The long‐term survival of Phytophthora cinnamomi in mature Banksia grandis killed by the pathogen

The ability of Phytophthora cinnamomi to survive long dry periods is the key to its persistence in the south‐west of Western Australia. It has been proposed that dead Banksia grandis are a significant long‐term reservoir for P. cinnamomi inoculum. To test this, 36 healthy B. grandis trees were inoculated in April 1999, and the presence of viable propagules in planta was determined between 2 and 34 months after tree death. By 10 months after inoculation, 75% of the trees had died, with the remaining seven trees dying by 22 months. The pathogen was more commonly recovered from bark than from wood, except from those trees that died at 22 months, and more commonly from above‐ground trunks than below‐ground trunks and roots until 8 months after plant death. In trees that died 12 months after inoculation, P. cinnamomi was recovered from 60% of trunk and root core samples at 3 months, declining to 33% at 10 months, 5.5% at 12 months and 0.1% at 34 months after tree death. In trees that died at 22 months, P. cinnamomi was recovered from 87% of trunk and root samples 2 months after tree death, decreasing to 0.5% by 33 months. This study suggests that the pathogen does not have a saprotrophic phase within dead B. grandis tissue, and B. grandis is unlikely to be a long‐term reservoir for P. cinnamomi. However, the manipulation of the density of B. grandis and the use of fire to facilitate the breakdown of dead Banksia trunks in the Eucalyptus marginata (jarrah) forest may reduce the spread and impact of P. cinnamomi.     

Pathogenicity of Phytophthora multivora to Eucalyptus gomphocephala and Eucalyptus marginata

Phytophthora multivora is associated with the rhizosphere of declining Eucalyptus gomphocephala, Eucalyptus marginata and Agonis flexuosa. Two pathogenicity experiments were conducted. The first experiment examined the pathogenicity of five P. multivora isolates and one Phytophthora cinnamomi isolate on the root systems of E. gomphocephala and one P. multivora isolate on the root system of E. marginata. In the second experiment, the pathogenicity of P. multivora to E. gomphocephala and E. marginata saplings was measured using under‐bark stem inoculation. In Experiment 1, the P. cinnamomi isolate was more aggressive than all P. multivora isolates causing significant loss of fine roots and plant death. Two P. multivora isolates and the P. cinnamomi isolate caused significant losses of E. gomphocephala fine roots 0–2 mm in diameter and significantly reduced the surface area of roots 0–1 mm in diameter. One P. multivora and the P. cinnamomi isolate significantly reduced the surface area of roots 1–2 mm in diameter. Two of the P. multivora isolates significantly reduced the number of E. gomphocephala root tips. In E. marginata, the length and surface area of roots 0–1 mm in diameter and number of root tips were significantly reduced by P. multivora infestation. Rhizosphere infestation with the P. multivora isolates and P. cinnamomi isolate on E. gomphocephala, and one P. multivora isolate on E. marginata, did not significantly influence the foliar nutrient concentrations. In Experiment 2, under‐bark inoculation with P. multivora caused significant lesion extension in E. gomphocephala and E. marginata saplings, compared to the control. We propose that P. multivora is inciting E. gomphocephala and E. marginata decline by causing fine root loss and subsequently interfering with nutrient cycling throughout the plant. The impact of fine root loss on the physiology of plants in sites infested with P. multivora requires further research.     

Role of periderm in resistance of Eucalyptus marginata roots against Phytophthora cinnamomi

Phytophthora cinnamomi Rands invaded phloem tissue preferentially in Eucalyptus marginata Sm. (Jarrah) roots of both primary and secondary structure. Rate of fungal growth in the “primary” roots was less than in roots with secondary thickening. In the main field study jarrah roots between 1.5–2.5 mm in diameter, were inoculated with P. cinnamomi at regular intervals, over 12 months and in most cases invasion of the phloem was halted within the first 8 weeks of infection. In the majority of roots which exhibited secondary structure, lesion shape reflected the pattern of periderm development. Some lesions were peripheral in the outer phloem as cylindrical periderms had formed close to the vascular cambium. Other lesions were limited to a sector of the roots; periderm had formed radially through the secondary phloem and a zone of suberization was often continuous through the cambial zone and xylem. Formation of periderm at the edge of lesions was a multi-step process with various changes occurring, including lignification and suberization of walls and accumulation of autofluorescent compounds in cell walls and vacuoles. The layers of suberized and lignified cells formed near lesion fronts, were necrophylactic periderms. Necrophylactic periderm formation generally preceded the establishment of periderm of the exophylactic type.     

