How to Manage Pests

UC Pest Management Guidelines

Almond

Silver Leaf

Pathogens: Chondrostereum purpureum

(Reviewed 3/09, updated 3/09)

In this Guideline:

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 shelf-like 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.

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. 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.

MANAGEMENT

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. Basidiocarps may form on infected wood after it is dead, so be sure to remove and burn any prunings, branches, or stumps of diseased trees. 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 Trichodema harzianum, which is available in a commercial formulation, to pruning cuts and other wood-exposing wounds.

Common name		Amount/Acre	R.E.I.+	P.H.I.+
(trade name)			(hours)	(days)
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. Always read the label of the product being used.
A.	TRICHODERMA HARZIANUM#
	(PlantShield HC)	3–24 oz	0	0
	MODE OF ACTION: Biological competition
	COMMENTS: Apply as soon as possible after pruning or whenever wood-exposing injuries occur (within 2 to 3 days).
+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.
#	Acceptable for use on organically grown produce.

PUBLICATION

UC IPM Pest Management Guidelines: Almond
UC ANR Publication 3431

Diseases

J. E. Adaskaveg, Plant Pathology, UC Riverside
W. D. Gubler, Plant Pathology, UC Davis
Roger Duncan, UC Cooperative Extension, Stanislaus County
J. J. Stapleton, UC IPM Program, Kearney Agricultural Center, Parlier
B. A. Holtz, UC Cooperative Extension, San Joaquin County

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