Department of Plant Pathology, Cornell University,
NY State Agricultural Experiment Station, Geneva, NY
Botrytis bunch rot (BBR) occurs sporadically in Mediterranean
climates such as California, yet it can still cause significant
crop losses in a challenging year, as 2006 so rudely demonstrated.
Botrytis is a chronic problem in New York, as it is in other
humid growing regions of the world. Therefore, we have devoted considerable
effort to better understanding its biology and control.
Some of this research has been supported by the California wine
industry through the American Vineyard Foundation, in addition to
the U.S. Department of Agriculture Viticulture Consortium-East.
Such support is gratefully acknowledged.
BBR is an amazingly complex disease, and is a virtual poster
child for the concept of the so-called disease triangle,
the idea that plant diseases are governed by multiple three-way
interactions among the host (grapevine), the environment, and the
causal pathogen (the Botrytis fungus).
Many of these interactions are poorly understood. This is fascinating
to a research plant pathologist, but frustrating to a vineyard manager,
who needs to determine when intensive disease control is necessary
and when its not. Such complexity notwithstanding, there are
a several basic concepts that go a long way to help understand this
disease, and devise a strategy to manage it.
Botrytis cinerea is very widely dispersed, and can overwinter
as resting structures on infected canes, or more commonly, in various
vineyard debris. Old cluster stems appear to be a particularly common
source of inoculum. Botrytis is a weak pathogen
that primarily attacks highly succulent, dead, or injured tissues,
or tissues that are senescing.
Berries damaged by insects, powdery mildew, or splitting, due to
pre-harvest expansion and/or rain, are common injury sites attacked
by Botrytis. Withering blossom parts and ripening fruit are
senescing tissues of particular importance to the initiation and
development of BBR.
The fungus thrives in high humidity and still air, hence the well-known
value of cultural practices such as leaf removal and canopy management
to minimize these conditions within the fruit zone.
Although the fungus does not grow well inside berries until they
start to ripen, it can gain entrance into young fruit through senescing
blossom parts, old flower trash sticking to berries
within the cluster, and scars left by the fallen caps.
Infection requires that tissues remain wet for at least several
continuous hours (rain). The ideal temperature for infection is
approximately 6075ºF, although it can occur at temperatures
outside this range under longer wetting periods.
Infections early in the season remain confined to a few cells and
latent (dormant) while berries are green, but given the right
conditions, some of them can resume activity, and progress to the
point that they rot the affected berries as the grapes approach
maturity. Once this occurs, the disease can spread rapidly from
berry to berry within clusters and, to a lesser extent, among clusters.
The likelihood and magnitude of disease spread depends on weather
conditions and certain physiological factors within the vine.
The following are some recent findings concerning various details
of this basic scenario, and the fungicides that are used to control
the disease. Although this work was conducted in the Finger Lakes
region of New York, the general principles should be applicable
to California as well, although certain specifics will obviously
Latent infections can be common following a wet bloom period, but
the vast majority of latent infections appear to remain inactive
through harvest, meaning that the fruit stays healthy. For example,
when weve inoculated Pinot Noir clusters with Botrytis
spores at bloom, weve been able to detect latent infections
in up to 70% of the young berries that weve sampled at various
However, we often end up with only 2% or 3% rotten berries at harvest,
especially in a dry autumn or a clone with loose clusters. On the
other hand, weve ended up with 25% to 40% rot at harvest in
clones with tight bunches, particularly in wetter harvest periods;
leading to the following point:
Serious Botrytis losses result from disease spread during
the pre-harvest period, after berries begin to ripen and become
highly susceptible to rot by the fungus. Thus, latent infections
established at bloom can play a critical role in disease development
if even a few infections become active and provide an initial foot
hold from which spread can occur under favorable pre-harvest
Sprays to control one, both, or neither of these two phases of disease
development may be worthwhile, depending on the specific vineyard
characteristics and weather conditions.
The potential importance of early disease establishment, and two
vine factors that promote subsequent disease spread, are illustrated
by the results of two following field experiments.
In the first experiment, we inoculated one, three, or five
individual Pinot Noir berries per cluster, 10 days after veraison,
by using a hypodermic needle to inject them with Botrytis
spores. This produced initial point sources of the disease
within clusters about a week later, which were meant to simulate
occasional activations of latent infections before harvest.
