Thursday, September 25, 2008

Organic Powdery Mildew Control in McLaren Vale

Powdery mildew on surface of a vine leaf.

Thoughts by James Hook



I am not a promoter of strict organic farming, although I see many advantages in taking techniques that are used and adapting them to conventional farming. There are positives to organic farming however these positives need to be balanced with the drawbacks. For example like organic farming has some techniques that require high-horsepower tillage equipment which uses diesel at a high rate. Conventional farming can do the same job, by using a synthetic product with a low powered four wheeled motorbike which is very efficient.

However, I can see the chemical treadmill that conventional farming has gotten us on. One of the key ways to break this cycle is to learn about the problem you are facing and combat it with knowledge rather than safety spraying.

The number one disease in McLaren Vale, Powdery Mildew, can be beaten with organic techniques.

When are grapes most susceptible to powdery mildew infection? The answer is from the beginning of flowering to the end of fruit set. Consequently the most critical time to spray against powdery mildew is from the beginning of flowering to the end of fruit set. Four weeks after the beginning of flowering (i.e. after bunch closure) is the last stage of susceptibility to powdery and generally does not pose a threat because the fruit is highly resistant to infection. After this point only leaves and bunch stalks can get infected.

Powdery Mildew is a slow creeping disease that is limited by sunlight. The best natural defense is keeping your vines open to sunlight. Open canopies that let sunlight into the fruit zone inhibit its growth naturally; these same open canopies have the advantage of suiting A grade red wine production. Open canopied red grape varieties have much less chance of developing Powdery Mildew infections than white varieties that require shading to stop sunburn.

By putting these two pieces of information together, Powdery doesn’t like sunlight and your vines are at most risk during flowering, we can make a conclusion on how best to control the disease.

You will have a much better change of organic control if you run your vineyard with techniques to let sunlight into the developing flowers and bunches. You will have a much better chance of control is you try organics on high quality red wine grapes with open canopies. If you are trying organics on whites try to keep sunlight in the bunch zone until the fruit is naturally immune.

1 comment:

Anonymous said...

I shifted from conventional powdery control to (uncertified)organic about 7-8 years ago.
I've had next to no infection since and find it more effective than Bafidan et al was.
The basic spray regime is two early sulphur sprays (it kills beneficial bacteria as well as pathogens so then only used later if serious infection threat), then alternate between powdered milk at 10:1 (sunlight through milk heniates the powdery spores I'm told), and microbe teas with Bacillus Subtillus (sprayed on mild evenings to give the microbes best chance of work eating pathogens through the night, they don't like sun or dry wind).
VSP helps with prevention as you describe, as does a couple of silica sprays(strengthens the cell walls of leaf and stem I believe)

Keep up the good work

Tony French-Kennedy