Author: Eunsung OhPublisher:
ISBN:
Category : Phytophthora diseases
Languages : en
Pages : 414
Book Description
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.
