Systemic fungicide treatments may have the potential to inhibit both P. ramorum trunk cankers and twig infections. If fungicide-treated tanoaks develop fewer or smaller twig infections, spore production will be reduced, lessening the likelihood of trunk infections. To benefit from the synergistic effects of reduced spore production and increased resistance to trunk infections, contiguous blocks of tanoaks need to be treated. If untreated tanoaks are left within a stand, spores produced on these trees could splash onto treated trees, which may reduce disease control.
In well-stocked tanoak stands, it may be more efficient and cost-effective to remove small understory tanoak seedlings and saplings instead of treating them with fungicides. Because spores from small (<1 m) understory tanoak regeneration or stump sprouts are unlikely to splash more than about 2 m, these small shoots only need to be removed if they are within about 2 to 3 m of treated tanoaks.
Because California bay is the primary source of P. ramorum inoculum in oak stands, fungicide treatment of all oaks within a block does not provide any advantage over selective treatment of individual oaks. For susceptible oaks, use of fungicides should always be combined with California bay removal, if possible, to reduce disease pressure. Where California bays are too large or significant to remove, fungicide use may be warranted to help protect high-value oaks that are closest to California bay trees. However, fungicides may not be sufficient to prevent SOD in trees that are exposed to large numbers of P. ramorum spores.