Year of Publication

2006

Degree Name

Doctor of Philosophy (PhD)

Document Type

Dissertation

College

Agriculture

Department

Plant Pathology

First Advisor

Dr. Lisa Vaillancourt

Second Advisor

Dr. John Hartman

Abstract

Diplodia pinea causes Diplodia tip blight on more than 30 different pine species. During the past 10 years, Diplodia tip blight has emerged as a serious problem in landscape and Christmas tree farms in this region. Surveys of diseased and symptomless Austrian pines revealed that latent infections of symptomless shoots by D. pinea were common. Latent infections may account for the recently observed rapid decline of mildly diseased pines in our region. To investigate the colonization habits of D. pinea within its host, molecular cytology was attempted and traditional histology was performed on naturally infected, diseased and asymptomatic Austrian pine tissues. I devoted much effort to developing a transformation system for D. pinea. Ultimately I did not succeed in this goal, but I was able to develop a highly efficient protocol for Agrobacterium tumefaciens-mediated transformation of another pathogenic fungus, Colletotrichum graminicola, in the process. The work that I did should help in future efforts to transform D. pinea, something that will be essential if it is to become a tractable system for the study of fungal latency. Traditional histological methods were more successful, and provided important information about the nature of latent infections. Very sparse epiphytic and subcuticular fungal growth was observed in healthy shoots, however, no fungal tissues were present within the shoots. In diseased and latently infected shoots, crevices created between the needle bundles and the shoots were filled with fungal material, and hyphae were observed colonizing the needle sheaths. Hyphae were also observed breaching the shoot epidermal layer in these crevices and colonizing the underlying periderm. D. pinea colonization was extensive in all tissues of diseased shoots early in symptom development. In contrast, localized pockets of degradation were observed in the periderm and adjacent cortical cells located around areas of needle attachment in asymptomatic, latently infected shoots. The mechanism that operates to prevent expansion of these infected pockets in the latently infected shoots is still unclear. Obvious signs of pine defense mechanisms were only observed in 2 shoots. My observations were consistent with the idea that colonization progresses into the vascular tissues, and that this results in symptom development. Vascular colonization may occur more readily if the host is stressed. My research lays the groundwork for future efforts to understand the nature of the transformation from latent to pathogenic infection.

append_1.pdf (115636 kB)
append_2.pdf (54215 kB)
cpt1.pdf (23446 kB)
cpt2.pdf (567 kB)
cpt3.pdf (27426 kB)
cpt4_1.pdf (99364 kB)
cpt4_2.pdf (108492 kB)

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