In vitro influence of phosphite on chlamydospore production and viability of Phytophthora cinnamomi

Chlamydospores of four isolates of Phytophthora cinnamomi were produced on mycelium grown on medium with or without 100 μg phosphite/ml, then tested for germination on medium containing no phosphite or 100 or 160 μg phosphite/ml. For each isolate when spores were produced on medium without phosphite, there was no effect of phosphite on germination. However, chlamydospores produced in the presence of 100 μg phosphite/ml showed significantly lower germination on media with no phosphite, 100 or 160 μg phosphite/ml than those produced in the absence of phosphite. For isolates known to be phosphite sensitive, the level of viable non‐germinated chlamydospores was higher amongst those produced on medium containing phosphite than amongst the controls, suggesting phosphite induces chlamydospore dormancy in some isolates of P. cinnamomi. Chlamydospores were produced uniformly across the radius of colonies on solid or liquid control media, but on media containing 100 μg phosphite/ml, chlamydospore production was lower in the centre of the colony and peaked at a point where mycelial morphology changed from tightly packed to sparse. This change in mycelial morphology and the peak in chlamydospore numbers did not occur when the pathogen was grown on liquid medium renewed every few days. In renewed medium, chlamydospores were evenly distributed across the radius of the colony.     

Susceptibility of the Quercus rubra root system to Phytophthora cinnamomi; comparison with chestnut and other oak species

The susceptibility of oak seedlings (Quercus palustris, Quercus robur, Quercus rubra) and chestnut seedlings (Castanea sativa) to Phytophthora cinnamomi was tested. The dynamics of infection was examined in plant material raised in a rhizotron. In the oak species, primary root tissues were susceptible whereas secondary cortical tissues showed some resistance to P. cinnamomi. Secondary cortical tissues of the tap root in C. sativa were susceptible. Inoculations with P. cinnamomi were performed both in situ and on excised roots of mature Q. rubra. In both cases, the resistance of Q. rubra roots and shoots was negatively correlated with diameter at the inoculation point. Small roots (l–5-cm diameter) were resistant, whereas collar and trunk were susceptible. In contrast to oak, small excised roots of mature C. sativa (0.7–2-cm diameter) were susceptible to P. cinnamomi. This may explain why P. cinnamomi does not induce a decline of the attacked oaks, but rather a trunk canker.     

Emergence,early survival,and growth of seedlings of six tree species in Mediterranean forest of Western Australia

This paper reports an experimental and comparative field study of germination, early survival, and height growth of six tree species in the northern jarrah forest of Western Australia. The species are Eucalyptus marginata and Eucalyptus calophylla (comprising the overstorey) and Banksia grandis, Allocasuarina fraseriana, Persoonia longifolia and Persoonia elliptica (all understorey).Seed weight does not correlate with germination success, survival or height growth. Germination success is greatest for B. grandis (90%) and zero for Persoonia spp. Survival of caged germinants is highest for B. grandis and E. calophylla; that of uncaged seedlings is least for E. marginata, E. calophylla and B. grandis because of browsing.Over 40% of seeds of E. marginata, E. calophylla and B. grandis do not germinate because they are harvested and presumably eaten. Viability of seeds of all species exceeds 70%. Germination is influenced by type and depth of litter but not by artificial shading or root competition from forest trees. Browsing by herbivores (kangaroos) is an important cause of death of seedlings of E. marginata, E. calophylla and B. grandis. Litter type and depth affect survival of E. calophylla, B. grandis and A. fraseriana but root competition and shading are unimportant. Height growth of E. marginata, E. calophylla and B. grandis is reduced by root competition and browsers further reduce height growth of B. grandis. Litter affects growth of E. marginata and B. grandis and shading influences growth of E. marginata and E. calophylla. Poor germination explains the low abundance of both Persoonia spp. in the jarrah forest but differential seedling establishment does not explain the adult representation of the other four species.     

