Silver Leaf

Silver leaf is caused by a fungus that infects wood and the water-conducting xylem through fresh wounds. A toxin produced by the pathogen is carried through the xylem to leaves, causing them to turn a silvery gray. As the disease progresses over a few years, leaves curl upward at the edges and turn brown. Eventually limbs, scaffolds, and the whole tree will die.

Dark brown discoloration of the heartwood in dead or dying limbs is a characteristic symptom of the disease. Spore-forming basidiocarps develop on the surface of trunks and branches that have been killed by the fungus. These are small, leathery structures that are often shelflike in shape and frequently form on the north side of affected trees. Their upper surface is grayish white and indistinctly zoned, and their lower surface is smooth and purplish. They may appear at any time of the year, but most often they are formed in fall. Spores are ejected from the basidiocarps' lower surface during rainy or moist weather and spread by wind. A basidiocarp can produce spores for 2 years. Sapwood-exposing wounds that have not healed over are susceptible to infection. Spores infect exposed xylem, and the pathogen remains confined to the xylem tissue until the infected branch dies.

Leaf symptoms are most easily identified in spring before leaves "harden off." Symptoms are most commonly seen in trees 3 to 5 years old, but the disease can affect trees of any age. It generally takes 1 to 2 years after infection before leaf symptoms are obvious.

The pathogen attacks a wide range of woody plants, including many indigenous to riparian habitats such as willow, poplar, birch, and oak. Silver leaf is most commonly found in almond orchards of the northern San Joaquin Valley and occasionally in other stone fruit species such as peach. The most commonly affected variety is Padre, followed by Butte.

Certain cultural practices help reduce the spread of silver leaf.

• Avoid excessive and improper pruning, including pruning of large branches that may require long periods for wound healing.
• Remove and burn any prunings, branches, or stumps of diseased trees, since basidiocarps may form on infected wood after it is dead.
• Prune young trees in late spring and bearing trees immediately after harvest to reduce the likelihood of infection during rainy weather.
• Integrate management of this disease with biological control treatments such as the application of Trichoderma harzianum, which is available in a commercial formulation, to pruning cuts and other wood-exposing wounds.