Seasonal response of ponderosa pine to inoculation of the mycangial fungi from the western pine beetle

Abstract

The hypersensitive response obtained by inoculating the two mycangial fungi of the western pine beetle, Dendroctonus brevicomis LeC, and the blue-stain fungus, Ceratocystis minor Hedge, into ponderosa pine was measured during three different seasons in the same 12-month period. The lesions were produced faster and were significantly longer in the fall than in the summer. Similarly, lesions were longer and were produced faster when inoculations made in the summer were compared with spring inoculations. However, the response to a nonpathogenic fungus, a Penicillium sp., was not different from the response to mycangial fungi, suggesting that lesion production is a generalized response to infection that isolates fungus-colonized tissue from the rest of the tree. Ethanol extracts of hypersensitive-response lesions produced during the spring did not reduce growth of the mycangial fungi or C. minor when bioassayed at concentrations similar to those found in host tissue. This is different from other bark beetle–host tree systems that have been investigated.