Silver leaf is a fungal disease of the xylem tissues of many various species of fruit trees. Caused by t he fungus Chondrostereum purpureum, it is commonly found in riparian areas containing hardwood trees such as willows, poplar, birch, and oaks.
Figure 1: Padre leaves showing symptoms of silverleaf (left) versus leaves from a healthy Padre tree (right).
A farm call this week brought me to an orchard showing signs of silver leaf infection. Silver leaf is so named because of its ability to make laves appear silver in color (Figures 1 and 2). This appearance is due to a toxin produced by the fungus that is carried through the xylem to the leaves. Once within the leaves, it creates a separation between the epidermis and palisade layer, creating an air gap that interferes with the normal interception of light. As the disease progresses, leaves curl upward at the edges and turn brown (Figure 3). Scaffolds will eventually collapse, shortly followed by tree death.
Figure 2: Close up comparison of a Padre leaf showing symptoms of silverleaf (left) versus a healthy leaf (Right).
Irregular cankers found within the xylem tissue of dead and/or dying branches is another characteristic symptom of the disease (Figure 4). In later stages of tree decline, spore-bearing conks or shelf like mushrooms form in the fall. Generally found on the north side, these mushrooms are gray to white in appearance and have a smooth, purplish lower surface. The spores from the mushrooms can infect neighboring trees, and are released upon rain events. Exited spores are moved by the wind and the mushrooms can produce spores for 2 years.
Figure 3: Severe infections of silverleaf are characterized by curled leaves and necrotic margins.
Wounds with exposed xylem or sapwood or vulnerable to infection. This includes pruning wound cuts, growth cracks, wind cracks, or growth inclusions that have not healed completely over. The most commonly affected variety is Padre, followed by Butte. Any age of tree can become infected, but trees between the ages of 3-5 are most commonly observed with the disease. This may be due to the amount of pruning cuts made to train the young tree. It can take up to 2 years after infection for symptoms to become obvious.
To prevent the infection of trees, tree injury should be avoided – especially during rainy periods. Avoid excessive pruning or removal of large branches that require a long time to heal. If cuts need to be made, make them as early in the fall as possible to avoid having susceptible wounds during the rainy period. Cuts made to trees in spring, summer, or fall can take one to several weeks to heal, while cuts made in the winter may take longer. In areas of high disease pressure, applications of the biocontrol fungus Trichoderma harzianum can be made to pruning cuts to help prevent infection by the silverleaf pathogen.
Figure 4: Cross section of a tree infected with silverleaf shows a distinctive ireegular shaped canker within the heartwood.
There is no way to cure a tree affected by silverleaf. Affected trees should be removed and burned before the conks form in the fall. Furthermore, try to remove as many roots as possible upon replanting. Once one tree is found within the orchard, there usually are a few more. Be sure to scout the orchard for other diseased trees.