Fig. 320. Advanced spring symptoms of potassium deficiency, including marginal and interveinal necrosis accompanied by downward rolling of leaf margins. (Courtesy W. Gärtel)
duced. Sometimes light-brown islands of dead tissue appear between the main veins of the basal leaves, and in extreme cases the leaf blades may wilt and abscise. Berries may be small. No specific patterns of organ malformation are associated with nitrogen deficiency. In commercial plantings, the first symptoms may appear after berry veraison because nitrogen is trans located from the leaves near the clusters into the berries. During long periods of cool, wet weather, vines may develop “cool-weather chlorosis,” which can be confused with nitrogen deficiency. Low temperatures reduce chlorophyll synthesis, and the chlorosis disappears as soon as temperatures rise. Mechanical injuries and root damage from soil pests such as nematodes and phylloxera may hinder the absorption and transport of nutrients and lead to symptoms that also may be confused with nitrogen deficiency. Excess nitrogen increases growth; internodes become long and large, and leaf blades become deep green, thick, and sometimes cupped. Shoot growth may be excessive and prolonged. Grapevines are more tolerant of high nitrogen when other micro-and macronutrients are in adequate supply and when the trellis system allows for adequate leaf exposure to light. Excessive rates of foliar urea can cause phytotoxicity that presents as foliar burn.
Phosphorus Phosphorus deficiency is an uncommon nutritional problem in grape production. Thus, most reports are based on symptoms obtained in nutrient culture. In such cases, shoot and root growth are reduced and leaves are small and dark green. Leaf margins turn down without rolling. Under severe deficiency, leaves develop red, punctiform spots. Only a few experimental results report crop increases achieved through phosphorus fertilization. Rather, a history of excess phosphorus fertilization is more common. Excess phosphorus may induce zinc and iron deficiencies. Phosphorus deficiency may be one component of the nutrient imbalance brought about by low soil pH called acidic soil sickness (see Acidic Soil Sickness). Fig. 321. “Black leaf” symptom of potassium deficiency seen in late summer on leaves that receive direct sunlight. (Cour tesy L. Cheng)
Potassium The symptoms of potassium deficiency vary with the stage of development of the leaf blades when the potassium content
Fig. 322. Symptoms of “acidic soil sickness” on various cultivars grown in low-pH soil (bottom) compared with leaves from those cultivars grown in the same soil to which lime was applied (top). (Courtesy T. Bates)
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