North American gall-inducing fungus
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Cystotheca lanestris, the live oak witch's broom fungus, is a species of mildew that infects buds and induces stem galls called witch's brooms on oak trees in California, Arizona, and Mexico in North America. Witch's brooms are "abnormal clusters of shoots that are thickened, elongated, and highly branched." This fungus infects coast live oaks, interior live oaks, canyon live oaks, valley oaks, and tanoaks, and is most commonly found along the coast. Research published in 2023 newly describes this fungus as also growing in association with "Quercus laceyi" and "Q. toumeyi."
According to the U.S. Forest Service Treesearch Department: 
 ;templatestyles src="Template:Blockquote/styles.css" / ;"C. lanestris" mycelium primarily grows on the lower leaf surface...The white mycelium turns tan to brown with age. The fungus can cause systemic infections that induce the formation of witches' brooms...Leaves on witches' brooms are small and senesce early...The entire broom becomes white when the fungus sporulates, and may either die out over the winter or produce either more brooms or normal shoots the following season.
While this fungus is technically considered a plant disease, most infections are harmless "except possibly in small seedlings". "Cystotheca lanestris" is considered widespread in California.
References.
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