Phytophthora syringae and First leaf Tree Infections

Figure 1: Young almond tree killed by Phytophthora syringae.

This year, Phytophthora syringae infections have been quite common. This is most likely due to the long, cool wet spring. Many new orchards have seen 3% or greater infection rate, especially in the variety Nonpareil. Figure 1 shows symptomatology of a tree affected by P. syringae. Leaves on these trees are lesion free, but tend to dry up, becoming crispy. Trees are not always killed, but they may be stunted in comparison to other trees within the orchard.  Examination of the trunk will typically yield amber "gumballs," which often mark the edge of the fungus canker (Figure 2). When the bark is removed, the canker becomes visible (Figure 3). This disease has been observed frequently, and is commonly referred to as "bundle rot."

Figure 2: Amber colored "gumballs" caused by tree infection with  Phytophthora syringae.

Although common this year,  infection by P. syringae does not typically cause such a large problem. When temperatures warm above 75F, the fungus within the canker dies, and the affected area heals over as the tree resumes normal growth. Weather this year has been very mild, with few days above 75F.

The fungus infects the trees through wounds that occur from digging, processing, delivering, planting, and rough weather. Upon infection, the fungus may remain latent until the tree breaks dormancy. If conditions remain favorable, the fungus continues to grow, eventually killing the tree. Most likely, we do not have a higher amount of this fungus within the orchard in comparison to any other year, but rather that the current cool and wet environmental conditions are promoting fungal growth, causing an increase in observed incidence.

Figure 3: Bark removal shows  fungal canker of an almond tree affected by Phytophthora syringae.

If tree loss or damage occurs within your block due to P. syringae. It is better to remove and replant if the the tree is dead, severely stunted, or if the trunk has been partially killed. It is advised to call your nursery as they often are willing to help with tree replacement. Some trees may be marginally affected. It is advised to keep these trees in the ground because as the temperatures warm, the fungus will die and the tree will recover. It is not advised to spray phosphanates or other anti-Phytophthora products in attempts to prevent tree infection.