Kathy Evans, Tasmanian Institute of Agricultural Research, August 2010.
Q&A on botrytis
How far do botrytis spores move?
Botrytis spores are dispersed to grapevine leaves and bunches in air currents, by
splashing water droplets and by insects (eg LBAM). PhD thesis research from New
Zealand suggested that 95% of airborne botrytis spores landed within 1.6 m of their
source, meaning that sources of spores from the vineyard floor and within the
grapevine canopy are likely to be the most important for grapevine. Important
sources of spores are therefore the dead and decaying plant material in close
proximity to grape bunches:
Previous season
Current season
Infected cane debris, bunch remnants, Infected, damaged leaves; decaying
tendrils, leaf petioles and blades
floral parts: caps, aborted berries; rotting
berries
Trials conducted in New Zealand also revealed that the number of decaying floral
parts (eg. aborted berries, flower caps) infected with Botrytis cinerea was correlated
positively to botrytis severity at harvest (R. Beresford, personal communication).
Even though movement of botrytis from a shelter-belt plant to grapevine is possible,
it is the spore sources very close to grape bunches (eg. decaying floral parts) that
contribute most to the risk of botrytis.
What is the effect of mid-row species on botrytis inoculum and risk?
The key principle for mid-row species and botrytis risk is the amount of decaying
floral parts on the mid-row species or other dead or dying tissues that can support
the growth of the botrytis fungus and hence the amount of botrytis spores provided to
air currents. The time when these mid-row species flower might also be important in
relation to whether or not spores are released to air currents when the grapevine is
highly susceptible to infection (eg. ripening, wounded berries). Again, spores from
decaying floral parts within the grape bunch, or bunch remnants in the fruiting zone,
are likely to present a greater risk than spores from sources further away.
Mulching?
Mulching can have a large influence on canopy development and bunch
microclimate, so it is important to first understand these effects in relation to
viticultural and wine-making objectives. Any benefits for botrytis control are
secondary but important in that mulch may play a role in the multiple measures
needed to reduce botrytis risk. In New Zealand, mulch reduced the sporulation of
botrytis on vine debris and it also increased soil biological activity. At the same study
site, the severity of botrytis was reduced through application of mulch from
aerobically composted grape marc or shredded office paper. In another New
Zealand study, a multi-component mulch reduced botrytis incidence in five of 12 field
trials conducted at four sites over three growing seasons.
The effect of mulching appears to be a reduction in the amount of botrytis (inoculum
potential) carried over from the previous season. Whether or not the reduction in the
‘spore load’ actually results in lower botrytis severity at harvest will depend on all the
other factors that contribute to botrytis risk (eg. weather, berry wounding, bunch
compactness and so on).
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Kathy Evans, Tasmanian Institute of Agricultural Research, August 2010.
When are botrytis infections occurring?
In regions
In Australia, there has been a heavy emphasis on application of protective fungicides
at 80% cap fall, which might be explained partly by the tendency of rain to fall early
in the season rather than later in a number of regions. Flowering is the first
opportunity for latent infections of fruit to become established, but fruit infection can
occur any time subsequently.
One way of identifying when infections are occurring commonly in a particular grape
growing region is to conduct small-plot fungicide timing trials over multiple growing
seasons. In Tasmania, it was found that fungicide applied at pea-sized berries
and/or pre-bunch closure (mid season) reduced botrytis severity significantly,
whereas crop protection at flowering was not always beneficial. Figures 1 and 2
show results from two growing seasons for Sauvignon Blanc at a site where
resistance to Rovral® was suspected, but not confirmed.
b
4
3.5
Mean
botrytis
severity
(%)
3
2.5
ab
b
Late
Non treated
2
1.5
a
1
0.5
0
Early
5%
capfall
Mid
80%
capfall
Pea
size
PBC
Veraison
Preharvest
-
-
-
-
-
-
-
Rovral
Rovral
-
-
-
No.
