Almond Shaker Damage

Severe shaker damage will reduce future yields and can kill almond trees.

Shaker damage is commonly observedacross orchard operations. This damage occurs when the force of the shaker isgreater than the strength of the bark, causing it to tear away from the tree.The obvious damage not only impacts tree vigor, but also provides anopportunity for infection by wood canker fungi which can kill the tree. If theshake is properly timed and executed, shaker damage can be greatly reduced (andeven eliminated) within an orchard.

A common misconception is thattrees that are kept too wet during the harvest period will be damaged by theharvesting process. Interestingly, this has never been supported in researchnor found to be consistently true across operations. Research in the mid 90’sby Gurusinghe and Shackel found that withholding irrigation during July throughharvest did not reduce bark damage of the trees. To further elaborate, theyfound no difference in bark strength with respect to shaker damage for almondtrees grown under various irrigation treatments. Irrigation treatments includeda wet treatment and dry treatment, in which mid-day stem water potential (SWP)was maintained at -9 bar and -20 bar, respectively, through the month ofJuly/August. Despite this range in tree water status, both treatments exhibitedthe same gradual increase in bark strength through July and August. Based onthis, the researchers concluded that water stress did not influence the timingor rate of tree bark strengthening.

Almond trees with shaker damage are often infected by the wood canker pathogen Ceratocystis fimbriata. Infected trees usually die within 5 years. Covering wounds with thiophanate-methyl may help reduce infection.

Based on the lack of direct effectof moisture status on tree bark strength, it is thought that the damageobserved in wet areas of the orchards is most likely due to the delays inripening. Research has found that decreased water stress (Goldhamer andcolleagues, 2006) and high nitrogen status (Saa and colleagues, 2016) both  delay ripening. These trees, when shaken atthe same time as the rest of the field, are often shaken harder to remove thenuts that aren’t ready to harvest. This harder shake often leads to barkdamage. This hypothesis is supported by observations from multiple operations,suggesting that this is the most likely cause of most shaker damage.

To reduce shaker damage, thefollowing practices should be considered:

1. Properly time harvest to reduce the force needed for nut removal;
2. Improve irrigation distribution uniformity to reduce over-irrigated areas of the orchard;
3. Maintain mid-summer leaf nitrogen levels within the range of 2.3-2.5%;
4. Develop a shaker operator training and maintenance program to make sure trees are being shaken correctly with proper settings on the equipment;
5. And consider starting to harvest later in orchards considered more susceptible to bark damage (e.g. young orchards) to allow for more time for bark hardening.

New shaker operators should betrained and supervised under an experienced operator  until they gain familiarity with theequipment. They should be instructed that damaging trees is preventable and ifdamage occurs on multiple trees (i.e. 1 out of 100 trees), they should stop the harvesting process for furtherevaluation.  

If damage occurs, some practices may help recovery. Damaged bark should be removed from the tree to help the formation of callous tissue. It is currently advised to make a clean cut when removing the bark. After the removal of the bark, research suggests that painting the wound with thiophanate-methyl may help reduce wound infection by Ceratocystis.  If mixing with paint, acrylic-based paint should be used over latex or oil.

After the damage is done, it cantake multiple years for the tree to recover. Damaged trees will stress easierand may advance through hull-split faster, increasing the risk of insect damageand stick tights. Furthermore, damaged trees will use less water and are proneto over-irrigation. This often leads to the surrounding trees become more susceptibleto bark damage due to the delayed ripening from the extra water. Reducing theapplied water within the damaged areas is advised.

Works cited or referenced:

Gurusinghe, S.H., Shackel, K.A. 1995.  The relation of cambial zone mechanicalstrength to growth and irrigation of almond [Prunus dulcis (Mill.) Webb.] trees. JASHS 120:170-176. 

Saa, S., E. Peach-Fine, P. H.Brown, et al. 2016. Nitrogen increases hull rot and interferes with hull splitphenology in almond (Prunus dulcis). Scientia Horticulturae 199: 41-48.

Goldhamer, D., M. Viveros, and M.Salinas. 2006. Regulated deficit irrigation in almonds: effects of variationsin applied water and stress timing on yield and yield components. IrrigationScience 24, 2: 101-114.