POWDERY MILDEW
By
Pete Anderson
The material in this paper is a compilation of information from U.C. Davis ,
Ohio State University , Oregon State, U.S. and Australian publications, various
internet web sites and personal experience.
WHAT IS IT? - It is caused by the fungus Uncinula necator. According to UC
Davis: "it is without a doubt, the most enduring and persistent disease
problem faced by grape producers, especially among California Vitis vinifera."
SYMPTOMS - can be described as: red blotchy areas appear on dormant canes;
susceptible varietals will grow "flag shoots" already showing signs
of major infection; on leaves, initial symptoms appear as chlorotic spots on
the upper leaf surface; as spores are produced, the colony takes on a white
powdery appearance; signs of the pathogen appear a short time later as white,
webby mycelium (Mycelium is the vegetative part of a fungus consisting of a
mass of branching threadlike hyphae. It is through the mycelium that a fungus
absorbs nutrients from its environment.); on fruit and rachises, the pathogen
appears as white, powdery masses that may colonize the entire berry surface;
on blossom clusters, the flowers may wither and drop without setting fruit;
on cluster stems they may wither, dry up and result in berry drop; on berries
they are often misshapen with rusty spots; on young shoots dark brown to black
patches may develop and remain as dark patches on dormant canes. Late in the
growing season, black specks which are cleistothecia can be observed. (Cleistothecia
are the sexual fruiting bodies of the fungus and the most important source of
over wintering inoculum) Some dormant canes may appear bleached with dark spots.
DISEASE CYCLE - The over wintering cleistothecia can be washed underneath the
bark. During the warm spring rains the spores contained in the cleistothecia
known as Ascospores are released. They are wind-dispersed to emerging leaves
and clusters, and can infect wet or dry tissue at temperatures of 50 degrees
F or higher. Infection will continue through the growing season. Mildew colonies
produce masses of white, powdery secondary spores, Conidia, which are wind-dispersed
throughout the vineyard and do not require rain for release or infection. This
repeating cycle of infection, spore production, spore dispersal, and reinfection
can continue to occur and reoccur in as little as 5 to 7 days.
CONTROL - UC Davis (http://www.apsnet.org/online/feature/pmildew/Top.html) has
developed a powdery mildew Risk Assessment Index (RAI) to monitor the disease
development. Basically, it provides that initiating the index is: after you
find powdery mildew, an epidemic will begin when there are 3 consecutive days
with 6 or more continuous hours of temperatures between 70 and 85 degrees F
as measured in the vine canopy. Starting with the index at 0 on the first day,
add 20 points for each day with 6 or more continuous hours of temperatures within
this zone; if the index reaches 60, an epidemic is under way and you are instructed
to begin using the spray-timing phase of the index. There are some considerations;
however, temperatures of 95 degrees and above will inhibit the growth of the
fungus and temperatures over 104 degrees will kill it. Also, free moisture,
especially rainfall is detrimental to the survival of the fungus. If the above
temperature and free moisture are not continual, the infection can start all
over again in 7 - 10 days.
For those just planting their vineyard, the above RAI guide is useful. For those
with an established vineyard already know the extent of their problem. With
the exception of those vineyard sites that are inland and receive temperatures
in the 100's during the growing season, everyone else most likely will have
to initiate a control program at dormancy and continue it until the berries
have reached 10 - 12 Brix sugar readings. (Note: this paper will not address
the "other fungal disease", Botrytis cinerea, but it generally does
not reveal itself until the sugar level is this high. That is when mycelium
of that fungus begin to feed on the sugar; the berries split becoming susceptible
to bacteria resulting in gray/sour rot not the "Noble Rot" famous
for France's sauternes.)
Control and management of the disease:
A. VINEYARD DEVELOPMENT - some control actions that can be taken prior to establishing
the vineyard:
1. Planting site selection that is open and with direct sunlight;
2. Orient rows in direction of the prevailing wind;
3. Select grape varietal with high resistance;
4. Plant vines and rows to reduce shading and increase air ventilation.
B. FUNGICIDES - have different modes of action and each one has strengths
and weaknesses. There are two ways fungicides work: one is as a Contact and
the other is as a Systemic.
