Symptoms and Signs
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.
Comments on the Disease
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.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(PlantShield HC)||16–32 oz||4||0|
|MODE OF ACTION: Biological competition|
|COMMENTS: Apply as soon as possible after pruning or whenever wood-exposing injuries occur (within 2–3 days).|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.|
|1||Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number.|
|#||Acceptable for use on organically grown produce.|