As almond trees enter dormancy in California, many growers will shift their focus to pruning. This practice, which coincides with the onset of winter’s cool, wet weather, often leads to the development of pruning wounds. Research conducted by the University of California Cooperative Extension (UCCE) shows that pruning wounds are primary sites of infection for fungal canker pathogens Botryosphaeria, Cytospora Ceratocystis, and Eutypa, each of which can cause serious damage to growers’ orchards.
The act of pruning removes protective bark from the tree, which in turn exposes cambial and vascular tissues that are susceptible to infection. These pruning wounds require time to heal and create an opportunity for pathogens to infect the sensitive tissues before the pruning wound is completely healed. To avoid the threat of fungal canker diseases due to pruning wounds, growers must be extra mindful of the weather, minimize standing water in the orchard and remove any infected branches.
Plant pathologists Leslie Holland and Florent Trouillias1 recently conducted research on pruning wound susceptibility to fungal infections in almonds. Their research aimed to answer several key questions regarding pruning wound susceptibility, protection, and to define management guidelines for almond canker diseases.
They discovered that fresh wounds faced the highest infection rates at 80-100% infection. Further, through their research they found, after pruning, allowing two weeks to pass before inoculation decreased infection rates by 60%, and allowing three weeks to pass decreased rates by 75%.
Ultimately, as time after pruning increased, wound susceptibility to infection decreased – the older the wound, the less susceptible it was to infection. This is particularly important to consider when rain events, standing water, and high humidity in your orchard create favorable conditions for fungal pathogens to occur.
Rain events, in particular, can aid in the spread of infectious spores, so pruning should be avoided if there is rainfall forecasted. However, if rain is forecasted in the weeks after pruning and yet the job must be done, prior to the rain event growers should consider applying a fungicide spray (such as Topsin-M® or Topsin-M + Rally®) to cover up and protect the pruning wounds. In addition, after any rain event growers must strive to minimize standing water in the orchard, and can cut checks in their berms, if necessary, to release standing water, which in turn will reduce moisture and humidity in the orchard.
Another key precaution growers can take to prevent disease is to ensure all infected wood andbranches are removed from the orchard in a timely manner. Cankers on infected branches, which are ripe for new infection, will continue to grow unless they are cut out. Growers can remove existing cankers by cutting branches several inches-to-a-foot below the canker’s outer ring to insure complete removal.
For more information on pruning wounds, fungal diseases and possible treatments, stay tuned for an upcoming, in-depth article from Trouillas. Growers are also encouraged to visit the Sacramento Valley Orchard website for information on this topic as well as other orchard management resources.
1 https://www.sacvalleyorchards.com/almonds/trunk-soil-diseases/pruning-wound-protection/
