Shothole borer damage on Almond – Varietal Differences in Response to Water Stress

The water crisis on the West side of the valley has presented opportunities to see problems not typically observed in almond production. Many of these issues tend to be directly related to tree stress caused by lack of water, failure of salt leaching due to decreased winter rains, and chronic water stress from prolonged drought. Typical observations of drought stressed trees range include yellowing leaves and leaf drop in mild cases, wilted new growth during moderate stress periods, and poor crop set, thinned canopy, and scaffold die back in chronic cases. A visit was made to an Aldrich-Monterrey-Nonpareil orchard located near Los Banos, CA. All varieties were on Nemaguard rootstock. This orchard has been under moderate drought stress for the past two years as determined by the use of a pressure bomb. This year, the grower was again short on water and had to water at about 80% evapotranspiration rate for the season. The grower was concerned about clear gumming coming from multiple wounds on his trees. Orchard observations included those listed above for orchards suffering from chronic drought stress: poor vigor, thinned canopies, and reduced crop load. On the affected trees, gumming was seen extending form the graft union up to the scaffolds. The issue was predominantly found on Aldrich trees, with some strikes on the Nonpareil, and very few, if any, strikes on the Monterrey. Upon seeing the damage (Picture 1), it was very clear that it was shothole borer, Scolytus rugulosus, an insect that invades and lays eggs in the branches/trunks of stressed trees. Borer insects are able to push their eggs through the bark and into the tree. A tree that is healthy enough to elicit a response then gums profusely in order to flush the egg out of the tree’s tissue (Picture 2). This gum

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