Graduate Thesis Or Dissertation
 

Infection of stressed ponderosa pines by Armillariella mellea from indigenous and artificial inoculum sources

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  • Armillariella mellea (Vahl. ex Fr. ) Karst. was grown aseptically in hazel stems (Corylus cornuta var. californica) which were used in the forest to infect 290 ponderosa pines (Pinus ponderosa Laws.) which had been "stressed" in five different manners plus two control groups. All trees were inoculated twice giving a total of 580 inoculation attempts. Stress treatments consisted of: 1) girdling lateral roots; 2) severing distal portions of lateral roots; 3) removal of fifty-percent of the cross-sectional area of the lateral roots; ) removal of the bottom two-thirds of the biomass of the live crown; and 5) stressing the roots by felling the tree. Controls consisted of inoculations of the lateral roots and root collars of unstressed trees. Infection occurred at one-hundred ten (19%) of the inoculation points, of these 97 were by the indigenous population of A. mellea rhizomorphs. Significantly more trees were infected at the root collar than at lateral roots, but lesion size was uninfluenced by location or treatment. More infections occurred on trees of excellent than poor vigor, but no differences in susceptibility could be detected based on position of the crown ( overtopped, intermediate, codominant, dominant). Once attacked, lateral roots or root collars showed similar resistance to A. mellea, even while under stress. During the earlier stages of infection establishment, disease is little influenced by stress, crown position and decreased host vigor. Prevalence of rhizomorphs of A. mellea in eight ponderosa pine stands was found to be influenced by silvicultural histories of the stands. The highest quantities were on sites clearcut in 1965 and sprayed for shrub control. Fewest occurred in young pole-sized stands about 70-years old. The results suggest that A. mellea rhizomorphs will increase with increasing manipulations by man. The greatest increase will occur when a stand is clearcut and little increase will be observed when a stand is thinned. Internal mycoflora of healthy and declining lateral roots of ponderosa pine respond to changes in host vigor before visual changes are apparent. Microfungi were observed in apparently healthy cambial and xylem tissues. As lateral roots declined in health the mycofloral populations changed. One-hundred twelve days after felling of the tree ten-percent of the xylem and five-percent of the cambium was sterile. The majority of the microfungi isolated from the healthy and declining lateral roots showed no antagonism towards A. mellea in culture.
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