I often receive questions about why the leaves of a grape vine turn yellow long before fall.

Although there are about 20 reasons why these symptoms show on leaves, I think the most common reason for yellowish leaves on a grape vine, is Magnesium (Mg) deficiency.

Not all grape vines show symptoms of Magnesium deficiency, but it is becoming more and more evident in vineyards as we learn more about the importance of Magnesium for optimum grape quality.

What is the function of Magnesium in a grape vine?

Magnesium form part of the chlorophyll molecule.  As you can recall from school, chlorophyll is a green pigment found in most plants, hence the green colour of chlorophyll-containing tissues such as the leaves of your grape vine.

As we all know by now; the leaves (and green parts) of the grape vine is where the carbohydrates are manufactured and transported to the rest of the grape vine.  These carbohydrates are necessary for optimum growth, the immunity of the grape vine and for the proper development of the berries.

The symptoms of Magnesium deficiency:

More often, the symptoms of Magnesium deficiency is overlooked when growing black or red grapes, as white varieties show Magnesium shortages more prominent.

 

 

Deficiency symptoms are seen on the older basal leaves and start with leaf margin yellowing that moves inward. The leaf veins stay green the longest and the areas between become pale green and often creamy white. In red grapes, there may be a reddish colouring that develops between the veins (like the picture below).

 

  If you are not familliar with these terms, have a look at the picture below:

 

 

Not only does Magnesium deficiency affect the leaves and the manufacturing of carbohydrates, but it can also lead to premature fruit drop at harvest (more reason to keep a look out for the symptoms!).

How to correct Magnesium deficiency:

The most common mistake grape growers make, when they diagnose a grape vine with a Magnesium deficiency, is add tons of Magnesium to the soil or foliar.

Over the past 20 years I have been growing grapes, I have found that most of the time, it is not a shortage of Magnesium that cause the problems (symptoms), but more often it is a pH problem or Potassium (K) / Magnesium imbalance.

The problem starts when you are growing grapes on more acid soils.  Magnesium is tied up to the soil particles of acid soils and become more and more unavailable to the grape vines, the lower the pH gets.

Now, the first thing you do before planting your grape vine, is to correct (raise) the pH of these soil, right?  But most of the times, grape growers don’t take into account that by adding elements like lime and Potassium to correct the pH, it can cause even more Magnesium to become unavailable to the plants!

Soils with very high levels of Potassium (K) (many of the organic growers will have this problem), will have little available Magnesium.  Why?  Because Potassium will displace Magnesium cations (Mg ++), making less  Mg anions available to the plant.

In other words, if you have a low pH soil (below 5.5 pH), and you want to raise the pH without binding the Magnesium, you should apply dolomitic lime (high in magnesium) at the rate of two to four tons per acre – BUT before you run out to by some lime, I do recommend you let your soil be analyzed first.  Magnesium soil test values between 100-250 ppm are considered adequate for growing grapes.

If the Magnesium deficiency symptoms are picked up during the growing season (normally between flowering and veraison), a petiole analysis (leave stalk analysis) of the grape vine is the right way to determine what is going on inside the grape vine.  A foliar spray of Magnesium sulphate will temporary correct the issue.  The most common recommendations are 15.0 to 20.0 lb. of magnesium sulfate/100 gal. sprayed as a dilute spray at 200 gal./acre.

 

Then again; get your vineyard soil analyzed do the corrections there, as Epson salt can cause foliar and fruit damage if mixed with other products.

 I hope that this article shed some light on the subject of the importance of Magnesium when growing your grape vines.

 

Have an excellent day my friend!

Danie

www.my-grape-vine.com

 

P.S.  Did you like this tip? 

Join the more than 2000 happy grape growers by becomming a member of the Complete Grape Growing System Today!

Share and Enjoy

Tagged with:

Filed under: growing grapeshow to grow grapes

Like this post? Subscribe to my RSS feed and get loads more!