Powdery Mildew (Erysiphe necator): Powdery mildew is
a fungal disease that that can infect all green parts of grapevines. It
overwinters as chasmothecia in cracks in the bark of vines. In spring, the
chasmothecia release spores with rain events ≥ 0.1″ when temperatures are ≥
50°F. The spores are blown to nearby tissue where they germinate and
initiate a primary infection. The optimal temperature for primary infection is
77°F, but infections can occur over a wide range of temperatures and free
water is not required beyond the initial rainfall to trigger spore release.
However, powdery mildew infections are favored by high humidity.
Powdery mildew colonies grow and sporulate most rapidly at temperatures of 73
to 86°F and the period of time required for infection and subsequent
sporulation can be as short as 5 to 6 days when temperatures remain in this
range. Cooler temperatures increase the latent period significantly, and
powdery mildew development is inhibited at temperatures ≥ 90°F. However,
the interior of a grapevine canopy is often cooler than the ambient air due to
transpirational cooling and shading. Powdery mildew infections can occur
through the state, but Blanc Du Bois, Black Spanish and some other hybrid
cultivars have a high level of resistance to powdery mildew and typically do
not require protection from powdery mildew during the growing season.
Powdery mildew leaf infections may vary in appearance based on the age of the
foliage when infected, age of the fungal colony and source of infection.
Colonies are first noticed as white circular area with a silvery gray to brown
tinge. On leaves, young colonies may be most visible by looking across the
leaf blade. As colonies sporulate they take on a white appearance, but
eventually become gray with age. Dead leaf tissue may be observed in older
colonized areas.
Early leaf infections may occur on the underside of leaves near trunks or
cordons where the overwintering chasmothecia were likely located. These
infections may go unnoticed, but often form chlorotic spots on the surface of
the leaves that resemble downy mildew. Later infections occur on the surface
of leaves as circular or irregular shaped colonies. Stem infections take on a
similar appearance as leaf infections, but turn dark in color as the season
progress. These colonies are eventually killed as periderm forms in late
season leaving dark colored blotches. Berry infections may be visible as a
whitish coating that covers a portion or the entire berry. Berry infections
that occur before bloom up to a week after berry set often cause the epidermis
of the berry to stop growing during expansion resulting in splitting of the
skin. These cracks can serve as a point of entry for spoilage organisms. The
most critical period for powdery mildew control in fruit is two weeks before
bloom through 30 days after bloom. Maintaining control of foliar infections
throughout the growing season is also important in order to maintain canopy
health for fruit ripening and for post-harvest recovery.
Because humidity plays an important role in powdery mildew infections,
vineyards near bodies of water and areas of the vineyard with poor airflow may
be at greater risk of infection. Improving airflow through the canopy with
proper vineyard design and canopy management can reducing humidity within the
canopy and enhance control of powdery mildew by reducing fruit and foliage
drying times after rainfall, increasing fungicide spray penetration, and
facilitating leaf and cluster exposure to sunlight (ultraviolet light is
lethal to powdery mildew).
Fruit
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Shoots
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