Pruning Wound Cankers Found in Almond Trees

Making pruning cuts on almond trees is risky business. Every cut provides the opportunity for fungal/bacterial pathogens to enter the tree (Think: a cut on you hand can allow bacteria to enter). Once the pathogens enter the tree and successfully colonize, poor tree performance and shortened orchard life can be expected.

Figure 1: Fungal canker associated with a pruning wound on a young tree. Upon bark removal, canker growth and damage is evident.

Fungi are the most commonly found pathogens invading tree wounds. In figure 1, a large pruning cut was made on a super-vigorous first leaf almond. The pruning wound became infected with the fungi Eutypa which grew through the tree causing a large canker. This canker did not become noticeable until the third leaf. Upon bark removal, it is clear that the canker grows outward from the point of origin. As the canker continues to grow, it has the ability to kill branches by girdling (Figure 2), weaken scaffolds which then break (Figure 3), and/or killing the tree by girdling the trunk (Figure 4). Botryosphaeria is another fungi that may cause this problem.

Figure 2: Scaffold killed by fungal canker girdling the vascular tissues of the branch.

Figure 3: Scaffold split caused by weakening of crotch angle by invading fungi.

Figure 4: Tree killed by wood fungal canker girdling the vascular tissue of the tree trunk. Bark removal shows the advancement of the canker.

Irregardless of the possible infection by fungi, trees need to be pruned to shape (young trees), remove unwanted branches, and remove dead/diseased tissue. Knowing this, how can we prune the tree and reduce the chance of fungal infection? Most fungi require moisture/high humidity/rain event to produce spores. These spores are usually transferred by wind-blown rain.

The "window of opportunity" for these fungi to infect almond trees occurs when the tree has an open wound. Large cuts (larger than a quarter) made on an almond tree may take up to 14 days to heal, while smaller cuts can take up to 10 days. This open wound can provide a point of infection for fungi until the cut tissue "heals." At this point it becomes resistant to most infections. To prevent these types of problems, it is advised that growers prune their trees when rain is not forecasted for the foreseeable future (7-10 days). For super-vigorous first/second leaf trees, consideration should be made to prune unwanted branches during a period of low humidity (late summer/early fall). Available pruning wound paints do not appear to be effective in preventing invasion by Eutypa and Botryosphaeria in almond.