To determine the effect of cluster architecture on disease spread
from these sources, we utilized clone 29 (very tight bunches) and
opened up some of the clusters by removing enough berries after
fruit set so that they resembled the Mariafeld clone (loose clusters,
generally less prone to bunch rot than other Pinot Noir clones under
In Figure I, Botrytis was able to spread extensively throughout
the unthinned clusters. One single rotten berry that appeared 21/2
weeks after veraison was all it took to end up with 50 additional
rotten berries at harvest (pre-harvest weather was wet that year).
In contrast, disease spread was minimal in the thinned clusters.
Additional experiments showed that the relative Botrytis
resistance long noted for the Mariafeld clone can be accounted for
simply by its loose bunches rather than some inherent chemical factor.
In the second experiment, clusters of a tight-bunched Chardonnay
clone were similarly thinned and inoculated. Additionally, some
vines received four weekly sprays of urea (8 lb urea in 100 gal
of water per acre), starting at veraison, to see if high berry nitrogen
content would affect disease spread.
This treatment increased the mean level of assimilable nitrogen
in the must to 303 mg/L, compared to 235 mg/L in the untreated plots
(determinations from 10 clusters per each of four replicate plots
per treatment; differences between the two treatments were statistically
significant at P = 0.05).
In Figure II, little disease spread occurred in the thinned clusters,
regardless of nitrogen treatment. In contrast, elevated berry N
did increase spread in intact clusters with either one or three
initial points of infection. The late season nitrogen applications
did not increase canopy growth. Therefore, it appears that their
effect on disease development was due to an increased susceptibility
of berries to fungal colonization as the nitrogen content increased,
rather than an increase in canopy density and associated microclimate
In Figure II, the analysis of variance showed a highly significant
(P<0.001) interaction between the thinning (opening up) treatment
and nitrogen treatment.
Factors that determine whether latent infections become active and
cause disease, or remain latent and symptomless, are poorly understood.
However, the activation of latent infections appears to be influenced
by both the weather and vine physiology. Experience and controlled
experiments both show that high atmospheric humidity during a several
day period prior to harvest is one such factor that promotes this
To examine this factor, we inoculated clusters of
potted Chardonnay vines at bloom in order to establish latent infections.
The vines were kept outdoors, protected from rain.While all clusters
were still symptomless, we moved them into a high humidity (92%
RH) chamber for 0 to 9 days either at veraison, or starting 10 days
In Figure III, the percentage of clusters with disease symptoms
(due to activation of latent infections) increased in proportion
to the duration of the high humidity treatment pre-harvest, but
was not affected by high humidity at veraison.
In addition to high atmospheric humidity, high soil moisture can
also promote activation of latent infections. The influence of this
factor is illustrated by the following experiment, in which potted
Chardonnay vines were once again inoculated at bloom to establish
latent infections and grown in a screenhouse, where they were protected
from rain and irrigated as needed to maintain moderate growth and
avoid drought stress.
At veraison, the vines were split into two groups: a) watered several
times per week to keep the soil moisture levels constantly high
(wet); or b) watered only when shoot tips began to wilt (dry). On
these occasions, water was provided to alleviate stress, but the
soil was not saturated.
Pots receiving the two irrigation regimes were interspersed among
each other randomly. Therefore, all clusters were subjected to the
same atmospheric environment, and only the soil environment was
Disease incidence was determined in two manners: 1) at the time
of harvest; and 2) after harvested clusters were incubated for four
days at 68ºF and 92% RH, to promote additional activation of latent
infections. There were seven replicate plants with six clusters
per plant for each of the two treatments.
In Table I, disease incidence (the best indicator of latent infection
activation), was more than three-fold greater at harvest in the
wet compared to dry soil treatment. However, most of this difference
was no longer evident after post-harvest incubation, by which time
there was no statistical difference between the two treatments.
