Water stress– One of 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 results in smaller berries. Periderm may form earlier on shoots with the water stressed vines. Water stress occurs when the vine is unable to uptake 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.
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 be infected by plant pathogens or necrotic microorganisms, such as grapevine crown gall.
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 and followed by applications of necessary nutrients is recommended.
Magnesium deficiency –Symptoms are usually expressed in older basal leaves. Leaves become yellow on along the margin, while the tissue around the main veins of the leaf remain dark green. Red cultivars express reddening rather than yellowing. Magnesium deficiencies are common in vineyards with 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.