Agriculture Reference
In-Depth Information
and any infected tissue. These surfaces may be completely
covered in fungal growth and spores if infection is severe.
shoot (the f lag shoot) with ash-grey to white spores and
fungal growth. Flag shoots are often distorted, stunted,
grey-coloured and bear infected leaves that curl
upwards. Spores are produced abundantly on these
shoots and provide the main source of disease in most
vineyards. The spores are dispersed by wind to
neighbouring foliage.
Severe infection delays berry maturity and may cause them
to split.
Source of infection and spread
The fungus has a two-season cycle of disease. Inoculum
(the source of spores) for the current season arises from
material diseased last season. The fungus overwinters
as hyphae (fungal threads) inside dormant duds or as
fungal resting bodies (cleistothecia) on leaf litter and/
or bark.
Cleistothecia form late in the previous season and appear
as tiny yellow (immature) or black (mature) specks on
the surface of severely infected leaves, stems and
bunches. In cool, moist conditions, cleistothecia release a
second spore type called ascospores. Soon after, a new
generation of grey spots may appear on the undersides of
leaves near spurs and cordons where the cleistothecia
overwinter.
Developing buds are infected early in the previous
growing season. They become substantially resistant to
infection when three to four weeks old. The fungus
remains dormant on the inner bud scales until it
resumes activity when the buds shoot next spring. As
the fungus spreads, it covers the emerging diseased
Powdery mildew is a dry weather disease and is favoured
by mild, cloudy conditions. Ultraviolet light reduces the
development of disease that grows best in humid overcast
or shaded conditions, which occur inside large canopies.
Free moisture (leaf wetness) is not required except for
cleistothecia to germinate. Temperatures of 21-28
°
C are
optimal, while temperatures above 35
C may discourage
growth, but rarely enough to slow the development of
disease.
°
Importance
Powdery mildew is a common disease of grapevines. It
may cause extensive leaf loss and a reduction in berry
quality if control is inadequate. Most risk of crop loss
comes from low levels of bunch disease, which can taint
wine and lead to fruit from an entire patch being
rejected.
Management
•
Monitor vineyards from budburst onwards, looking for
leaf blotches and fl ag shoots.
Apply the
three T's
: use the right
t
reatment, at the right
t
iming, with the right spraying
t
echnique. Good spray
coverage is critical for good control.
•
Apply recommended fungicides early in the season -
three sprays within the first 40 days from budburst
(or at least three before flowering) reduce initial build
up of disease and protect young buds from infection.
This leads to good control of disease this season and
to less carryover inoculum for the next.
•
Orientate vineyards and use training systems to allow
good movement of air and penetration of light in the
canopy and reduce excessive shading.
•
Fig 9.28 Leaf symptoms of powdery mildew showing distorted leaves.
Inset: infected growing tip.
