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International audience; A greenhouse inoculation experiment and field study were conducted to determine the cause of an outbreak of Septoria musiva cankers on Populus balsamifera in a northern Alberta plantation. Four clones of P. balsamifera, five clones of putatively resistant P. deltoides, and one susceptible hybrid poplar clone, Northwest, were inoculated with seven isolates identified as S. musiva. Four of the isolates were from P. balsamifera in Alberta and the others were from P. deltoides in Quebec. Results indicated that disease severity was similar for Alberta and Quebec isolates (P = 0.243) and that P. balsamifera had the greatest mean disease severity (x-bar = 4.20), P. deltoides had the lowest (x-bar = 2.76), and Northwest was intermediate (x-bar = 3.45). A genetic analysis comparing six polymorphic polymerase chain reaction restriction fragment length polymorphism loci and the mitochondrial small subunit ribosomal DNA sequences of the seven isolates indicated that the Alberta population was made up of at least three distinct genotypes. Canker incidence and age on Northwest and 56 different clones of P. balsamifera in a plantation were recorded. Canker incidence (P = 0.726) and the canker age distributions (P = 0.994) were similar for the two species. In conclusion, contrary to what has been reported in the literature, P. balsamifera appears to be quite susceptible to Septoria canker.

Most artificial inoculations of Populus spp. stems with Septoria musiva have required host wounding to induce canker development; in the absence of wounds, frequencies of cankers have been low. Three greenhouse inoculation experiments were conducted to demonstrate the reliability and repeatability of an inoculation method that did not require wounding. In the first, 16 clones of hybrid poplar were inoculated with three isolates of S. musiva to compare responses following wounding and inoculation with mycelium (wound inoculation) with responses following inoculation of nonwounded trees by spraying with a conidial suspension (spray inoculation). Stem disease severity among clones following spray inoculation was correlated with stem disease severity following wound inoculation. A significant clone–isolate interaction was detected with spray inoculation but not wound inoculation. In the other two greenhouse experiments, 29 clones of hybrid poplar and 69 clones of Populus balsamifera were inoculated with a spore suspension mixture of three isolates. In both cases, the experimental error was similar to that obtained in previous experiments, in which trees were wound inoculated, and was adequately small to permit detection of differences in responses among clones. Ultimately, field studies will be needed to determine the best inoculation method for predicting stem responses to this pathogen under field conditions. However, relative to wound inoculation, spray inoculation of nonwounded trees has the advantage of yielding faster results, permitting inoculation with a mixture of isolates, and not circumventing potential mechanisms for resisting penetration. The ability to infect stems without wounding creates opportunities for numerous types of epidemiological and disease control studies that are difficult to conduct with wound inoculation.

Summary To test the hypothesis that galls resulting from infection by western gall rust [Endocronartium harknessii (J. P. Moore) Y. Hiratsuka] on lodgepole pine [Pinus contorta Dougl. ex Loud.] create hydraulic constrictions that reduce tree growth, the hydraulic conductivity (Kψ; m2 s−1 Pa−1), leaf area (AL) and sapwood area (AS) were measured on 6- to 15-year-old trees having galls encircling 0–100% of the stem. Increases in gall encirclement were accompanied by decreases in Kψ, foliar nitrogen concentration, AL and AS, although the AL:AS ratio remained constant. Dye flow patterns through galled stem sections indicated that water moved primarily through the non-galled portions of the stem. In a second field study, xylem and phloem wounds were applied to 13-year-old galled and non-galled trees on either the galled or non-galled side of the tree. Although both galls and xylem wounds reduced Kψ, radial and height growth were unaffected. In aggregate the two experiments showed that although trees are quite tolerant to small galls and short term reductions in Kψ, the disruption of water movement by larger galls may be sufficient to initiate a chain of events leading to tree death, especially in very dense stands with high levels of intraspecific competition.

Fourteen clones of hybrid poplar were inoculated with 19 isolates of Septoria musiva Peck under greenhouse conditions to determine the magnitude of the clone, isolate, and clone × isolate interaction effects. Septoria musiva isolates were collected from five geographic areas, two symptoms (canker and leaf spot), and two host types (native species and hybrid poplar). The hybrid poplar clones were classified by parent type ( Populus deltoides Bartr. ex Marsh., Populus laurifolia Lebed. × Populus nigra L., and P. deltoides × (P. laurifolia × P. nigra)). There were no significant differences among geographic areas (p = 0.443), symptoms (p = 0.842), or hosts (p = 0.304) of origin for the 19 isolates nor significant differences among the three parent types (p = 0.089). Clone, isolate, and clone × isolate interaction effects were all significant, accounting for 65%, 15%, and 18%, respectively, of the explained variation. These results indicate that clones rather than parent types should be the focus of resistance screening programs and that the pathosystem should be stable given the relatively small clone × isolate interaction. These results also indicate that a single isolate should be sufficient for preliminary screening of disease resistance in hybrid poplars.

