Breeding Port-Orford-cedar for resistance to Phytophthora lateralis, a causal
agent of root disease, begins by screening, through artificial inoculation,
phenotypically resistant trees selected from natural stands. The successful program
selected tolerant or resistant POC parent trees for the purpose of disease management.
Candidate resistant POCs were used in my dissertation to: 1. validate screening
methods such as stem- and root-dip inoculation; 2. test for increased virulence of P.
lateralis; and 3. evaluate detection techniques. The results showed that the established
screening methods were appropriate, and no evidence of changed virulence was found.
A PCR technique was more reliable than other techniques for detection of P. lateralis
in seedlings. An additional test for foliar infection showed that initial penetration
through wounds and natural openings was possible.
POC seedlings and rooted cuttings from resistant and susceptible families were
used to demonstrate resistance mechanisms. In order to explain the mechanisms at the
cellular level, the susceptible response of POC seedlings to P. lateralis was first
observed with light microscopy. Zoospores encysted on lateral roots, germinated, and
penetrated by means of appressoria. Direct penetration between epidermal cells was
common but penetration through epidermal cell walls was also observed. The hyphae
colonized the root cortex inter- and intracellularly. Wound inoculation on stems
resulted in inter- and intra cellular hyphal growth in cambial, sieve, and parenchyma
cells in the secondary phloem.
Several resistance mechanisms were observed: 1) there was a difference in
zoospore attraction between susceptible and certain resistant POCs revealed by
microscopic observation, direct count of encysted zoospores, and quantitative real-time
PCR; 2) the frequency of encystment, penetration, and colonization of resistant
seedlings was much lower than susceptible seedlings, but no differences in infection
pathway were observed by means of light or electron microscopy; 3) collapsed cell
walls were present in resistant POCs showing increased cell wall thickness, wall
appositions, and electron dense materials. / Graduation date: 2005
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29749 |
| Date | 30 November 2004 |
| Creators | Oh, Eunsung |
| Contributors | Hansen, Everett M. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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