Resistance to the western gall rust fungus, Endocronartium harknessii, was observed in 3-, 10-, 20-, and 33-month-old greenhouse-grown seedlings of lodgepole pine (Pinus contorta var. latifolia). Three sites of resistance were identified: epidermal, cortical, and cambial. In cases of epidermal resistance, penetration of the epidermis occurred, but infection was prevented by an apparent hypersensitive response. Cortical resistance occurred where infected cells in the cortex were successfully isolated by necrophylactic periderm and the infected tissue was exfoliated with the rhytidome. In cambial resistance, infections progressed to the vascular cambium where infected cells and cambial initials were inactivated. This resulted in abnormal secondary xylem development, characterized by a zone of pathological tissue extending from the pith to the epidermis. In a number of infected seedlings, cambial function was restored and infected lesions were overgrown. Live mycelium was often maintained in cortical lesions and in some cases reinvaded healthy cortical cells. These latent-type infections resulted in the initiation of gall formation up to 1 year after initial resistance to infection occurred. Key words: tree improvement, western gall rust, pine stem rust.
Temporal changes in three factors (shoot susceptibility, spore availability, and spore germinability) were evaluated to estimate their effects on the infection of lodgepole pine (Pinus contorta var. latifolia) by Endocronartium harknessii. Germinability of rust spores from sori free of hyperparasites remained above 80% during the growing season and thus would not have significantly limited infection. In contrast, there was a slight, but statistically insignificant, increase in estimated susceptibility as infection increased from 94% of maximum at 45% shoot elongation to maximum susceptibility at 90% shoot elongation. This was followed by a precipitous decline in estimated susceptibility to 57% of maximum when shoots were 95% elongated. Spore availability was 24% of maximum when shoots were 45% elongated, and thus may have been limiting at the beginning of the growing season. At the end of the growing season, spore availability declined prior to, or simultaneous with, the decline in susceptibility. The combined effect of both factors drastically reduced the number of infections estimated to occur once shoots are more than 95% elongated. As shoots elongated, relatively more infections formed higher on the shoot, supporting the hypothesis that periderm is important in limiting infection.
Survival and growth of planted white spruce (Picea glauca (Moench) Voss) were assessed at year 15 in boreal mixedwood stands of northern Alberta, Canada, in stands that were deciduous-dominated prior to logging or were conifer-dominated. Three overstory retention levels (0%, 50%, and 75% retention) and four site preparation treatments (mound, high speed mix, scalp, and no treatment) were evaluated. In deciduous-dominated stands, planted spruce performed best in the 50% retention; here, stem volume was at least double that of any other retention treatment after 15 years. In contrast, spruce had reduced growth in coniferous-dominated stands in both 50% and 75% retention treatments compared with the 0% retention. Survival of planted spruce was unaffected by level of retention, but survival was lower in coniferous-dominated stands than in deciduous-dominated stands; in the coniferous-dominated stands, survival was better with mounding and mixing and lowest with scalp treatments. All height variables tended to be greater in the mix and mound site preparation treatments. Finally, the best estimates of future total growth (regenerated spruce and deciduous combined) in the coniferous-dominated stands were in the clearcut treatment. In terms of regenerated spruce growth, the best estimates occurred in the deciduous-dominated – 50% retention stand planted with soil mixing–mounding treatments, where projected growth of spruce was comparable with that of open-grown and tended stands in Alberta’s boreal forests.
Uptake and translocation of maltol into 6-week-old Pinusconforta (Engelm.) var. latifolia seedlings was followed by means of gas chromatography. Maltol concentrations of 6.0–7.3 mg/g seedling dry weight (46–55 μg/seedling) were observed over the first 3 days following a single application of 50 mg maltol/seedling. A split-plot design was used to determine if maltol could reduce seedling infection by Endocronartiumharknessii. Whole plots consisted of two inoculum levels (7 or 30 mg of spores per tray of seedlings) and subplots consisted of four levels of maltol (0, 2, 10, or 40 mg/mL) applied 2 days before inoculation to one-half tray of seedlings at the rate of 5 mL/cavity. The highest concentration consistently and significantly reduced seedling infection relative to the untreated control, although it also caused some phytotoxicity.