Septoria musiva , the causal agent of Septoria canker, has caused widespread damage to plantations of hybrid poplar across North America. A greenhouse experiment was conducted to evaluate the size of interactions between the genotype factors of (i) clone, (ii) isolate, and (iii) the clone–isolate interaction and the environmental factor of water stress. Four clones of hybrid poplar were inoculated with four isolates of S. musiva and subsequently exposed to two levels of water stress (stressed and unstressed). Tree height, root collar diameter (RCD), and disease severity were measured 56 d after inoculation. Water stress reduced height and RCD growth by approximately 30% but did not affect disease severity (P = 0.258). Of the explained variability, 97.5% was attributable to genotype effects (89% from clone, 0% from isolate, and 8.5% from clone–isolate interaction). The remaining 2.5% of the variation was accounted for by the genotype – water stress interaction. These results suggest that clonal effects will be more important determinants of disease severity under greenhouse conditions than genotype – water stress interactions.

Seasonal changes in aspen (Populus tremuloides Michx.) resistance to shearing force were described to determine their apparent contribution to seasonal changes in cattle utilization. Shearing resistance of current-year, 1-year-old, and 2-year-old stem-age classes were measured from late spring to early fall. For all stem-age classes, resistance to shearing force increased with time, partially due to increasing stem diameter. Depending on stem-age class, resistance to shearing force increased by 200%-2000% between the first and last measurements of a given year. Increased resistance of aspen stems to shearing force in late summer may reduce their selection by cattle. Grazing management that defers cattle entry until midsummer could reduce damage to re-generating aspen associated with cattle foraging in cut blocks.

Infection of lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) by western gall rust has been shown to decrease with tree height and age, but the effects of those two factors have not been separated. Five replicate artificial inoculations were done on a total of 327 trees of different ages in two height classes. Temperature and percentage of spore germination at the height of inoculation, shoot development (stem elongation at the time of inoculation as a proportion of final shoot elongation), main stem leader length at the time of inoculation, tree height, and tree age were measured. Modeled percentages of infected trees and the number of galls per 10 cm of shoot length decreased by 85% and 88%, respectively, as tree age increased between 2 and 10 years, indicating the undesirability of early, aggressive precommercial thinning of lodgepole pine stands in areas where western gall rust is common. By controlling and (or) statistically accounting for inoculum, microclimate, and phenological factors, it was possible to demonstrate that changes in susceptibility with tree age are sufficient to account for the reduction in infection with tree height.

Fungal colonization of aspen (Populus tremuloides Michx.) roots was examined in boreal mixedwood sites that were mechanically site prepared 8–10 years earlier for white spruce (Picea glauca (Moench) Voss) regeneration using disc trenchers or ripper plows. A survey of root wounds determined that Armillaria sinapina Bérubé & Dessureault and Armillaria ostoyae (Romagn.) Herink were both wound pathogens of aspen; however, A. sinapina was more frequently associated with wounds than was A. ostoyae. Armillaria ostoyae was more common on unwounded root tissues. Sixty percent of wounds infected by A. sinapina were not compartmentalized and the likelihood of an A. sinapina infection did not increase with increasing wound size. Pathogenic fungi other than Armillaria were rarely associated with root wounds. Sever wounds were associated with furrows; scrape wounds were located both along and between furrows irrespective of the site-preparation technique (ripper plow vs. disk trencher).

The impacts of partial cut systems on nutrient availability and understory vegetation are poorly understood. To examine these responses, white spruce dominated stands in the boreal mixedwood of Alberta were clear-cut or partial-cut and the forest floor treated by slash burning, mixing, mounding, or scalping in a split-plot design. Soil nutrient availability (ion exchange resin), net N mineralization (in situ incubations), and vegetation (density and cover) responses were assessed. With the exception of higher Mg availability in the clearcuts, differences in nutrient availability were driven by forest floor disturbance and not harvest method. Relative to controls, burning increased availability of NH4+, NO3–, and P, and scalping increased Ca and Mg but diminished K. Controls had low levels of NO3–. The mixing treatment substantially reduced net N mineralization. In terms of vegetation, partial cuts reduced root suckering by Populus spp. (Populus tremuloides Michx., Populus balsamifera L.) relative to clearcuts. Burning and mounding stimulated fireweed (Epilobium angustifolium L.) cover, while scalping increased Populus spp. sucker density. In contrast, mixing largely reduced vegetation establishment, likely because of the destruction of roots and rhizomes and reduced N supply. Nutrient availability and vegetation establishment were more strongly controlled by forest floor disturbance than by partial canopy retention.

