important to
remember that phosphonates will not eradicate the
pathogen or eliminate disease, but remain an
excellent, cost-effective option for control of
phytophthora diseases.
Conclusions
Effective disease control is rarely achieved through
the application of a single disease-control method. In
order to limit the risks associated with outbreaks of
disease we need to use a number of different
approaches in an integrated manner. Starting with
disease-free planting material, site preparation and
establishing good drainage will not only limit
phytophthora disease severity but, also, the
improved soil health will benefit the host plant
directly. The planting of resistant material, if
available, is a highly cost-effective way to control
disease, but these trees will also benefit from
improved drainage and good soil health. Chemicals
can be used as a last option, as their use often
involves a significant cash outlay for equipment and
fungicides. The use of fungicides also requires
knowledge about optimal timing of sprays, rates of
application, additives and application methods, in
order to be applied effectively. Throughout this
monograph we have tried to give practical advice on
how to integrate the different components of disease
control in an effective manner to reduce losses due to
Phytophthora.
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