|Pinus taeda (loblolly pine), a most widely cultivated timber species in the southern U.S., creates 110,000 job opportunities and contributes 30 billion dollars per year to the economy. However, insect-vectored root-infecting ophiostomatoid fungi, Leptographium terebrantis, and Grosmannia huntii are potential threats to sustainable P. taeda forest management in the southern U.S. Understanding the intra-species response of P. taeda to these fungi is critical to mitigate the potential problem due to these fungi. Thus, the objectives of my research are: (i) to determine the intraspecific tolerance/susceptibility of P. taeda to L. terebrantis and G. huntii, (ii) to understand whether intraspecific tolerance of P. taeda to L. terebrantis and G. huntii remain same regardless of the tree growth stage (iii) to understand the interaction of the vascular-inhabiting fungi and P. taeda under drought conditions, (iv) to determine the antibiosis potential of these fungi by plant growth-promoting rhizobacteria (PGPR), (v) to understand whether PGPR can induce resistance of P. taeda families to these fungi, (vi) to determine the intraspecific variation in virulence of L. terebrantis, (vii) to determine the growth potential of most virulent L. terebrantis at different inoculum densities in P. taeda wood segments, and (viii) to determine the growth potential of various blue-stain fungi on P. taeda stem segments.
In study 1, seedlings from 94 P. taeda families were artificially inoculated at the stem with L. terebrantis and G. huntii and family responses were studied. In study 2, the roots of the mature P. taeda trees from 4 families were inoculated with these two fungi to understand the intraspecific response of mature trees. In study 3, P. taeda families were exposed to drought and simultaneously inoculated with vascular-inhabiting fungi and impacts were studied. In study 4, PGPR strains and ophiostomatoid fungi were plated together in a dual agar plate and the antibiosis potential of PGPR strains to fungi was studied. In addition, induced systemic resistance of P. taeda to L. terebrantis and G. huntii were studied by inoculating PGPR in soil and fungi in stems of P. taeda seedlings. In study 5, most virulent L. terebrantis was inoculated to P. taeda stem segments at different inoculum densities. In study 6, various blue-staining ophiostomatoid fungi were cultured in P. taeda stem segments to study their growth potential.
The results suggest P. taeda families vary in tolerance to ophiostomatoid fungi with potential for selection of relatively tolerant families. Moreover, this intra-species variation in tolerance is an inherent character of P. taeda, regardless of the tree growth stage. The growth and productivity of P. taeda seedlings decrease and fungal pathogenicity increase under severe drought. Specific strains of PGPR have the ability to inhibit the growth of blue-staining fungi in vitro. Specific PGPR strains have the capacity to induce systemic resistance of P. taeda during fungal infection. Fungal growth and blue-staining potential are high when fungal inoculation points are closer. Growth and staining potential of L. terebrantis in P. taeda stem segment is higher compared to G. huntii and G. alacris. This study provides meaningful insights into P. taeda and ophiostomatoid fungal interaction.