Tuesday, October 14, 2014

Fungicides for Grapes

The three most common fungi affecting grape vines in our area are downy mildew (Plasmopara viticola), powdery mildew or oidium (Uncinula necatora) and botrytis or grey mould (Botrytis cinerea).
Downy Mildew oil spots on upper and white down on under side of leaf

Powdery mildew

Botrytis affected bunch

We spray commercial fungicides in an effort to prevent these diseases plus one that cures any outbreak of downy mildew.
In a protective spray program, fungicides are applied to the grapevines at regular intervals largely dictated by the weather eg. lengthy or intense rain incidents can reduce the time between sprays.
In curative programs sprays are applied only after conditions that are conducive to disease outbreak have occurred eg. 10:10:24 for downy mildew where >10mm of rain over 24 hours when the temperature is >10°C.
Sprays differ with regards to their mobility within the plant.
Some sit on the leaf surface where the spray droplets have fallen and do not move.
So any unsprayed area or new growth is not protected.
Some spread out over the leaf surface and move inside the leaf tissue. These are known as locally systemic or translaminar. New growth is not protected.
Others spread out over the leaf surface and move inside the leaf tissue and enter the plant through the stem and move upwards via the xylem to new growth. These are systemic. New growth (within reason) is protected.

How do the chemicals sprayed work or, more technically, what is their mode of action (MOA) ?
Fungicides kill by damaging the cell membranes of the fungus or inactivating critical enzyme or protein production or interfering with vital metabolic processes such as respiration.
So fungicides are grouped according to their MOA and may or may not have similar chemical structures.
What is the extent of the activity of the various chemicals?
Some are active only against a single enzyme or protein of the fungus or just at one point in its metabolic process. These are known as single site fungicides.
Others are active on numerous functions of the pathogen and are known as multi site fungicides.
It is important to be aware of the MOA of single site fungicides. Continually targeting pathogens at the same 'site' eg. overusing the same chemical, can eventually lead to resistance.
Multi site fungicides do not have this disadvantage.
For our preventative program we use the multi site activity copper oxychloride (M1) against downy mildew, wettable sulphur (M2) against powdery mildew, chlorothalonil (M5) against botrytis (and downy mildew). We also use a systemic phosphonate (33) as a downy curative when weather conditions have been particularly difficult, not an unusual situation in our warm maritime climate.
The basic plan is copper and sulphur every two weeks, chlorothalonil at 80% cap fall and just before bunch closure.
A full list of AWRI approved chemicals for 2014/15 as well as a spraying strategy to reduce the risk of resistance is available here.

No comments: