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 response is a rapid response to a wound, not an infection, and therefore clear gum is exuded.
Removal of the bark from around a gumball reveals a small hole (Picture 3). This hole was probably formed by the female beetle laying her eggs (Picture 4).
Shothole borers can kill trees, but typically damage this severe is not present in a vigorous orchard. If eggs laid within the tree hatch, the borer larvae will feed on plant tissues, creating a gallery which could girdle the tree and cause tree collapse (Picture 5). Eventually the larvae will mature and emerge leaving exit holes which resemble damage done by a shotgun (Picture 6). Of another concern is the introduction of fungi who gain entrance through the wounds caused by beetle entrance/exit. These fungi may also girdle limbs and cause branch or tree death.
Control strategies for of the shothole borer are very limited: chemical controls can work, most are not registered for use, and the others appear to be more of a hassle than worth in an orchard setting. Infested branches should be removed from the orchard and burned. Traditionally, insecticide mixed with latex paint was applied tot he trunks of the trees. This practice is no longer available for use: the insecticide previously used are not labeled for this practice. Therefore, the main factor in reducing shothole borer damage in an orchard setting is to reduce tree stress. Stress trees attract shothole borers either through by increases infrared radiation or the emittance of kairomones.
In this orchard, the tree stress was most likely due to lack of water. Using a pressure bomb, six trees of each variety ( Aldrich , Nonpareil, and Monterrey) where pressure bombed in order to determine the differences in varietal susceptibility. The pressure chamber average results were as follows (Remember: the more negative, the more stressed): -17.3 bars, -16.5 bars, and -15 bars for Aldrich , Nonpareil, and Monterrey, respectively. According to the UC general guidelines, all of these trees are moderately stressed, with the Aldrich approaching severe stress. It is also worth to note that the variety that is least affected by shothole borer was also the variety that was least stressed. This provides evidence for the stressed tree hypothesis proposed above.
This brings to light another question: Why do varieties within the same orchard, same irrigation system/program, and same soil profile differ so greatly in their tree stress? This question remains to be unanswered but probably has something to do with one or more of the following possibilities:
1. Varieties differ in their temperature ranges for stomatal activity. Some varieties may have stomates active during higher temperatures causing a higher rate of water loss (i.e. Aldrich varietal may maintain stomatal activity when temperatures reach 90 degrees F while Monterrey varietal shuts down stomates at 88 degrees F).
2. Varieties differ in their vigor. Some varieties may “pump” more photosynthate into the root system, thus increasing the root mass and depth, providing more access to soil moisture.
3. Varieties may differ in their tolerance of water stress. Varieties react differently to a mild, moderate, severe water stress, resulting in a reduction of time that stomates are open, thus reducing water loss (i.e. Aldrich varietal may maintain stomatal activity when moderately stressed while Monterrey varietal shuts down stomates when moderately stressed).
What do you all think?