Water stress – One of the first symptoms of water stress is the cessation of shoot elongation. Shoots on the water-stressed vines tend to be short, and flowers may abort or result in smaller berries.  Periderm may form earlier on shoots of water-stressed vines.  Water stress occurs when the vine is unable to take up enough water to support the canopy and crop load. Drought is the main cause of water stress; however, excess water, which can kill roots by soil oxygen deprivation, can also cause water stress-type symptoms.  Check your local weather history as well as the site conditions, such as soil drainage. Severe drought stress can cause leaves to yellow, brown, and eventually dry and fall off.

(Photo credit: Mizuho Nita)

Magnesium deficiency –Symptoms are usually expressed in older basal leaves. Leaves become yellow along the margin, while the tissue around the main veins of the leaf remains dark green. Red cultivars express reddening rather than yellowing. Magnesium deficiencies are common in vineyards with a high potassium to magnesium ratio and/or low pH in soils (<5.5 pH). Nutrient test of vines and soils, followed by application of necessary nutrients and/or soil pH adjustment, is recommended.

(Photo credit: West Virginia University Extension – Fertilizing Grapes)

Mechanical damage – Trunks may become damaged by mechanical means. Feeding activities by animals, girdling from wires or vegetation, or human activities such as a use of weed eater or improper removal of sucker shoots, can injure the vine.  The injury itself can result in water stress-type symptoms (short internodes, yellow discoloration of leaves, etc.), but the injured trunk is often subject to infection by plant pathogens or necrotic microorganisms, such as grapevine crown gall.

(Photo credit: Mark Battany)

Potassium deficiencies – Symptom is leaf reddening (red-fruited cultivars) or yellowing (white-fruited cultivars) that starts on the margins of the leaves and progresses inwards interveinally. Symptom development starts on basal leaves and moves upward, and often becomes visible near or around veraison when berries become the “sink” of the nutrients. Severe potassium deficiencies can result in curling and burning of leaf margins. Nutrient testing of vines, followed by applications of necessary nutrients, is recommended.

(Photo credit: West Virginia University Extension – Fertilizing Grapes)