Boron deficiency of stone fruit is rare in the San Joaquin Valley (survey), even though B deficiency has been found in many vineyards in the area. Symptoms that have been observed include shoot dieback and the subsequent development of many side shoots. Leaves on these shoots are small, thick, misshapen and brittle. Defoliation often follows. B deficiency can also have a dramatic effect on fruit set. In our sand tank study, we observed greatly diminished fruit set but no other symptoms. Based on this relationship we have suggested the B deficiency threshold for mid summer leaf samples should be set at 25 ppm rather than the currently published value of 18 ppm for peaches and nectarines (Refining Deficiency Levels). If leaf levels (or dormant shoot samples) drop below the threshold, correction can easily be achieved with foliar or soil-applied fertilizers containing B.
Boron toxicity can occur in peaches if summer leaf samples exceed 100 ppm. Symptoms include necrotic spots on the underside of the midrib. Cankers can also develop on the midrib or on petioles and young twigs. The fruit is distorted with sharp sunken areas and poor kernel development. Although some soils and irrigation water sources in California contain high levels of B, it is rare in the San Joaquin Valley (survey). However, toxicity can result from the over application of B containing fertilizers.