Genetic interactions in the radiata pine-western gall rust pathosystem

Vogler, D. R. 1991.

Department of Plant Pathology, University of California, Berkeley 94720.

Ph.D. dissertation, University of California, Berkeley. 206 pp. (1991)


Abstract

The radiata pine-western gall rust pathosystem is endemic to the western United States, and may provide a useful model for other wild forest pathosystems. The host (Pinus radiata D. Don) is confined to three populations on California's central coast and two islands off the coast of Baja California, but has been planted extensively world-wide; it is fast-growing and easily propagated by rooted cuttings. The pathogen (Peridermium harknessii J. P. Moore) is an autoecious rust fungus that infects several hard pine species and causes discrete galls on its hosts. This study reports on several aspects of the pathosystem, including isozyme structure and nuclear division in the pathogen, vegetative propagation of the host, clonal variation for susceptibility to P. harknessii in field trials, and clone-isolate interactions in greenhouse inoculations.

A population genetic study of the western gall rust fungus (WGR) was conducted using isozymes as genetic markers. Starch gel electrophoresis of 404 aeciospore isolates collected from several pine species revealed that WGR comprises two distinct zymodemes (multi-locus electrophoretic types) in the western United States. Within zymodemes, all 15 loci studied were monomorphic (0.95 criterion), although rare variants (frequency <0.03) were found at three loci. All Pacific Coast isolates and many from the Sierra Nevada, Cascade, and Rocky Mountains were in zymodeme I, which was characterized by single bands, indicating haploidy or homozygosity at all loci. Fifty percent of isolates from Pinus contorta Dougl. in the Sierra Nevada and Rocky Mountains were in zymodeme II, in which double or triple bands were observed at six of 15 loci; isozyme phenotypes at the other nine loci were identical to those of zymodeme I. Generally, all isolates within a forest stand were in the same zymodeme and, where isolates from both zymodemes were present, recombinant genotypes between zymodemes were not observed. The uniformity of isozyme structure and the lack of recombinant genotypes are inconsistent with sexual reproduction, suggesting that P. harknessii is asexual.

Nuclei, stained with a DNA-specific fluorochrome, were counted in mature, ungerminated and in germinating aeciospores from both zymodemes of WGR. Spores of eight isolates from zymodeme I (collected from several pine hosts in Montana, Oregon, and California) were predominantly binucleate, while spores of 14 isolates from zymodeme II (collected from Pinus contorta in California, Idaho, and Nevada) were predominantly uninucleate. Thus, the number of nuclei in the aeciospores is a convenient marker for zymodemes I and II. In germinating aeciospores of zymodemes I and II, there was an arithmetic increase in the number of nuclei over time. The results were consistent with expectations for mitotic division in both zymodemes, regardless of whether mature, ungerminated spores were predominantly uninucleate or binucleate. There was no evidence for nuclear fusion or meiosis in any isolates (with one anomalous exception), suggesting that zymodemes I and II are both asexual.

Radiata pine was propagated vegetatively by cutting succulent shoots from donor hedges, treating them with fungicide and a rooting hormone, and setting them in rooting medium under intermittent mist until roots formed. Cutting donors included clones of the three mainland populations (Ano Nuevo, Monterey, and Cambria) and two Mexican island populations (Guadalupe and Cedros Islands), inter-population hybrids, and Australia-New Zealand selects. Within a set of 31 clones, 97.1% (6454/6646) of all cuttings rooted. Radiata pine populations and clones exhibited significant differences in percent rooting and time to root; generally, Guadalupe clones were the slowest and Monterey clones the fastest rooters. For some clones, rapid rooting and field susceptibility to WGR were associated, but multiple regression analysis did not support a linear relationship between susceptibility and rooting success across all clones. Short, thin, succulent cuttings rooted at a significantly higher percentage than longer, thicker, partially hardened-off cuttings, indicating rooting success can be improved by careful selection and handling of cutting tissue. Potted hedges of lodgepole pine (Pinus contorta), Bishop pine (P. muricata D. Don), and incense-cedar (Libocedrus decurrens Torr.) also served as cutting donors; generally, these species rooted as successfully as radiata pine.

In January 1988, stecklings of twenty-four radiata pine clones were planted in blocks in four plantations at Russell Reservation, Lafayette, CA; branch and stem galls were counted after two years of exposure to WGR inoculum (i.e., in 1990). Clones were derived from all five native populations, from inter-population hybrids, and from Australia-New Zealand selects. The goal of the experiment was to determine whether clones would exhibit similar levels of resistance or susceptibility to WGR as had their seedling ortets in a 1982 experiment at the same site. Generally, clones maintained similar levels of susceptibility between experiments, with some exceptions; many of these anomalies may be because results of the 1982 experiment were based on the response of a single seedling ortet, while in the 1988 experiment eight ramets of each clone were exposed to WGR at four sites. Perhaps most important, clones that were extremely susceptible or extremely resistant after two years in the 1982 experiment (i.e., in 1984) were generally consistent in response to WGR in the 1988 experiment. Four of five clones that were uninfected in 1984 were the least-infected clones in 1990, and three of these five were again uninfected. Six of eight clones that were severely infected in 1984 were again among the most severely infected clones in 1990.

Ramets of thirty-one radiata clones were inoculated with 22 single-gall aeciospore isolates of WGR collected from galls on Pinus radiata, P. muricata, and P. contorta at several locations in California, including Russell Reservation (11 isolates; zymodeme I), widely-separated forest stands on the Pacific Coast from Cambria to Eureka (8 isolates; zymodeme I), and the Sierra Nevada (3 isolates; 2 from zymodeme II). The experiment was designed to test whether differential clone/isolate interactions occurred in this pathosystem. Inoculated ramets were observed monthly up to 12 months to record early host responses, first symptoms of infection, and definitive gall formation. Clones exhibited a range of susceptibility to inocula (55.1 to 91.0% of all ramets/clone infected), and inocula exhibited a range of infectivity to clones (infecting 48.1 to 93.8% of all ramets/isolate). Relative levels of clonal susceptibility in greenhouse inoculations were generally consistent with results of the 1982 and 1988 field experiments, but all clones proved substantially more susceptible in greenhouse inoculations. Among 646 clone/isolate interactions tested, only one resulted in no infected ramets. Early resistance infection types were absent, and no evidence for differential interactions between clones and isolates was observed for infection.

Results of the greenhouse experiments suggest that the radiata pine-WGR pathosystem is non-specific, with quantitative variability for virulence in the pathogen and susceptibility in the host. However, ontogenetic resistance may be involved in this pathosystem in nature, since clones that were uninfected after two years in the field were nevertheless readily infected in the greenhouse under conditions optimal for infection. Most early host symptoms were not consistent indicators of subsequent clone infection, and those that were reliable were too infrequent (<15% of all symptoms) to be useful. Most infected ramets (94.4%) developed galls within eight months, but some clones (including three of four from Cedros Island) developed galls more slowly; furthermore, particular isolates (including both from zymodeme II) were associated with slow gall formation. In some clone/isolate interactions, galls did not develop after exhibiting definitive symptoms of infection, suggesting a possible late resistance response preventing further growth of the fungus.


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