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 per...

Moltzan, B. D., Blenis, P. V., and Hiratsuka, Y. 2001. Effects of spore availability, spore germinability, and shoot susceptibility on gall rust infection of pine. Plant Dis. 85:1193-1199. 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.

Vingt-quatre peuplements de pin de Murray (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.), âges de 26 a 32 ans et eclaircis entre l'âge de 15 et 23 ans, ont ete examines dans le centre-ouest de l'Alberta. L'âge du peuplement au moment de l'eclaircie etait le meilleur predicteur individuel de la formation de galles sur les arbres qui en etaient exempts lors de l'eclaircie (infection posterieure a l'eclaircie). La hauteur du peuplement etait aussi un bon predicteur de l'infection posterieure a l'eclaircie apres un ajustement pour les effets de la localisation du peuplement et les annees d'infection severe. Le nombre de galles par arbre au moment de l'eclaircie etait le meilleur predicteur de l'incidence des galles sur tous les arbres, incluant ceux qui avaient des galles au tronc au moment de l'eclaircie (infection totale). Les infections au tronc diminuaient avec l'âge et la hauteur des arbres. Pres de 60% des galles au tronc couvraient 40% ou moins de la circonference du tronc. Il n'y avait pas de relation entre le degre d'encerclement et la hauteur ou l'âge des galles. Dans les endroits ou la rouille-tumeur est commune, les amenagistes devraient considerer la possibilite de retarder l'eclaircie.

Endocronartiumharknessii (J.P. Moore) Hirat. populations never approach the level where all susceptible Pinuscontorta Dougl. ex Loud. var. latifolia Engelm. tissue is infected, even in years when environmental conditions are favourable for infection. A simple simulation model was developed to determine whether a hyperparasite could regulate the rust population and maintain it at an equilibrium level far below host saturation. The model assumed that galls begin sporulating and first become susceptible to parasitism 2 years after formation and requires specification of infection rates for the rust and the hyperparasite, mortality rates for normal and parasitized galls, and the degree to which aeciospore production is reduced following hyperparasite infection. Analysis of the model showed that the hyperparasite could regulate the rust population, producing a stable equilibrium, but only if the various rates were such that healthy galls, in their lifetime, produced enough aeciospores to give rise to more than one new gall, while galls that were parasitized during the first year of aeciospore production produced, in their lifetime, fewer than one new gall. The model of hyperparasite regulation provides one possible explanation of the dynamics of the gall rust pathosystem. It shows that control measures such as sanitation and breeding for resistance may reduce the effectiveness of the hyperparasite in regulating the system and, thus, may fail to achieve the anticipated goals.

To determine whether spore dispersal gradients of Endocronartiumharknessii (J.P. Moore) Y. Hiratsuka were present adjacent to heavily infected pine stands, spore samplers were placed 0.5 m above ground, at five distances, 2–80 m from the edge of an 18-year-old stand and were operated for a total of 33 days over 2 years. Six weather parameters were recorded inside the stand, and multiple regression was used to determine the relationship between weather and parameters of spore dispersal. To determine whether gradients of lodgepole pine (Pinuscontorta Dougl. var. latifolia Engelm.) seedlings infected by gall rust were present adjacent to heavily infected pine stands, seedling infection was determined in 75-m2 plots centered 2.5–120 m from the stand edge, in five such plots in the center of that stand, and in transects of two-tree plots through a second stand. Spore concentration decreased with increasing distance from the stand edge in accordance with the power law model. Spore concentrations and the slopes of spore dispersal gradients were significantly affected by wind values, although there was considerable unexplained variation. There was little evidence for the existence of disease gradients.

Lodgepole pine (Pinuscontorta var. latifolia Engelm.) seedlings from 23 open-pollinated pine families from five stands in west-central Alberta were inoculated with western gall rust (Endocronartiumharknessii (J.P. Moore) Y. Hiratsuka) spores from four locations within 69 km of those stands. Six weeks after inoculation and at 3-week intervals for 21 weeks thereafter, seedlings were examined for symptoms of infection and galls. Eighteen months after inoculation, most trees in a sample of 1321 had either fully developed galls or no galls; partial gall formation was uncommon. Frequency of gall formation was highly correlated with the area under the disease progress curve (AUDPC) of symptom appearance and the AUDPC of gall appearance. It was also correlated, although less strongly, with the frequency of seedlings that had initially shown symptoms of infection but had no galls 21 weeks after inoculation. Regardless of whether AUDPC of gall appearance, frequency of gall formation, or failure to develop galls following infection was used as the criterion of resistance, the results were similar. Family effects were greater than stand effects, and rust-source effects were smaller than family effects and not always significant. There was no interaction between rust-source effects and either stand or family effects.

