Relative Susceptibility of Four Melampyrum Species to Cronartium flaccidum

The relative susceptibility of Melampyrum arvense, M. pratense, M. sylvaticum and M. nemorosum to Cronartium flaccidum was investigated by artificial inoculations. Uredinia and telia were artificially produced on plants from sown seed or transplanted from the wild in the greenhouse. This is the first report in which C. flaccidum telia were artificially produced on leaves of whole plants of M. arvense and M. pratense, the first report of successful artificial disease establishment on M. arvense and the first report of telia production on the upper surface of Melampyrum leaves. As an earlier unreported observation, telia were occasionally formed on both upper and lower leaf surfaces of M. arvense, M. sylvaticum and M. pratense. Host species, spore source and their interaction each affected some aspects of disease development. Melampyrum arvense and M. sylvaticum were the most susceptible hosts, while M. nemorosum was least susceptible. Melampyrum arvense should be considered a potentially important alternate host of C. flaccidum with its limited area of distribution.     

Cronartium flaccidum fruitbody production on Melampyrum spp. and some important alternate hosts to pine

Four Vincetoxicum species, Paeonia anomala and Melampyrum sylvaticum known to act as alternate hosts for Cronartium flaccidum, and a potential host Melampyrum pratense were inoculated either in vitro or in vivo with various aeciospore sources of C. flaccidum. All fresh C. flaccidum aeciospore sources variably produced uredia or telia on Vincetoxicum hirundinaria, Vincetoxicum mongolicum, Vincetoxicum fuscatum, Vincetoxicum nigrum, P. anomala, nine geographic provenances of M. sylvaticum, and two provenances of M. pratense. This is the first time that C. flaccidum telia were artificially produced on M. sylvaticum and the first time that M. pratense was shown to act as alternate host for C. flaccidum. The results suggest that M. sylvaticum is relatively susceptible to C. flaccidum, whereas M. pratense is almost resistant to this rust. The low host-specificity of the Finnish population of C. flaccidum was demonstrated, as V. hirundinaria from Italy was susceptible to the rust.     

Susceptibility of Pedicularis spp. to Cronartium ribicola and C. flaccidum in Finland

Aeciospores of Cronartium ribicola and C. flaccidum were collected from several locations in Finland and used to inoculate Pedicularis spp. and some known and suspected alternate hosts in 2008–2009. In all trials, C. ribicola formed uredinia and telia on leaves of Ribes nigrum. No uredinia or telia of C. ribicola formed on older leaves of Pedicularis spp. but both uredinia and telia were found on young leaves of P. palustris ssp. palustris. Cronartium flaccidum produced uredinia and/or telia on leaves of P. palustris ssp. palustris, P. lapponica, Vincetoxicum hirundinaria and Melampyrum sylvaticum. Neither rust infected P. sceptrum‐carolinum, Vaccinium myrtillus, Calluna vulgaris or Ledum palustre. Similar to rusts in Asia and North America, the results showed that European C. ribicola exhibit more variable host reactions and wider alternate host ranges than earlier described. Pedicularis palustris may play a role in the spread of Cronartium in natural forests.     

Distribution and frequency of Cronartium flaccidum on Melampyrum spp. in permanent sample plots in Finland

Abstract

In 2006, the distribution and frequency of pine stem rust Cronartium flaccidum was studied on Melampyrum spp. in permanent sample plots of an EU forest health monitoring program and of a BioSoil program in Finland. The frequency of Melampyrum spp. infected with C. flaccidum was low, but the rust was recorded in new locations of eastern Finland. No uredinia of C. flaccidum were observed in samples collected from over 700 plots. Telia were common on Melampyrum sylvaticum but were occasionally also found on Melampyrum pratense and Melampyrum nemorosum. Melampyrum pratense was the most common species growing in plots on mainly dry sites. Melampyrum sylvaticum was seldom detected, suggesting that alternate hosts are absent from northern Finland or that the current sample plot network is too sparse for effective disease monitoring in the north. No relationship between C. flaccidum, Melampyrum spp. and rust incidence in host trees was observed, which implies that the autoecious Peridermium pini is a more likely pathogen than C. flaccidum locally. Because plots containing M. sylvaticum occur almost solely in southern Finland, the plots with M. sylvaticum reflect the best changes in rust epidemics in that area. In the future, nutrient-rich sites containing M. sylvaticum may serve as a source of rust epidemics. It is unlikely that C. flaccidum will spread to dry sites via M. pratense.     

Finnish Cronartium ribicola does not infect alternate hosts of Cronartium flaccidum

Susceptibility of the main alternate hosts of Cronartium flaccidum, Vincetoxicum spp. and Melampyrum spp., to C. ribicola was investigated by artificial inoculations on detached leaves in the laboratory and on whole plants in the greenhouse. Neither uredinia nor telia developed on the leaves of either Vincetoxicum hirundinaria, Vincetoxicum nigrum, Melampyrum sylvaticum, Melampyrum pratense, Melampyrum nemorosum, Melampyrum arvense, Melampyrum cristatum or Melampyrum polonicum following inoculation by the 26 aeciospore sources collected from Pinus strobus, Pinus monticola, Pinus flexilis and Pinus peuce in 2000–2003. The results suggest that Finnish C. ribicola does not infect the main alternate hosts of C. flaccidum. Cronartium ribicola and C. flaccidum can thus be identified reliably by inoculations on their main alternate hosts.     

