In the Vineyard

Prepare for mildew in May

Don’t be lulled into a false sense of security if the new shoots look good in early spring. By Gary Strachan


Implementing control measures early in the season is the key to preventing a powdery mildew epidemic later in the summer.

t’s really easy to postpone that first spray. Pruning is complete, and lime sulfur was applied before bud break. The canes have stopped weeping , the weeds are under control and the world looks good. What could possibly go wrong?

Consider that it’s early May. The weather is warming and the new shoots are pushing rapidly. They look soft and green. This could be the start of a luscious infection of mildew. In an average year May temperatures are below optimum for a mildew infection to initiate, but sometimes an early May warm spell could have daytime temperatures in the mid-20s, perfect for a mildew infection. Don’t forget that the early stage of infection is invisible to the naked eye. Don’t be lulled into a false sense of security if the new shoots look good in early spring. The infection can come back to bite you later in the summer.

New growth is especially vulnerable to infection. It hasn’t yet laid down defences that appear in more mature canes. If you are spraying oil or sulfur, remember that these are physical barriers to infection and there will be no protection for new growth that occurs between sprays. Keep a close eye on daytime temperatures and growth rate because it makes good sense to spray more frequently during a rapid growth period.

You will sleep better in late summer if you know your hard work in spring avoided disease that could wreck your crop.

There are other tools that can assist resistance to infection. A good long-term strategy is to infect your vineyard with mycorrhizae. This is a synergistic filamentous fungus that infects vine roots.

A physiological response of vines to fungal infection is that they release phytoalexins. This is analogous to the human immune response to infection that releases interferon and antibodies.

The response will not impart immunity to infection but it will enhance resistance. In past columns I’ve written about the other advantages of mycorrhizae in the vineyard. Always remember that a mildew infection is enhanced in the presence of moisture. An east-facing slope will dry more

rapidly in the morning than a west-facing slope. Also recall that

ultraviolet light is lethal to fungi; therefore open, unshaded canes are more resistant to infection. Ultraviolet light also induces vines to produce phytoalexins. The open canopy also allows more breeze to pass through and carry moisture away. All of the above

methods of control are acceptable organic or biodynamic strategies. Another obvious

control is to choose grape varieties with inherited resistance to infection.The BC Best Practices Guide has a table which lists the mildew resistance of many varieties commonly planted in the province. Some hybrid grapes are so resistant they can be grown without sprays.

Growers who practice integrated pest management (IPM) have another tool. There are many systemic mildew sprays that have both a retroactive and a residual effect. A retroactive effect provides the ability to suppress an infection that has already occurred.

A residual effect provides inhibition to infection for plant tissue that has grown since the last spray. Systemic sprays can be a good choice when something has gone wrong and your back is against the wall. They might save the crop if an infection has gone out of control, but a visible infection can only be inhibited, not reversed. Systemic sprays can also be useful as a pre-bloom spray because the interval between sprays is typically greater than with barrier sprays such as oil or sulfur. This allows bees to access the vineyard unmolested. Most domestic grapes are self pollinating but bees nevertheless collect pollen and can enhance fruit set.

Systemic sprays come with a warning that fungi can develop resistance to the sprays and the choice of spray material must be rotated according to biochemical grouping.

This is in contrast to barrier sprays, which impart a physical resistance to fungal infections. There is no chance of fungal resistance to barrier sprays, thus they can be used repeatedly without rotation.

There is a huge advantage if an infection can be controlled in early summer. Mildew infections overcome infection barriers and pierce plant surfaces. Even if the infection is controlled, the scar remains and can facilitate further infections such as botrytis or the various species of a sour rot infection.

This is a synergy to avoid. Once a fungal infection is established, plant sap is released and bacterial secondary infections such as Acetobacter can start to release volatile acidity.

In an extreme case, wave goodbye to your crop. Diligence in spring time can provide a huge payback.

British Columbia FRUIT GROWER • Spring 2018 25

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