Chondrostereum purpureum

Chondrostereum purpureum is a plant pathogen which causes the Silver leaf desease of fruit trees; plums are especially vulnerable.

In the past the name Stereum purpureum Pers. was widely used for this fungus, but according to modern taxonomy it is only distantly related to Stereum, actually belonging to order Polyporales whereas Stereum is in order Russulales. .

Characteristics
After starting as just a crust on the wood, the fruiting structure develops undulating intergrowing brackets up to about 3cm broad, which have a tough rubbery texture. The edges and fertile lower surfaces show a fairly vivid violet colour while the fungus is growing, and the upper surfaces have a grey aspect (sometimes with zonation) and are covered with whitish hairs. After a week or two the fructification dries out, becomes brittle, and turns a drab brown or beige. Infected wood can be recognized because it is stained a darker tint.

The spores are rounded cylinders approximately 5-8µm x 3-4µm in size. The hyphal structure is monomitic with clamp connections.

It is often found on old stumps and dead wood, but can also be a serious parasite of living trees. As well as plum trees it attacks many other broad-leafed species (other Prunus, apple, pear, willow, poplar, maple, hornbeam, plane, oak, elm, lilac, and many others). Occasionally it also infects conifers (fir, spruce, Thuya, ...). Geographically it is roughly speaking just as widespread as its hosts - it is common in woods, orchards and tree plantations in temperate climates.

A biological control agent
C. purpureum is commercially available as a method of combatting forest "weed" trees such as red alder, aspens, and others (see U.S. patent 5587158 and Canadian patent 2451038). The fungus is applied directly to the weed trees in a nutrient paste which can be stored and handled conveniently. According to a report of the Canadian Pest Management Agency, the use of this control method will only have a limited impact on non-target trees since the fungus spores are ubiquitous anyway and healthy trees are resistant to attack.