Phosphite impact on the in vitro production and viability of selfed oospores by Phytophthora cinnamomi

Four isolates of Phytophthora cinnamomi were able to produce selfed oospores when cultured in vitro on modified Ribeiro’s minimal medium. Phosphite inhibited the production of selfed oospores at 100 μg phosphite/ml, lower concentrations had no effect. Selfed oospores were considered dormant as 16–47% of selfed oospores were found viable using viability staining techniques such as thiazolyl blue tetrazolium bromide and 4′,6‐diamidino‐2‐phenylindole·2HCl yet they did not germinate on a range of media. Phosphite up to 100 μg/ml did not change the proportion of viable selfed oospores. This study shows that phosphite does not affect the production of selfed oospores.     

Assessment of Australian native annual/herbaceous perennial plant species as asymptomatic or symptomatic hosts of Phytophthora cinnamomi under controlled conditions

Phytophthora cinnamomi is a necrotrophic pathogen of woody perennials and devastates many biomes worldwide. A controlled perlite–hydroponic system with no other hyphae‐producing organisms as contaminants present allowed rapid assessment of ten annual and herbaceous perennial plant species most of which have a wide distribution within the jarrah (Eucalyptus marginata) forest in Western Australia where this pathogen has been introduced. As some annuals and herbaceous perennials have recently been reported as symptomatic and asymptomatic hosts, laboratory screening of some of the field‐tested annuals and herbaceous perennials and additional species was used to further evaluate their role in the pathogen's disease cycle. Nine of the species challenged with the pathogen were asymptomatic, with none developing root lesions; however, Trachymene pilosa died. The pathogen produced thick‐walled chlamydospores and stromata in the asymptomatic roots. Furthermore, haustoria were observed in the roots, indicating that the pathogen was growing as a biotroph in these hosts.     

Influence of site factors on the impact of Phytophthora cinnamomi in cork oak stands in Portugal

Although decline of cork (Quercus suber) and holm oak trees (Quercus rotundifolia) has been described in Portugal in the late years of the 19th century, its development has become a motive of high concern during the last two decades. The presence of Phytophthora cinnamomi in cork and holm oak stands was surveyed in four different regions of the country (Trás‐os‐Montes, Alentejo, Ribatejo and Algarve) during 1995–98. Tree decline severity, sudden death and site characteristics were assessed in 56 sites representing varied conditions. The pathogen was isolated from oak roots and rhizosphere samples in 27 of those places. Other plant species from natural vegetation were sampled in three active disease centres. This survey showed that 56% of the surveyed species of shrub flora were infected with P. cinnamomi, which was detected mainly on species belonging to the families Ericaceae, Cistaceae and Leguminosae. Recovery of P. cinnamomi was more frequent in shallow soils (Leptosols and complex Leptosols and Luvisols). These soils are more common in the south (Algarve), where decline has a high impact. Soils with low fertility and low mineral nutrient levels, particularly phosphorus, seemed to favour infection. Site aspect and topographic tree situation were also evaluated. Sites facing south showed higher occurrence of P. cinnamomi, which was also more frequent in slopes and valleys than on hilltops. In Algarve, a relationship could be established between the crown status and the presence of P. cinnamomi in roots and rhizosphere. Different morphotypes of P. cinnamomi could be distinguished in vitro, and their occurrence in the field was correlated with particular site characteristics. Further research needs and management strategies to limit the extension of the disease are discussed.     

Susceptibility of Eucalyptus grandis and Acacia mearnsii seedlings to five Phytophthora species common in South African plantations

Eucalyptus grandis and its hybrids, as well as Acacia mearnsii, are important non‐native trees commonly propagated for forestry purposes in South Africa. In this study, we conducted pathogenicity trials to assess the relative importance of five commonly isolated Phytophthora spp. (Phytophthora alticola, P. cinnamomi, P. frigida, P. multivora and P. nicotianae) from the plantation environment on E. grandis and A. mearnsii seedlings. Overall E. grandis was more susceptible to the tested Phytophthora spp. than A. mearnsii. Phytophthora cinnamomi was the only pathogen that had a significant negative effect on both the host tree species, leading to a reduction in root and shoot weight as well as to death in the case of E. grandis. Phytophthora alticola and P. nicotianae exclusively affected E. grandis and A. mearnsii, respectively. This study updated the current knowledge on the pathogenicity of Phytophthora spp. on two important non‐native commercially propagated tree species from South Africa.     

