Black Rot (Guignardia bidwellii): Black rot is an
important fungal disease that has the potential to cause total crop loss if
left uncontrolled. It overwinters in mummified berries that were infected in
the previous season, and in cane lesions. In spring, black rot spores are
released with rainfall events ≥ 0.1″, and grape tissue can be infected when
leaf wetness persists and temperatures are ≥ 50°F. The duration of leaf
wetness required for infection ranges from 24 hours at 50°F to as little
as 6 to 7 hours at 70 to 80°F. All young growth is susceptible to black
rot infection, and grape berries can remain susceptible for up to 6 to 7 weeks
post-bloom. The most critical period for controlling black rot infections on
fruit is from approximately 2 weeks before bloom to 30 days after bloom. Black
rot can occur throughout the state of Texas and all commercial winegrape
varieties are susceptible to the disease.
Black rot leaf infections appear as tan regular to irregular shaped lesions
approximately 2-10 mm in diameter, and they become reddish brown as they
mature. The spore producing bodies, called pycnidia, may be visible inside
lesions as small black pimples often arranged in a ring. Shoot, tendril,
petiole, and pedicel lesions appear as elongated dark colored depressions that
turn black. Lesions may girdle the tissue restricting water and nutrient flow
beyond the infection resulting in death of the tissue. Berry infections begin
as a small round, tan colored spot that becomes surrounded by dead tissue. The
infection eventually encompasses the entire berry which shrivels and becomes
hard and dark colored. The mummified berries remain firmly attached to the
cluster and will often persist to the following season if not removed.
Sanitation is an important aspect of any black rot control program. Mummified
berries are capable of producing large numbers of spores in close proximity to
susceptible tissue. Thus, mummies should be completely removed from the
vineyard and destroyed during dormant pruning or dropped to the vineyard floor
where they can be buried with cultivation before budbreak. Even small numbers
of mummies left in the trellis can cause significant damage as they can
continue to produce spores throughout the growing season.
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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