Timing
1
Early
2
Mid
-
-
3
Late
-
-
-
4
Non
treated
-
-
-
Bravo Scala
Captan Switch
Figure 1. Fungicide timing trial in 2006/07, Sauvignon Blanc, southern Tasmania.
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Kathy Evans, Tasmanian Institute of Agricultural Research, August 2010.
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c
5
Mean
botrytis
severity
(%)
bc
4
3
ab
2
ab
1
a
0
Early
Mid
Late 1
Late 2
Non
treated
No.
Timing
5%
capfall
80%
capfall
Pea
size
PBC
Veraison
Preharvest
1
Early
-
Switch
-
-
-
-
2
Mid
-
-
-
Switch
-
-
3
Late 1
-
-
-
-
Switch
-
4
Late 2
-
-
-
-
Rovral
-
5
Non
treated
-
-
-
-
-
-
Figure 2. Fungicide timing trial in 2008/09, Sauvignon Blanc, southern Tasmania.
Note that Switch® was applied at veraison for experimental purposes only. Always
check the restrictions on fungicide use.
In vineyards
Given the complex interactions between the vine (eg. berry skin thickness,
wounding), the pathogen (eg. amount & proximity of inoculum) and the weather, it is
still not possible to make reliable predictions about the timing of single infection
events for botrytis. What is possible, however, is to predict the relative risk of
botrytis during key stages of vine development. Refer to Kathy Evan’s ‘Expression
of Interest’ document on the prototype Botrytis Decision Support Model (BDSM).
What about compost teas?
Aerated compost tea (ACT) is a watery extract of compost that contains aerobic
(oxygen-loving) micro-organisms and nutrients. Other nutrients are sometimes
added during extraction to promote microbial growth. There are many anecdotal
reports that ACT, when applied to the crop canopy, reduces the incidence or severity
of a range of plant diseases, but few scientific reports backing the many claims.
A former TIAR PhD student, Alice Palmer, conducted research on one particular type
of compost tea, which was produced according to a particular method. When this
compost tea was applied multiple times in commercial vineyards it reduced the
incidence of latent B. cinerea in Chardonnay bunches and the sporulation severity on
Riesling bunches. However, more research is required to test this type of tea over a
wide range of viticultural environments to determine how consistently the tea
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Kathy Evans, Tasmanian Institute of Agricultural Research, August 2010.
suppresses botrytis. There is also a lack of information on how the application of
these micro-organisms in ACT might impact (or not) on wine making and wine quality.
Like any type of biological control, this control measure, if adopted, should be
integrated with other approaches (eg. canopy management) to ensure botrytis risk is
reduced to an acceptable level.
What’s the latest on induced resistance?
Induced resistance is a state of enhanced defensive capability of a plant cell in order
to mobilise defence responses before or upon pathogen attack. This ‘priming’ means
that the plant is capable of a faster and/or stronger expression of normal defence
responses. Interaction of plant cells with chemical ‘elicitors’ or beneficial microorganisms can lead to the generation of active oxygen species, which lead to signals
being sent around the plant to produce chemicals associated with plant defence.
The end result might be the strengthening of cell walls in leaves and berries to limit
botrytis infection and/or the production of anti-microbial stilbenes (trans-resveratrol
and its derivatives: piceid, pterostilbene and ε-viniferin). Stilbenic compounds are
accumulated in leaves and grape skins in response to UV radiation or microbial and
chemical elicitors. It is important to note that stilbene levels in berries decline
naturally during berry ripening with a concomitant increase in susceptibility to
infection by the botrytis fungus. Pathogenesis-related (PR) proteins, such as
chitinases and glucanases, also accumulate in response to pathogen infection or
elicitors of induced resistance. These PR proteins appear to contribute to grapevine
defences by degrading structural components in the pathogen (fungal) cell walls.
