I have received numerous question about grapes drying out before they are ready for harvest or that pea-size berries crack open. So, I have decided to write an article about Powdery Mildew and also include some of the pictures send by them – enjoy!

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Wondering what the white powder-like mildew on grapes is? It is called oidium and it is caused by a fungus called Uncinula necator. This fungus only attacks grape plants and a few of their related species. It is safe to say that it is a widespread fungal disease that can cause total crop loss and or reduced fruit quality, wine quality and vine growth. Oidium’s severity will vary from season to season but it does require treatment each and every season.
The powdery mildew can be seen on all parts of the grape and vine. The foliage, fruit, flower parts and the canes. The first place that you are going to see it is usually on the undersides of the basal leaves. At first, it appears as a whitish or greenish white powdery patch. You may notice mottling or a distortion on the severely infected leaves. Curling and withering may also be noted.

The lateral shoots are incredibly susceptible to the fungus. The blossoms if infected may not turn to fruit. The berry is most susceptible to being infected in the first three to four weeks after bloom. The rest, though, the shoots, petioles and other parts are susceptible throughout the season.

If the infection takes place early, it can reduce the size of the berry and decrease the sugar content as well. You will also notice that the infected berries will have what appears to be a netlike pattern on them. They may crack open and dry up or just never ripen. On the canes, you can see old infections because they will show up as brown areas. As the fungus grows on the grapes and vines and begins to produce spores you will see that the tissue that is infected with have an ash grey powdery look.

Although a bit out of focus, you can clearly see

the brown areas on the shoots and stems of the grapes

The organic grower is going to take into consideration things such as: the location of the vineyard, design of vineyard, row orientation, choice of variety of grape (due to susceptibility factors), canopy structure, irrigation, water and nutrition and shoot removal done early in the season.

There are chemical treatments that can be used as well to help treat and get rid of the powdery mildew. The application of fungicides should start with early shoot growth and continue until bloom. It is important to establish good control early so that the disease is prevented from becoming the powdery mildew epidemic of the summer. Fungicides that are used most often include sulphur, Nova, Lance, Sovran, Flint or Milstop.

Doing a dormant spray of lime sulphur is very effective when it comes to suppressing any over wintering population of the mildew. Applying in the early spring before the buds break will kill the powdery mildew, covering any dormant vines is very important. Then there is the post-harvest spray. These are also beneficial, and the date of your harvest will help you determine the necessity to keep foliage and canes protected.

There are some cultural things that you can do to help control the disease and possibly prevent it. Make sure that you are selecting proper rootstocks, training systems and fertility. Make sure that you are practicing timely sucker control. Cut the canes back close to the top wire of the trellises. Make sure that you are removing leaves so that bunch rot does not occur, this allows the fungicides to better cover the clusters. You can also choose one of the very few grapes that are not susceptible to this fungus to grow the grapes that are not susceptible such as Auxerrois, Malvoisie, Melon, Pinot Gris and Semillon.

Choose carefully, plan wisely and spray at the appropriate times to make sure that the powdery mildew does not attack and destroy your crop.

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