Agriculture Reference
In-Depth Information
Control
Monitoring the vineyard is a key to the success of powdery mildew management.
After budburst the vineyard manager should check flag shoots (young shoots with
only one or two leaves expanded) for signs of powdery mildew infection. Fun-
gicides for the control of powdery include chemicals that inhibit the synthesis of
sterols, which are components of the fungal cell membrane. Without these sterols,
the fungus is unable to function and dies. There are also a number of other chemical
groups that are registered for powdery mildew control. These include the strobi-
lurins (e.g. Cabrio) that interfere with fungal respiration. Sulphur based sprays are
also effective and these have been used in viticulture for over a century. A strategy
termed the '2:4:6 strategy' has been employed for powdery mildew control, where
fungicides are applied at two, four and six weeks after budburst. Once powdery
mildew is established in the spring, it can be difficult to eradicate.
Downy Mildew
Symptoms and Infection
Like powdery mildew, downy mildew also infects the leaves, but the symptoms are
different. Downy mildew manifests itself initially as yellow oil-like spots on the
upper surface of the leaf. Surrounding the oil spot there is often a darker halo, these
spots will enlarge until they may eventually merge to cover the whole leaf. On the
underside of the leaf there will be a white mildew of growth. Pre-veraison berries
are also susceptible, infection results in an arrest in berry development.
Berries then stop growing and develop a purple colouration before dropping
from the bunch. Downy mildew was first introduced to Australia in 1917 in Vic-
toria, and then quickly spread to other areas. The disease is indigenous to North
America and was probably introduced to Europe on phylloxera-resistant rootstocks.
Like powdery mildew, downy mildew is an obligate parasite; it cannot be cultured
in the laboratory on artificial media. Downy mildew is caused by Plasmopara vi-
ticola (Gindro et al. 2012 ) and strictly speaking this organism is not a fungus, it
belongs to a group of organisms that are in the kingdom Chromista.
Control
Monitoring the vineyard is important in downy mildew control. Infection is fa-
voured when there is at least 10 mm of rain while the temperature is 10 °C or more
over a 24 h period. This observation led to the development of a model known as the
'10:10:24 model' to predict disease outbreaks. Many vineyards in downy mildew
prone areas monitor the weather in order to predict likely disease outbreaks.
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