Death in the forest and the case of the missing vector

Fusarium circinatum, which causes a disease known as pitch canker, infects wounds on susceptible pine species. In the southeastern U.S., where pitch canker was first described, weather-related injuries and silvicultural practices were considered the most important causes of wounds. However, when pitch canker was discovered in California, it affected trees in landscapes and native forests that were not subject to management practices and where weather induced injuries were rare. Consequently, it was unclear how wounds that would serve as infection courts were being created. One clue that insects might be involved came from the fact that twig beetles (Pityophthorus spp.) could be reared from branch tips killed by the pitch canker pathogen. One problem with postulating a role for twig beetles as wounding agents or vectors was the fact that these insects seek out dead or dying host material to colonize immediately after emergence and therefore would not be expected to visit healthy branches. This conundrum was resolved by demonstrating that twig beetles could not identify weakened branches prior to landing and that if they landed, by chance, on a healthy branch they would create small wounds, presumably for the purpose of assessing the suitability of the substrate for colonization. If spores of the pathogen were present on the surface of the branch, these wounds became infected by F. circinatum.