What Causes the Vineyard to Turn Yellow After It Blooms?

Soil Deficiencies

• When the entire vineyard turns yellow slowly, or large portions of it, the cause is most likely deficiency of minerals in the soil. You will want to have your soil tested, as it could be too little nitrogen, potassium, iron or magnesium. Elemental deficiencies can be resolved through the addition of fertilizer high in the required element.

Insects

• Many insects plague grape growers, but can be handled readily. An infestation requires sterner measures. Signs of a leaf hopper include leaf yellowing in spots on the leaf, gradually spreading to the entire leaf. Grape phylloxera are tiny soft yellow insects similar to aphids that produce leaf yellowing and galls. There is no good way to get rid of them, but they can be prevented with irrigation and fertilizer to help the vineyard stay unstressed.

Disease

• Immediately after blooming, the young grapes are vulnerable to downy mildew, which affects both the berries and the leaves. The leaves will look moldy on the underside and splotched with yellow from above. Downy mildew and powdery mildew can be treated with a fungicide spray at or immediately after bloom time. Pierce's Disease, a bacterial infection spread by leaf hoppers, causes the vines to yellow and then turn brown as they die. Vines should be removed, as there is no cure.

Drought

• Lack of water causes yellowing of the leaves, stunted grape growth and loss of leaves. Poor irrigation practices can lead to only a few vines being affected, or the whole vineyard can show signs of water loss stress in a very dry year. Modern irrigation allows for targeted water delivery to each and every vine with drip irrigation. Sprinkler systems waste water, as the water will evaporate before reaching the roots, and can foster diseases by leaving the leaves damp.