Comparisons of spatial pattern,structure, and tree composition between virgin and cut-over jarrah forest in Western Australia

Several old growth (unlogged) and regrowth (logged) stands in the northern jarrah forest of Western Australia were studied in respect of spatial pattern of tree species, segregation between tree species, distribution of trees of each species by diameter class, and tree species composition. The species are Eucalyptus marginata and Eucalyptus calophylla (overstorey) and Banksia grandis, Allocasuarina fraseriana, Persoonia longifolia and Persoonia elliptica (understorey).Most populations of the species are aggregated at small but random at large scale. This pattern probably originates from non-random seed fall. Eucalyptus marginata and B. grandis are segregated, probably for the same reason. Manipulative experiments showed that interspecific competition does not prevent establishment of B. grandis seedlings. On a local scale, heterogeneity in several surface soil properties does not help explain spatial patterns.The diameter-class distributions indicate that regeneration of all species occurs irregularly. This probably results directly from the release of dormant advance growth following temporary reduction in overwood competition induced by disturbance such as wildfire or logging. Banksia grandis is not a rare or scattered component of old growth jarrah forest. A single logging of jarrah trees does not necessarily alter the density or diameter class distribution of B. grandis.     

Testing systemic fungicides for control of Phytophthora oak root disease

Potassium phosphite (PP) formulations registered as fertilizers are now prohibited in Spain. Therefore, we evaluated the systemic fungicide fosetyl‐aluminium (fos‐al) in comparison with PP, against root rot caused by Phytophthora cinnamomi in Quercus woodlands. The direct effect of both systemic fungicides was evaluated in vitro on P. cinnamomi mycelial growth. Protection of cork and holm oak against infection was also evaluated in planta. Metalaxyl was included in both in vitro and in planta experiments for comparison purposes. At 100 μg/mL, PP totally inhibited colony radial growth, in comparison with 75% achieved by fos‐al. At doses recommended by manufacturers, with fos‐al and metalaxyl applications, root symptoms remained similar to the uninfected control levels. Based on these results, fos‐al is a candidate substitute product for PP in Quercus woodlands for control of Phytophthora oak root disease.     

Treatment of oak decline using pressurized injection capsules of antifungal materials

Quercus ilex and Quercus suber trees growing at several sites in Extremadura, Western Spain, that were showing symptoms of oak decline were injected with potassium phosphonate, quinosol or carbendazim using a low-pressure method of trunk injection composed of a pressurized capsule system. A team of four people injected between 120 and 189 trees per day, depending upon the density of the undergrowth vegetation. This labour cost represented, approximately, 15–20% of the total cost of the treatment. The potassium phosphonate-injected trees showed a significant improvement in vegetative growth within 2 years of the injection treatment, and they also showed some recovery from the decline symptoms during the third year. Only one injection treatment of an average of 3.5 capsules (corresponding to 24.5 g phosphonic acid) per tree of approximately 36 cm in diameter, was necessary to reduce the disease severity significantly. Indirectly, these results corroborate the implication of Phytophthora cinnamomi in oak decline within Spanish Quercus woodlands.     

Involvement of Phytophthora species in white oak (Quercus alba) decline in southern Ohio

This study was initiated to investigate the possible role of Phytophthora species in white oak decline (Quercus alba) in southern Ohio at Scioto Trail State Forest. Surveys demonstrated the presence of four species of Phytophthora including one novel species. By far, the most common species was P. cinnamomi; P. citricola and P. cambivora were isolated infrequently. In few instances, P. cinnamomi was isolated from fine roots and necroses on larger roots. No special pattern of incidence was found, but P. cinnamomi was more commonly isolated from greater Integrated Moisture Index values suggesting moist lower bottomlands favour this Phytophthora species. When tree crown condition was examined relative to the presence of Phytophthora, no significant association was found. However, roots of declining P. cinnamomi‐infested trees had 2.5 times less fine roots than non‐infested and healthy trees, which was significantly different. The population densities of P. cinnamomi from declining trees were significantly greater than from healthy trees, suggesting increased pathogen activity that has the potential to cause dieback and decline and possibly the cause of a reduced fine root amount found on declining trees.     