Large black stem galls occur sporadically on trembling aspen (Populus tremuloides) in western Canada. Although little is known about their cause or structure, trees having these galls are less likely than surrounding aspen to have advanced decay caused by the fungus Phellinus tremulae. The anatomy and histochemistry of black galls and associated branch galls were studied and compared with normal wood and bark. Light microscopy showed that the cambium of black galls produces greater numbers of cells per growth ring and that growth rings are two to three times wider than normal. Vessel elements and fibers are unusually small and misshapen. Gall xylem has characteristics associated with wounding or infection: ray cells filled with phenolic deposits, and vessel elements occluded by tyloses and granular material. Frequent radial strands of undifferentiated callus tissue surrounded by necrophylactic periderms indicate sites of cambial damage of unknown cause. White areas within dark-colored gall xylem of some samples were free of most of these abnormalities, suggesting that a persistent agent is required for continuing tumor growth. Thickened outer bark harbored a variety of saprophytic fungi, especially hyphomycetes. Surface and internal morphology of black galls was also compared with similar stem galls caused by poplar budgall mites (Aceria parapopuli) and was found to be different. Bacteria, fungi, or mites were not obvious within living tissue, and further studies are necessary to determine the etiology of black galls. Key words: Populus tremuloides, poplar, black gall, wood anatomy.
Scytalidium uredinicola Kuhlman et al. is considered an important mycoparasite of western gall rust caused by Endocronartium harknessii (J.P. Moore) Y. Hiratsuka (= Peridermium harknessii J.P. Moore). The temporal occurrence of S. uredinicola and its impact on the ability of western gall rust spores to germinate were determined. Scytalidium uredinicola was not isolated from either external or internal tissues of 3- to 5-year-old galls at any time of the year. However, it was isolated from the external, but not internal, tissues of 6- to 9-year-old galls in most months and from both external and internal tissues of 10+-year-old galls in all months. On galls older than 10 years, S. uredinicola overwintered beneath the periderm and colonized developing rust sori prior to peridial rupture in April. Spore viability of western gall rust was consistently reduced by 85% throughout the period of sporulation over two sporulating seasons. Because S. uredinicola requires considerable time to become persistent and perennial, artificial establishment of the hyperparasite in the early stages of gall rust epidemics might be required to make it an effective biological control agent.
Hypocotyls of Pinus densiflora, a species known to be resistant to western gall rust, were inoculated with Endocronartium harknessii and examined by light and electron microscopy. Host cells, when initially infected, were apparently unaffected, as were the haustoria within them. Seedlings were observed to respond to infection in two ways. In the first type of response, infected cells showed signs of necrosis by 9 days after inoculation, although infecting haustoria appeared normal. By 18 days, most cortical cells in the centre of the infected region were necrotic, as were the haustoria within them. Infected cells at the colony margin still appeared healthy, indicating that host cell necrosis lagged behind infection and only occurred after the haustorium was established. Four weeks after inoculation, a ring of suberized and lignified endodermal cells separated the infected cortex from the uninfected vascular tissue and appeared to prevent further inward growth of the fungus. The second response type involved production of encapsulations around haustoria. Encapsulations appeared to have formed after haustoria senescence and were eventually followed by host cell death.
The effects of inoculum concentration, subculturing of the pathogen, and time of disease screening on the development of blackleg disease caused by Leptosphaeria maculans were studied using the Brassica napus canola cultivars Quantum (resistant) and Profit (susceptible). Disease development was not affected by inoculum concentration in the range of 5 × 105 - 4 × 106 pycnidiospores/mL or subculturing of stock cultures of L. maculans up to 71 times on V8 juice rose Bengal agar. Disease development, however, showed seasonal variation in the greenhouse. While the disease severity values for the resistant cultivar Quantum increased during the summer, those for the susceptible cultivar Profit remained consistent throughout the year.
Eighteen lodgepole pine (Pinus contorta Dougl. ex Loud.) stands, 19-32 years in age, were surveyed for Scytalidium uredinicola Kuhlman et al. in west-central Alberta. The purpose of the study was to determine how hyperparasite incidence was affected by gall age, gall size, and the occurrence of western gall rust caused by Endocronartium harknessii (J.P. Moore) Y. Hiratsuka (Peridermium harknessii J. P. Moore). Such information was deemed valuable for evaluating the likelihood that S. uredinicola was functioning as a density-dependent constraint in the pathosystem and assessing the potential for using this fungus as a biological control agent. The hyperparasite was present in all stands, albeit at generally lower levels than previously reported, but there was little evidence that its incidence increased with increasing incidence of gall rust. The odds of hyperparasitism increased by a factor of 1.3 for each year of gall growth, following adjustment for gall size effects, and by a factor of 2.1 for each 1 cm increase in gall size, following adjustment for gall age effects. Since gall age and gall size were positively correlated, there was a strong positive relationship between the unadjusted effects of both age and size and the incidence of S. uredinicola. However, although the incidence of S. uredinicola consistently increased as galls became older and larger, low levels of the hyperparasite on small and young galls may limit the effectiveness of S. uredinicola as a biological control agent.
