No abstract

Bark beetle-vectored ophiostomatoid fungi, Leptographium terebrantis, is inoculated on the roots and lower stems of stressed Pinus species during the feeding activity of bark beetle. To determine the exact host response following inoculation, it is critical to challenge the host with a realistic amount of fungal inoculum. Thus, we designed a series of stepwise experiments using L. terebrantis colonized toothpicks which focused on the inoculum transfer from the toothpicks to excised Pinus taeda stem segments and living saplings, respectively, at different inoculum densities. The toothpicks served as a substrate for fungal growth and sporulation and the inoculation showed their utility in eliciting host's response to the pathogen. The inoculated fungus caused blue-stain and sapwood occlusions in P. taeda stems and saplings, respectively. The volume of occluded, visually damaged sapwood increased by 1.96 cm 3 per radial inoculation point on average. Fungal colonized toothpicks can be used as a suitable alternative to agar discs for studying bark beetles vectored fungi and their host interactions.

The complex interaction of various biotic and abiotic factors may put the overall stand health ofPinusspp. at risk. A study was designed to determine the combined impact of drought and vascular-inhabiting fungi (Leptographium terebrantisandGrosmannia huntii) inPinus taeda. Seedlings from twoP. taedafamilies were planted and watering treatments, (i) normal watering, (ii) moderate drought, and (iii) severe drought, were applied. One month following the initiation of watering treatments, seedling stems were artificially inoculated withL. terebrantisandG. huntii. Drought and fungal interaction significantly affected lesion length/seedling height, occlusion length/seedling height, and seedling fine root biomass.Leptographium terebrantiswas more pathogenic under moderate and severe drought than normal watering condition, whereas the pathogenicity ofG. huntiiremains unaltered. The susceptibility of the families to vascular-inhabiting fungi remained the same under different watering treatments. Drought and specific vascular-inhabiting fungi may negatively impactP. taedastand health.

Variation in the pathogenicity of 42 isolates of Leptographium terebrantis to Pinus taeda was examined. Stems of 1‐year‐old P. taeda seedlings were artificially inoculated with the fungal isolates. Eight weeks following inoculation, L. terebrantis isolates caused dark necrotic lesion and sapwood occlusion in the seedling stems. The fungal isolates varied in their ability to cause lesion and tissue occlusion. Lesions caused by fungal isolates were, however, significantly longer than the control. Results suggest that different isolates of L. terebrantis may not put the health of P. taeda at equal risk.