|Figure 1: Fungal infection of a pruning
wound that occurred during first
leaf pruning on Padre
Pruning wound cankers have been observed on all major varieties and are the most damaging to orchards. Cankers that develop on the primary (Figure 1) or secondary scaffolds (figure 2) of young trees will lead to the loss of these scaffolds and eventual tree death. Sometimes infection occurs within the wind cracks found in the base of the scaffolds. Removal of the infected scaffold may be possible, but all of the diseased wood must be removed or the infection will remain. To stave off infection, proper structuring and tying of trees to reduce wind breakage, and pruning when the weather forecast is clear, is advised. Delayed spring pruning or late fall pruning to avoid the rains may be necessary. Pruning cuts should not be cut flush with the tree, but rather be made outside of the branch collar as this increases the rate of healing. The varieties Aldrich, Carmel, and Padre appear to be more susceptible than Nonpareil or Butte. This disease has been noted to have a high occurrence around rivers, sloughs, and other riparian areas. It is important to note that once the tree is infected, there are no fungicide or nutrient sprays that can cure this disease.
|Figure 2: Fungal infection of a
pruning wound made on a
secondary scaffold in Padre.
Control strategies for these three diseases have been difficult to develop due to the complexity of the fungi’s lifecycle. Initial infection is thought to occur through airborne spores that move in to an orchard from a residual population. Recent research has shown that uninfected trees can harbor populations of these fungi. It is thought that the fungus has the ability to infect and survive on dead bark, remaining dormant until conducive conditions occur for tree infection. Since these fungi are found on many species within the environment, it is difficult – if not impossible – to provide adequate control through application of fungicides. Strategies that prevent infection (i.e. proper pruning, reducing excessive vigor) should be utilized to help reduce the severity of these diseases within the orchard.
Modern farming practices may make these wounds more or less susceptible than what was previously described. Previous research has shown that pruning wounds larger than 0.5 inch diameter may be susceptible for three to five days. Larger cuts are susceptible longer. Covering wounds with paints did not appear to increase wound healing, but may prevent the wound from fungal infection. Wound susceptibility and the feasibility of protecting tree wounds and trunks with fungicides are currently being researched.