Host removal as a potential control method for Phytophthora cinnamomi on severely impacted black gravel sites in the jarrah forest

Removal of living plants from an area of Eucalyptus marginata (jarrah) forest on black gravel sites infested with Phytophthora cinnamomi significantly reduced subsequent pathogen recovery. Vegetation, including trees and annual and herbaceous perennial plants, was killed on the sites by herbicide application. To determine whether this treatment efficiently eliminated P. cinnamomi, soil samples were seasonally collected and baited to test for the presence of the pathogen. There were no recoveries on treated sites in autumn, 28 months after removal of all vegetation by herbicide application. To test whether this was the result of the complete elimination of the pathogen or whether inoculum remained, regrowth on sites was not controlled after this period leading to the re‐establishment of annual and herbaceous perennial species, some of which are hosts of P. cinnamomi. Recovery of P. cinnamomi after plant regrowth on the formerly treated sites indicated that for complete pathogen removal, sites need to remain free of vegetation for longer than 28 months. Overall, however, this study confirms that the pathogen is a weak saprophyte, and withdrawal of host material for a period of time may make eventual rehabilitation of these sites possible.     

Involvement of Phytophthora spp. in chestnut decline in the Black Sea region of Turkey

Chestnut blight caused by Cryphonectria parasitica is a serious disease of Castanea sativa in the Black Sea region of Turkey. During disease surveys, dieback and decline symptoms were observed on trees without apparent blight and ink disease symptoms. Black necroses, similar to those caused by Phytophthora infections, were noted on some of the chestnut coppices and saplings in one nursery in Ordu and led to an investigation into this disease complex. Only symptomatic plants showing dieback symptoms were investigated. Soil samples together with fine roots were collected from two directions, north and north‐east, approximately 150 cm away from the main stems. Phytophthora spp. were baited with young chestnut leaves. Three Phytophthora spp., P. cambivora, P. cinnamomi and P. plurivora, were identified from 12 soil samples collected from 73 locations, while from the nurseries, only P. cinnamomi was obtained. Phytophthora cinnamomi was the most common species, obtained from seven locations in five provinces and from four nurseries having similar symptoms mentioned above in different locations. Phytophthora cambivora and P. plurivora were less frequently obtained, from three to two stands, respectively. Phytophthora cinnamomi and P. cambivora were the most aggressive species when inoculated at the stem base on 3‐year‐old chestnut saplings, killing six saplings of eight inoculated in 2 months. The three Phytophthora species were first recorded on chestnut in Black sea region of Turkey with the limited samples investigated in a large area about 150 000 ha chestnut forest.     

Root recovery rates for Phytophthora cinnamomi and rate of symptom development from root rot on Abies fraseri trees over 7 years

Phytophthora root rot on Abies fraseri trees was monitored from 2001 to 2007 within the disease front of a 12‐year‐old Virginia plantation where trees had been dying of the disease since 1994. After a slow increase in early foliage symptom development from July 2001 to September 2002, the frequency of A. fraseri trees with early symptoms accelerated for about 15 months. While the slow increase occurred during a 18.7% lower than normal rainfall period and the acceleration occurred during a 31.2% higher than normal rainfall period, the percentage of trees with early symptoms continued to increase during the mid‐winter months (December–February) when the estimated mean minimum daily soil temperature (25 cm depth) was unfavourable (<10°C) to Phytophthora cinnamomi pathogenic activity. The time required for trees to progress from early foliage symptoms to completely dead foliage, from November 2000 to October 2007, was highly variable, ranging from 4 to 35 months. Root recovery rates for P. cinnamomi, assayed on a selective medium, were 6.4 times greater for symptomatic foliage trees than for asymptomatic foliage trees in this deep, silt‐loam soil. Following an atypical cold period in February 2007, when the mean minimum daily soil temperature was 0.8°C, symptomatic roots yielded only a low level of germinable propagules of P. cinnamomi. Further, during an atypical midsummer in 2007 (June–August), when the soil water potential was at or below ?9 bars for 68 of 92 days, symptomatic roots yielded no germinable propagules of P. cinnamomi. Addition of thiophanate‐methyl to the selective medium aided P. cinnamomi isolation by inhibiting many undesired pythiaceous colonies growing from symptomatic roots.     