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.

Six-week-old Pinus contorta seedlings were inoculated with spores of the western gall rust fungus Endocronartium harknessii. Of 660 seedlings inoculated, 174 showed early symptoms and were sampled for histological observation in the 7 weeks following inoculation. Pigmentation of epidermal cells was the first externally visible response to infection, generally occurring 14–28 days after inoculation. Developing symptoms were extremely variable, ranging from little or no visible response to severe necrosis. The appearance of external symptoms was largely due to changes occurring in underlying infected cortical cells such as the production of phenolic compounds, necrophylactic periderm, or necrotic tissue. Intercellular lignin deposition was observed in infected tissue and was thought to be associated with necrophylactic periderm formation. Suppression of normal exophylactic periderm development was evident in virtually all infected tissue zones. Infection of the vascular cambium occurred as early as 21 days after inoculation and resulted in the subsequent formation of distorted xylem tracheids typical of gall tissue.

Hybrid genotypes that arise between plant species frequently have increased susceptibility to arthropod pests and fungal pathogens. This pattern has been attributed to the breakdown of plant defenses ('Hybrid susceptibility' hypothesis) and (or) to extended periods of susceptibility attributed to plant phenologies in zones of species overlap and (or) hybridization ('phenological sink' hypothesis). We examined these hypotheses by assessing the susceptibility of parental and hybrid Populus host genotypes to a leaf spot disease caused by the fungal pathogen Septoria musiva. For this purpose, 214 genotypes were obtained from morphologically pure zones of P. balsamifera and P. deltoides, and from an intervening zone of overlap and hybridization on the drainage of the Red Deer River, Alberta, Canada. Genotypes were identified as P. balsamifera, P. deltoides, or hybrid using a suite of 27 species-specific SNP markers. Initially the genetic structure of the hybrid zone was characterized with 27.7% of trees classified as admixed individuals. To test the hybrid susceptibility hypothesis, a subset of 52 genotypes was inoculated with four isolates of S. musiva. Levels of susceptibility were P. balsamifera > F1 hybrid > P. deltoides. A further 53 genotypes were grown in a common garden to assess the effect of genotype on variation in leaf phenology. Leaf phenology was more variable within the category of hybrid genotypes than within categories of either parental species. Leaf phenology was also more variable for the category of trees originating in the hybrid (P. balsamifera - P. deltoides [hybrid and parental genotypes combined]) zone than in adjacent pure zones of the parental species. The results from the inoculation experiment support the hybrid intermediacy hypothesis. The results from the common garden experiment support the 'phenological sink' hypothesis. These findings have greatly increased our understanding of the epidemiology and ecology of fungal pathogens in plant hybrid zones.

Hourly measurements of spore release by Endocronartium harknessii (J. P. Moore) Y. Hiratsuka on Pinus contorta Doug. var. latifolia Engelm. were obtained during 2 years. Two spore traps were placed beside both of two sporulating galls and electronic data loggers were used to record environmental data. On rainless days, most spores were trapped between 0900 and 2200, when vapor pressure deficit, temperature, light intensity, and wind velocity were high. A similar pattern of spore release occurred under simulated daytime and nighttime conditions in a growth chamber. On rainy days, most spores were released, on average, over a longer time interval than on dry days. Within 9 h of being placed in a dew chamber, the previously intact peridia on a single gall had ruptured, presumably from pressure caused by spore production. Further evidence that spore production is stimulated by low vapor pressure deficit was obtained by moving 11 galls alternatively between humid and dry conditions. Thus, it is proposed that the diurnal periodicity of spore release on dry days results from the production of spores during the night and the subsequent passive release of those spores during the next day.

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.

(1988). Axenic Culture of Endocronartium Harknessii. Mycologia: Vol. 80, No. 1, pp. 120-123.

Incidence of western gall rust infection in each of 10 replicated forest openings having diameter height ratios of 2:1, 4:1, and 6:1 was determined along two perpendicular 1-m wide transects. Infection frequencies were not significantly different among the different-sized openings. No pine regeneration was present in openings with diameter:height ratios of 0.5:1 or 1:1, thus precluding direct comparison of our results with those obtained from three other tree host–parasite studies in which disease incidence was greatest in small openings.