Evidence that induced resistance leads to reduced botrytis severity at harvest in
commercial vineyards is limited. New Zealand researchers found that a spray
program that used the fungal biocontol agent Ulocladium oudemansii (BOTRY-Zen®)
and the chemical elicitor 5CSA suppressed botrytis rot on Chardonnay vines to less
than 1% severity at harvest when the untreated control bunches expressed 14%
severity. Unfortunately, the 5CSA resulted in reduced berry weight, leaf chlorosis
and detectable chemical residues in grapes. There was also a reduction in total
phenolics in Cabernet Sauvignon treated with 5SCA at 3-weekly intervals from fruitset to harvest. These same researchers have had more success with chitosan, a
naturally occurring polysaccharide, which can have direct antimicrobial activity as
well as eliciting induced resistance. A spray program using BOTRY-Zen® early
season and chitosan (formulated as ARMOUR-Zen®) mid- to late season reduced
botrytis severity significantly in Chardonnay and Sauvignon Blanc grapes when
compared with non-treated controls (T. Reglinski et al. 2010, Plant Pathology).
More recently, French researchers reported application of non-pathogenic
rhizobacteria such as Pseudomonas spp. to grapevine roots and demonstrated
induced systemic resistance (ISR) to B. cinerea infecting grapevine leaves
(controlled-environment study). Drenching the soil of Chardonnay vines with a
particular bacterium at bud burst resulted in small but statistically significant
reductions of the percentage of berries per cluster with grey mould.
It is not possible to review here the numerous others studies conducted on induced
resistance and biosuppression of Botrytis cinerea in grapes. The examples
described previously simply give a ‘snapshot’ of the state of the science.
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Kathy Evans, Tasmanian Institute of Agricultural Research, August 2010.
The best outcome of induced resistance is reduced disease incidence or severity
rather than total disease control. Any type of biosuppression tends to work best
under conditions of low to moderate disease pressure. Even full spray programs for
botrytis fail when weather conditions are highly conducive for a full-on botrytis
epidemic. It is still not known how often ‘foliar’ chemical elicitors need to be applied
to maintain the plant in a ‘primed’ state. Obviously this approach becomes less
sustainable the more frequently the spray rig has to be carted around the vineyard.
The timing and amount of PR-proteins induced by some elicitors might also have
detrimental effects on wine quality as the proteins can contribute to wine ‘haze’.
The grape berry cuticle and cell wall naturally resist penetration by the botrytis
fungus whilst tannins and phenolics in the cell wall inhibit fungal enzymes. Indeed,
the berry cuticle is a very effective barrier to botrytis infection and the fungus only
gains entry by wounds or microfissures in the berry skin. Importantly, any wounding
of the berry (eg insect damage, powdery mildew infection) will provide the fungus
with a direct entry point and may mask any benefit derived from induced resistance.
Nevertheless, any viticultural practice that can improve soil microbial activity and
microbial diversity in the soil and above-ground plant parts can potentially contribute
to induced resistance and integrated disease control.
What are the limits for fungicide efficacy?
Researchers in Germany applied various fungicides to Müller-Thurgau at veraison in
the growing seasons of 2003 to 2005. The fungicides tested were boscalid,
fenhexamid, pyrimethanil, tolylfluanid, and cyprodinil + fludioxinil. These fungicides
remained effective in preventing infection of Müller-Thurgau berries by B. cinerea
(following artificial inoculation) for periods ranging from 19 days in 2003 to 44 days in
2005. The timing of the loss of efficacy for all fungicides in any particular year was
similar. While none of these fungicides can be used at veraison in Australia, the
German research illustrates that fungicide residues can remain effective for a long
time, but that the length of time will be dependent on seasonal conditions.
Disclaimer
This document contains information only. It does not constitute professional advice
or a service. The TIAR expressly disclaims any form of liability to any person in
respect of anything done or omitted to be done that is based on the whole or any
part of the contents of this document.
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