Does Foliar-Applied Phosphorus Influence Vine P Status, Mycorrhizal Fungi, or Fruit Yield and Quality of ‘Pinot Noir' Grapevines?

Foliar application of phosphorus (P) fertilizers in Western Oregon vineyards was previously linked to reduced root colonization by arbuscular mycorrhizal fungi (AMF) at veraison. Vines grown in low P soils typical of most Oregon vineyards may benefit from additional P supplied to the canopy. Alternatively, vines may be negatively affected by foliar P use if lower root colonization by AMF reduces uptake of other nutrients or increases vine water stress. The impact of foliar P fertilizer use on vine growth, nutrient status, AMF colonization, drought stress, and fruit yield and quality was studied in two ‘Pinot noir’ vineyards over three years. Three treatments (phosphite fertilizer, phosphate fertilizer, water control) were applied to the foliage three times over the growing season using a standard air-blast sprayer at a commercial vineyard. Two treatments (phosphite fertilizer, water control) were applied using a hand sprayer at a smaller research vineyard. Fertilizers were applied to foliage on the same days and at the same rate and volume per acre at both sites. The level of P applied to the foliage in each year was equivalent to 50% of the yearly canopy accumulation of P for vines grown in the region. Foliar P applications increased leaf and petiole P concentrations and reduced arbuscular colonization of roots at the research vineyard, but not at the commercial vineyard. This may be a result of greater precision of application at the research vineyard. Foliar P use had no effect on vine growth, drought stress, yield, or fruit quality attributes at either site. These results indicate that there is little benefit to applying P fertilizers to the canopy of grapevines, even in low P vineyards. The slight reduction of AMF root colonization that did occur at the research vineyard in response to foliar P applications did not result in a measurable increase in vine water stress or significant effect on other vine parameters.