Experienced grape growers saw it coming. New growers hadn’t a clue. Regardless, my crystal ball told me that with
months of rain would come severe outbreaks of downy mildew in vineyards.
Grape downy mildew has reached epidemic levels in some vineyards
in Kentucky and possibly the Midwest. The
pathogen that causes the disease (Plasmopara
viticola) is a water mold, which warrants special considerations for
disease management. Let’s begin with a
quick overview of water molds (oomycetes, to be exact).
Water Molds
Water molds are different from true fungi. In fact, they are not related. Most importantly, water molds require free
water to complete their life cycles. Initial
infections often occur during rainy spring weather as temperatures begin to
warm. After infection, pathogens release
large numbers of “swimming” spores (zoospores) that move in films of water
(damp leaves or moist soil). This is the
repeating stage of disease that leads to epidemics if wet conditions persist. Spores are spread by splashing water and
wind-driven rain. In addition to downy
mildew pathogens, many root rotting pathogens (Phytophthora and Pythium)
are water molds.
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| Figure 1 - Early symptoms of grape downy mildew include yellow spots on upper sides of leaves. |
Downy Mildew
Downy mildew symptoms are quite distinct. Upper leaves are the first to develop noticeable
symptoms. Yellow circular to blotchy spots
can quickly enlarge and become bright yellow (Fig 1). As disease
progresses, leaf tissue becomes reddish brown, and centers of spots becomes
necrotic (dead tissue) (Fig 2). Extreme disease
conditions result in a coalescing of spots into large necrotic areas.
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| Figure 2 - As downy mildew spreads, leaf tissue becomes necrotic. |
The most characteristic symptom of downy mildew is the “downy”
fungal masses that develop on undersides of leaves (Fig 3). If weather remains rainy or humid/foggy, pathogens
will begin producing spore capsules (sporangia) on microscopic branched
structures (sporangiophores). This
branching gives the disease its fluffy, downy appearance. Sporangia can spread to healthy plant parts
by wind or rain, and then open to expose infective, swimming zoospores. This repeating cycle is dependent upon
temperature and availability of free water.
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| Figure 3 - The most characteristic symptom of downy mildew is the "downy" fungal masses on undersides of leaves. |
Downy mildew pathogens also produce another spore type, thick-walled
overwintering spores (oospores), at the end of the season. These spores drop to the ground and
overwinter in leaf debris or in soils.
In spring, they germinate just as grape begin to bloom. Thus, early fungicide protection is critical
in order to combat the initial phase of disease. Moreover, an effective disease management
program (described below) will help eliminate some of the inoculum (oospores)
that before they overwinter.
Damage Caused by Downy Mildew
Effects of infection are two-fold. First, diseased leaves fail to properly
photosynthesize, while severely infected leaves drop, leading to inadequate
energy production (Fig 4). Secondly,
grape berries may become infected, leading to yield and quality losses (Fig 5).
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| Figure 4 - Advanced symptom development can lead to reduced photosynthesis and leaf drop. |
Fruit are susceptible to infection from bloom until 3 or 4
weeks after bloom. After that, they become
resistant to infection. Berries may not
develop symptoms until several weeks after infection. Affected fruit become soft and brown and do
not mature. Like foliar infections,
infected berries become covered with downy fungal growth when conditions are
wet or humid. While fruit become
resistant to infection as they mature, cluster stems (rachis) do not. Thus, infections in these cluster stems can
spread internally to berries. Additionally,
young shoots, leaves, and tendrils remain susceptible to infections throughout
the growing season.
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| Figure 5 - Grape berries become infected between bloom and 3 to 4 weeks after bloom. Symptoms, however, may not develop until several weeks after infection. |
Disease Management
Growers must combine cultural and chemical practices to
effectively manage downy mildew in grape vineyards.
Cultural practices are important for both conventional and organic
growers. Maintaining dry foliage is
important. Plant spacing, pruning,
tucking, and other practices that improve air circulation will help foliage dry
faster, and thus, conditions become less conducive for disease
development. Surface and internal
vineyard drainage can also help reduce moisture levels within canopies. Next, sanitation should not be
overlooked. To the extent possible, remove
diseased leaves, fruit, and other plant parts from vineyards. This will help to prevent fallen debris from
becoming a home for overwintering pathogens.
Some species and cultivars have some resistance to downy mildew. See page 36 of the Midwest Small Fruit and
Grape Spray Guide ID-94
for a partial listing of tolerant grapes.
Fungicides are a vital part of management of downy
mildew. Protectant sprays should begin at
bud break and continue throughout the growing season. Keep in mind though, that fungicide
applications between bud break and 3 to 4 weeks after bloom are the most
critical. When selecting fungicides,
remember that the downy mildew pathogen is a water mold, not a true fungus. Therefore, not all fungicides will be
effective against infection. Refer to
Effectiveness of Grape Fungicides PPFS-FR-S-18
or the Midwest Small Fruit and Grape Spray Guide ID-94
for up to date fungicide information.
Additional information on grape production and disease
management can be found online at the UK Department of Plant Pathology’s
website. http://www2.ca.uky.edu/agcollege/plantpathology/extension/pubs.html#Smallfruit