This leveling off between the two treatments was due
to the pronounced increase in disease incidence for clusters from
the dry, but not the wet, treatment following incubation and represents
the activation of multiple latent infections during the post-harvest
Thus, it appears that the wet-soil treatment provided conditions
that were more conducive to the activation of latent infections
than did the dry treatment while clusters were on the vine, but
that many latent infections in the dry treatment remained viable
and activated upon exposure to high atmospheric humidity during
As for most fungi, Botrytis spores require a film of water
in which to germinate and initiate infections. Thus, rainy pre-harvest
weather sets off a chain reaction, in which earlier latent infections
are activated by high atmospheric humidity and high soil moisture.
High humidity promotes spore production by the fungus from debris
and newly-diseased berries, and free water on the berry surfaces
promotes germination and new infections by these spores.
Cultural practices such as canopy management and leaf removal are
widely practiced and effective components of BBR management programs.
Dr. Doug Gubler (University of California, Davis, who first demonstrated
the value of leaf removal), emphasizes that this operation should
be undertaken during the very early stages of berry development,
at fruit set, to help avoid sunburn damage. In encouraging California
growers to remove leaves early, Gubler recently noted that some,
have been pulling leaves later and later and are running into
sunburn problems, [leading to] secondary fungal infections such
Removal or destruction of vineyard debris, particularly old cluster
stems, is theoretically useful and something to keep in mind insofar
as it is practical. However, fungicide sprays targeted specifically
at BBR may be necessary on susceptible cultivars and/or clones,
particularly in a wet year. The questions are, which materials and
Not surprisingly, the optimum timing for spray applications is heavily
weather-dependent. This probably accounts for the sometimes contradictory
opinions and data regarding the relative importance of sprays at
the times when they might potentially be applied, such as bloom,
pre-bunch close, veraison, and pre-harvest.
A bloom application is designed to prevent the initial establishment
of infections through susceptible blossom parts, or infected blossom
debris trapped within the cluster (see Photo II), and can be important
on susceptible varieties during wet flowering periods, especially
if pre-harvest conditions are wet also.
Veraison and pre-harvest sprays are designed to prevent both initial
infections through injured berries and, especially, the spread of
active infections through the ripening clusters. These are often
the most important spray timings in humid climates with regular
summer and autumn rainfalls.
Although most effective when applied before an epidemic
is in progress, such sprays can still help to slow down the rate
of spread if applied relatively soon after the disease first appears
and threatens to take off.
Unfortunately, most fungicides that control other diseases are relatively
ineffective against Botrytis, either providing no significant
control or requiring substantially higher rates than required for
other diseases. Similarly, most Botrytis-specific fungicides
provide no significant control of other disease-causing fungi (including
other causes of bunch rot), although there are important exceptions.
Do not forget that all of the Botrytis fungicides are at
moderate to high risk of resistance development. This makes it very
important to rotate among groups of fungicides used to control this
disease, even if you make only occasional applications. Dont
use the same one year after year without substituting something
else along the way. Following is a brief review and some new information
regarding the fungicides available for Botrytis control.
Before discussing specific materials, lets define a few of
the terms that will be used with respect to fungicial activities:
Protectant Provides control when applied before
the infection process begins.
Post-infection Provides control when applied for some
period of time after the infection process begins,
but before symptoms appear. This period of time is often limited
to one to several days, but for some materials can be significantly
longer with respect to control of latent infections.
Anti-sporulant Significantly reduces the production
of spores from infected tissues, although other symptoms may appear
These represent two different fungicides in the same class
of compounds (the anilinopyrimidine or AP fungicides),
so rotating between them will provide no benefit in
terms of resistance management.
These compounds appear to have equivalent properties and provide
the same general levels of control at labeled rates, although Scala
(not registered for application on grapes in California) has been
a bit weaker under high disease pressure in a couple of New York
trials. In general, these have been the most consistent performers
in our field trials over the years, and provide both protective
and post-infection activity. (See Table I.)
In recent tests where weve inoculated Chardonnay or Pinot
Noir clusters with Botrytis spores at bloom, and then sprayed
at veraison (before the latent infections became active), both materials
eradicated over 90% of latent infections, even though they had been
established almost two months earlier.
