Downy Mildew (Plasmopara viticola): Downy mildew is
a fungal disease that thrives under warm, wet conditions. The fungus
overwinters in leaf litter on the vineyard floor and in the surface layers of
soil where infected leaves have decomposed. Primary infections can occur with
as little as 0.1″ of rainfall at temperatures of ≥ 50°F, when spores are
rain-splashed onto susceptible tissue. Once the primary infection site is
capable of producing spores, at least 4 hours of darkness at temperatures >
55°F and a relative humidity of 95 to 100% is required for the induction
of sporulation. The entire process from induction to the actual release of
spores can occur in as little as 4 hours at optimal temperatures (64 to
72°F). Spores may then be blown by wind to new tissue where secondary
infections can occur if moisture from rainfall or heavy dew is present. The
length of wetting required for secondary infections can be as little as 1.5 to
2 hours at 59 to 77°F. The first primary infections often occur two to three
weeks before bloom to two weeks post-bloom, making this a critical period for
preventing the initiation of an epidemic. Downy mildew is most commonly a
problem in the Eastern half of the state and in wet years, and all commercial
winegrape varieties are susceptible.
Downy mildew can infect all green parts of grapevines including shoots,
leaves, and clusters. Leaf infections first begin as slightly discolored,
shiny areas on the upper surface of the leaf that become yellowish, taking on
an “oil spot” appearance. Downy mildew lesions may be circular to irregular in
shape, and progress from yellow to brown as the tissue begins to die. Spore
producing structures called sporangiophores may be visible on the underside of
the leaves opposite of lesions on the upper surface, particularly after a damp
night. The sporangiophores appear as cottony, white colored growth.
Infections of flower clusters, shoot tips, and petioles can result in girdling
and abscission. Young berries are highly susceptible to down mildew becoming
gray in color when infected, and sporulation may be visible under humid
conditions. Symptoms may be noticeable within 5 to 6 days after infection if
temperatures remain warm (64 to 79°F), but take longer to appear under
cooler conditions. When warm, wet conditions persist, downy mildew outbreaks
can be explosive, resulting in defoliation and total crop loss if left
uncontrolled.
Cultural control practices for downy mildew should focus on reducing leaf and
cluster wetness. As with all fungal diseases, improving airflow through the
canopy with proper vineyard design and canopy management can enhance control
by reducing fruit and foliage drying times after rainfall, and increasing
fungicide spray penetration.
Fruit
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Shoots
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