Field identification and characterization of manganese toxicity in 'Western Schley' pecan trees

Adult 'Western Schley' pecan trees with delayed budbreak and dieback of current year's shoots during and immediately following budbreak were observed in a pecan orchard located in Cochise County in Southeast Arizona. Soil and leaf tissue analyses indicated that this condition was a result of Mn toxicity. Leaf and soil were sampled from trees with varying degrees of toxicity symptoms during the period from 2004 to 2007. In the spring of 2008 twenty trees with different grades of delayed budbreak were classified as unaffected, moderately affected, and severely affected according to the intensity of the disorder to determine leaf Mn concentration and content, leaf area, leaf weight, shoot growth, fruiting shoot percentage, and fruits per cluster corresponding to these conditions. Severely affected trees had higher leaflet Mn concentration (4034 ± 1482 ppm) as compared with unaffected trees (1620 ± 646 ppm). Fruiting shoot percentage was reduced from 86 ± 8.4 % in unaffected trees to 7 ± 6.2% in severely affected trees. Shoot growth and leaf area were also influenced. Severity of the disorder increased as soil pH decreased (down to 5.26) and diethylene triamine pentaacetic acid (DTPA) extractable Mn increased (up to 42 ppm). By midsummer, leaf size and weight were similar in severely affected and unaffected trees. Chlorophyll index was not affected by the intensity of the disorder. During budbreak, trees with leaf Mn less than 1620 ± 646 ppm had a normal growth and reproductive characteristics. Trees with leaf Mn concentrations higher than 2782 ± 1203 ppm in May had reduced shoot growth (reduced from 13 ± 4.2 to 2 ± 0.8 cm) and the percentage of shoots with fruit was reduced from 86 ± 8.4 to 7 ± 6.2.