1. Contact fungicides cover the outside of the plant, killing the fungus spores
as they germinate. Some contact fungicides act as protectants and need to be
applied before infection. They also need to be reapplied if there is a rain
as they can be easily washed off the vine. Some contact fungicides act as eradicants
and should be applied as soon as signs of the disease are observed. These do
not require a Pest Control License to apply for a non-commercial vineyard.
a. Examples of Contact fungicides are:
Sulfur (dry or wettable/ some research has indicated it has a diminished effect
when applied when temperatures are under 60 degrees); AQ 10 (this is a biological
fungicide with a fungal parasite as the active agent); Kaligreen (potassium
bicarbonate); Serenade is a biological fungicide which has a bacterium, Bacillus
subtilis, that helps prevent the powdery mildew from infecting the plant. Agricultural
oil (Neem, JMS Stylet, Ultrafine, Saf-T-side/ this is an eradicant, but, should
not be applied within 7 - 10 days of a sulfur application or high temperatures
as phytotoxicity can occur ). Kocide (micronized dry flowable copper mixture)
These copper ions are toxic to fungi and bacteria because of their ability to
destroy proteins in plant tissues. However, because copper can kill all types
of plant tissues, the use of copper fungicides carries the risk of injuring
foliage and fruit of most crops.
2. Systemic fungicides are absorbed into the plant and kill the fungus early
in the infection process by blocking biological pathways in the fungus. Since
a minor change in the pathway is all that is needed to bypass the block, resistance
can become a problem. These do require a Pest Control License to apply. The
major issue of these fungicides is the fungus has developed a resistance and
they should be applied alternately with other fungicides. Two classes of Systemics
are Sterol Inhibitors and Strobilurin fungicides:
a. Sterol inhibitor (ergosterol biosynthesis inhibitor) fungicides (Rally, Rubigan,
Elite and Procure) inhibit the formation of ergosterol This prevents cell wall
formation and stops growth of the fungus. .
b.Strobilurin fungicides (Abound, Flint, Sovran, Pristine) were recently developed.
This is a methoxyacrylate class of chemistry which is related to naturally occurring
products found in a group of forest mushrooms called "pine-cone mushrooms."
Some are locally systemic and many have an extremely broad spectrum of activity
against all four classes of fungi, something that is extremely rare with most
classes of fungicides. These fungicides are environmentally friendly. Some strobilurins
such as Abound are locally systemic, moving into and up the leaf toward the
tip. Other strobilurins such as Sovran and Flint have only slight uptake by
the leaf and are redistributed on the plant through vapor action. Most fungicides
in this class are strongly bound to the cuticle (leaf surface) and so are not
easily washed off the leaf. They inhibit fungal respiration.
C. CULTURAL CONTROLS
1. Canopy management (e.g., shoot removal and positioning); opens canopy, enabling
complete fungicide coverage with lowest effective rates.
2. Sanitation; removal of diseased shoots and old clusters.
3. Integration of weather information via the mildew index; optimizes and often
reduces
treatments.
D. SPRAY PROGRAM - each vineyard site is unique and the disease management
program should be tailored to each particular site. The following plan is designed
for home vineyards which do not have a pest control license to apply controllable
chemicals. A plan of action based on the incidence of disease is most important
and can be broken down into Stages of the Growing Season:
1. Dormancy - following pruning and before bud break, apply a liberal spray
of lime-sulfur and oil (JMS Stylet, Ultrafine), which also is a surfactant,
solution drenching the entire vine and ground below. This will help control
much of the over wintering cleistothecia and reduce the size of the early infection
although it will not reach the hyphae inside the dormant buds.
2. Budbreak-Start of Bloom - Apply a wettable sulfur spray every 7 - 14 days
depending on infection history and current assessment. If infection is detected,
apply an eradicant (oil). DO NOT APPLY OIL SPRAYS IN LESS THAN 7 DAYS OF A SULFUR
SPRAY AND DO NOT APPLY OIL IF THE TEMPERATURE WILL EXCEED 90 DEGREES AS PHYTOTOXICITY
WILL OCCUR IN BOTH CASES.
3. Bloom - Veraison - continue a sulfur spray program and/or other contact sprays
(AQ10,Kaligreen,Serenade, Oil). Frequency of spray application will depend on
incidence of disease.
4, Veraison - Harvest - when the berries soften and change color STOP SULFUR.
There is a good possibility elemental sulfur will remain on the peel or in the
rachis which can produce hydrogen sulfide (H2S) during fermentation. Continue
spray program. Powdery mildew rate of berry infection is diminished when the
sugar reaches 12-15 brix, but, can still infect leaves and canes.
5. Post-Harvest - if the infection rate is severe, the application of Kocide
(a copper fungicide spray) after leaf fall will reduce the number of cleistothecia.
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