(NOTE: The clusters in these experiments were sprayed individually
by hand, so coverage was absolutely thorough and superior to what
would be expected under commercial conditions.) The resistance risk
for the AP fungicides is high, so rotation is very important. These
products provide no significant control of any disease other than
The other work horse against Botrytis over the
past decade. Elevate is not related to any other fungicide on the
market, so is an excellent rotational partner. In New York field
trials, weve seen the same control whether Elevate was rotated
with Vangard or either one was used alone. Conventional wisdom has
it that this is a protective fungicide, with no post-infection activity.
In contrast, we have several years worth of data to indicate
that Elevate does have some post-infection activity, and that it
can suppress or eradicate latent infections if applied before the
infections start to become active. Elevate provides no significant
control of any disease other than Botrytis. Resistance risk
The combination product of a strobilurin (pyraclostrobin),
plus a compound representing a new chemistry on grapes (boscsalid).
Both components are active against Botrytis (boscalid is
the more active of the two), which is good from a resistance-management
Application rate is important. When used at the rate of 8.5 oz to
10.5 oz per acre it is highly effective against powdery mildew and
several other diseases, but control of BBR is only fair under anything
more than moderate pressure.
There is no mention of Botrytis on the label at this standard
use rate, so the company makes no claim concerning activity
against this disease. However, Pristine is labeled for suppression
of Botrytis at a rate of 12.5 oz per acre, and has provided
good to very good results in a limited number of tests that weve
conducted using this rate. In New York, I would consider 12.5 oz
per acre to be marginal under very rainy conditions post-veraison,
although it will certainly be much better than applying nothing
and may be adequate under marginal pressure.
Pristine has a supplemental label for control of Botrytis
at 18.5 to 23.5 oz per acre. It has provided very good to excellent
control at a rate of 19 oz per acre in several trials that we have
run under very rainy conditions. It also has a much broader spectrum
of activity against other fungi than do the Botrytis-specific
materials mentioned above, and it gave very good control of the
numerous non-Botrytis rots that developed in our plots during
a very wet September in 2006.
The re-entry interval (without protective clothing) is 24 hours
at rates up to 12.5 oz per acre, but jumps to five days at the higher
Botrytis-specific materials will provide some control of
other rot organisms by reducing the amount of damaged fruit susceptible
to secondary invasion, but these are not the only infection courts
open to such invaders. Therefore, a broad-spectrum material such
as Pristine will provide additional control of non-Botrytis
rots via direct effects on such organisms, which is why this is
such a good material for general bunch rot control.
Pristine provides both protective and limited post-infection activity,
although the post-infection activity has not been as great as that
for the APs or Elevate in our tests. The resistance risk for both
the strobilurin and non-strobilurin component of this product is
high, so rotation is important, although the mixture of two unrelated
compounds should help reduce the resistance risk.
Note that Flint is also a strobilurin fungicide, so Pristine and
Flint should not be rotated with each other unless a non-strobilurin
fungicide is also included in the rotation.
An excellent powdery mildew fungicide when used at 1.5 to 2.0 oz
per acre, it has provided very good to excellent control of Botrytis
when applied at the 3-oz per acre rate labeled against this disease.
It is primarily a protective fungicide. It does suppress spore production
from subsequently-diseased tissues when applied post-infection.
Resistance risk is high.
This was the only true Botrytis fungicide available
in the U.S. for nearly 20 years, so rotation just wasnt an
option. As a consequence, resistance appears to be not uncommon
in many regions where it was used regularly for many years. However,
in locations where use has been much more limited, resistance is
much less likely, and the material often performs well. Dr. Gubler
reports that resistance to this material has not been a problem
in California vineyards.
In the absence of resistance, Rovral is an excellent Botrytis
fungicide, with both protective and limited post-infection activities.
It should be a good rotational partner where resistance is not a
major concern, but should be used with caution otherwise. It provides
no significant control of any disease other than Botrytis.
Although others have, weve never obtained any control
of Botrytis with Stylet Oil or the various potassium bicarbonate
products, and I would be very hesitant to rely on them for this
purpose under any sort of disease pressure.
Stylet Oil and the bicarbonates both control powdery mildew, which
helps to limit Botrytis by eliminating one injury site where
this fungus might enter, but direct activity against B. cinerea
is questionable. We have obtained significant control with Serenade,
but it has never been as effective as the standard products discussed