Photosynthesis, Transpiration, and Water Use Efficiency of Mature Grape Leaves Infected withUncinula necator(Powdery Mildew)

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Abstract
Photosynthesis and transpiration of mature leaves of susceptible Vitis vinifera ''White Riesling'' and less susceptible V. labruscana ''Concord'' were related to levels of infection by U. necator that developed in the field. Photosynthetic rate was reduced more in ''White Riesling'' than in ''Concord'' at similar levels of U. necator infection and U. necator-induced tissue necrosis, due primarily to greater palisade layer destruction in ''White Riesling'' than in ''Concord'' leaves. Because transpiration was relatively unaffected by infection, water use efficiency in both cultivars decreased with increasing infection. In ''Concord'', visual ratings of tissue necrosis provided better estimates of photosynthesis reduction than did visual ratings of U. necator infection. In ''White Riesling'', both methods gave similar estimates of damage.