Relative susceptibility of oaks to seven species of Phytophthora isolated from oak forest soils

Isolates of Phytophthora cambivora, P. cinnamomi, P. citricola, P. europaea, P. quercetorum and two unidentified species were tested for their pathogenicity to eastern US oak species by root and stem inoculations. Experiments were conducted during two different periods and included 1‐, 2‐ and 20‐year‐old oaks grown under greenhouse and field conditions. Species of Phytophthora were pathogenic in varying degrees to the oak species tested. All species were pathogenic to fine and taproots of at least one oak species. The fine root damage caused by the species of Phytophthora ranged from 9 to 55% when compared to the controls. Roots were more susceptible during the fall inoculation period than the summer. With exception of Phytophthora sp1 and P. quercina‐like, all species of Phytophthora were pathogenic to oak stems with P. cinnamomi and P. citricola being the most aggressive. Quercus montana and Q. rubra were the most susceptible oak species to stem inoculation. Lesion sizes were considerably larger when 20‐year‐old trees were inoculated. Generally, no significant differences in lesion sizes were detected in greenhouse tests when the summer and fall inoculation periods were compared. However, on 2‐year‐old field‐grown seedlings, lesion sizes were considerably smaller or not significantly different from controls during the fall inoculation period, suggesting lower, late season temperatures may restrict lesion development.     

New insights into the survival strategy of the invasive soilborne pathogen Phytophthora cinnamomi in different natural ecosystems in Western Australia

Despite its importance as one of the most notorious, globally distributed, multihost plant pathogens, knowledge on the survival strategy of Phytophthora cinnamomi in seasonally dry climates is limited. Soil and fine roots were collected from the rhizosphere of severely declining or recently dead specimens of 13 woody species at 11 dieback sites and two dieback spots and from healthy specimens of five woody species at four dieback‐free sites in native forests, woodlands and heathlands of the south‐west of Western Australia (WA). Phytophthora cinnamomi was recovered from 80.4, 78.1 and 100% of tested soil, fine root and soil–debris slurry samples at the 11 dieback sites, in some cases even after 18‐month storage under air‐dry conditions, but not from the small dieback spots and the healthy sites. Direct isolations from soil–debris slurry showed that P. cinnamomi colonies exclusively originated from fine roots and root fragments not from free propagules in the soil. Microscopic investigation of P. cinnamomi‐infected fine and small woody roots and root fragments demonstrated in 68.8, 81.3 and 93.8% of samples from nine woody species the presence of thick‐walled oospores, stromata‐like hyphal aggregations and intracellular hyphae encased by lignitubers, respectively, while thin‐walled putative chlamydospores were found in only 21.2% of samples from five woody species. These findings were confirmed by microscopic examination of fine roots from artificially inoculated young trees of 10 woody species. It is suggested that (i) the main function of chlamydospores is the survival in moderately dry conditions between consecutive rain events and (ii) selfed oospores, hyphal aggregations, and encased hyphae and vesicles in infected root tissue of both host and non‐host species are the major long‐term survival propagules of P. cinnamomi during the extremely dry summer conditions in WA.     

Lesion formation and host response to infection by Armillaria ostoyae in the roots of western larch and Douglas‐fir

The process of lesion formation and host response to natural infection by Armillaria ostoyae were studied in the roots of western larch (Larix occidentalis) and Douglas‐fir (Pseudotsuga menziesii ssp. glauca) trees in the three age classes, 6–8, 18–19 and 85–95 years. The characteristics of lesions on infected roots were recorded and bark samples were dissected from infection points and lesion margins in the field and stored in liquid nitrogen for macroscopic study in the laboratory. Infection in the roots of 6‐ to 8‐year‐old trees advanced freely, overcoming any host resistance, quickly girdling the root collar and killing the trees. In 18‐ and 19‐year‐old trees, however, 43% of infections on western larch and 27% of the infections on Douglas‐fir roots were confined to lesions bounded by necrophylactic periderms with multiple bands of phellem. Host response was similar in 85‐ to 95‐year‐old trees, but the percentage of confined lesions was higher than in younger trees. The results suggest that larch shows resistance to A. ostoyae at a younger age and with greater frequency than Douglas‐fir.