Cronartium flaccidum produces uredinia and telia on Melampyrum nemorosum and on Finnish Vincetoxicum hirundinaria

Uredinia and telia were produced on Melampyrum nemorosum through inoculations with Cronartium flaccidum aeciospores collected from northern Finland, proving that M. nemorosum serves as the telial host of C. flaccidum. Melampyrum nemorosum plants were either grown from seeds collected from several geographical provenances or transplanted from natural forest stands. They were inoculated with several geographical aeciospore sources of C. flaccidum in a greenhouse. Vincetoxicum hirundinaria, M. pratense and M. sylvaticum were used as positive controls. Detached leaves of Melampyrum spp. and V. hirundinaria were also inoculated but uredinia and telia formed only on V. hirundinaria. Most of the aeciospore sources from northern Finland produced abundant uredinia and telia both on M. nemorosum from two geographical locations and on V. hirundinaria in the greenhouse. Statistically significant differences were observed in the dimensions of artificially produced telia, teliospores and urediniospores among the tested alternate hosts and spore sources. The results suggest that M. nemorosum is susceptible to C. flaccidum and therefore may play a role in southern Fennoscandia as an alternate host but it has potentially a greater role in central and eastern Europe, where M. nemorosum occurs commonly.     

Pathogenicity of Thekopsora areolata from seed orchards in Finland on Prunus spp. and Picea abies

Pathogenicity of cherry‐spruce rust, Thekopsora areolata, was investigated by inoculations with aeciospores from seven Norway spruce, Picea abies, seed orchards that had suffered from successive severe rust epidemics in the 2000s in Finland. Detached leaves of Prunus spp. were inoculated in the laboratory using aeciospores from cones of various ages. In the greenhouse, live Prunus padus plants were inoculated, and possible autoecism of the rust was tested by inoculations of Picea abies seedlings. Thirty‐five spore sources from the seed orchards formed uredinia on Prunus spp. in the laboratory 2 weeks after incubation, but no telia developed. In the greenhouse, uredinia developed on live P. padus, but no rust symptoms, cankers or sporulation were detected on Picea abies. Thus, no evidence of autoecism was observed among the T. areolata populations and therefore all populations, suggesting all those tested from the Finnish seed orchards were heteroecious.     

Nasa,Nemesia and Euphrasia: new alternate hosts of Cronartium spp.

Susceptibility of 18 alternate host species to Cronartium flaccidum or C. ribicola was tested. Alive test plants were inoculated in the greenhouse and the formation of Cronartium uredinia and telia was followed on the plants for 8 weeks. Cronartium flaccidum formed uredinia and telia on Bartsia alpina and Euphrasia stricta (Orobanchaceae), Nasa triphylla and N. urens (Loasaceae), Nemesia floribunda (Scrophulariaceae), Tropaeolum majus (Tropaeolaceae), Veronica daurica (Plantaginaceae) and Vincetoxicum hirundinaria (Apocynaceae). Single uredinia or telia developed also on N. urens and E. stricta inoculated with C. ribicola. For the first time, Cronartium are reported to sporulate on members of Loasaceae and Scrophulariaceae, N. urens and N. floribunda.     

Susceptibility of Ribes spp. to pine stem rusts in Finland

Susceptibility of Ribes spp. to three pine stem rusts, Cronartium ribicola, Cronartium flaccidum and Peridermium pini, was investigated by inoculations both in laboratory and in greenhouse conditions, and by observing sporulation on Ribes spp. leaves under natural and artificial inoculum in 16 experiments. Twenty‐seven Ribes spp. cultivars were inoculated in 2000–2004 using 41 sources of C. ribicola aeciospores from a wide geographic range, and six Pinus spp.; 51 sources of C. flaccidum and 11 sources of P. pini from Pinus sylvestris. The results were very similar both after artificial inoculations and observations under natural inoculum in repeated experiments over the years. Cronartium ribicola uredinia and telia developed frequently or moderately on nine Ribes nigrum cultivars but were not present on two cultivars. Sporulation developed on five Ribes rubrum cultivars but was absent on two cultivars. Three Ribes uva‐crispa cultivars were only weakly susceptible to C. ribicola, while all R. alpinum cultivars were resistant to the rust. The other Ribes spp. tested, Ribes niveum, Ribes aureum, Ribes odoratum, Ribes sp. × nigrolaria, Ribes glandulosum and Ribes × culverwellii Jostaberry, were all susceptible to C. ribicola. Cultivars of R. nigrum were more susceptible than those of any other species. No significant variation in virulence of the aeciospores was observed between and among Ribes hosts over the years. Cronartium flaccidum and Peridermium pini did not form any uredinia or telia on Ribes spp. in any of the experiments.     

Alder rust,Melampsoridium hiratsukanum Ito,identified from Wales,UK and British Columbia,Canada

An epidemic rust, Melampsoridium hiratsukanum, originating from East Asia has spread to Europe over the last two decades. We report the first observation of this rust in the British Isles on leaves of Alnus incana and A. glutinosa. In addition to M. hiratsukanum, a native M. betulinum–like rust co‐occurred on the same leaves as a mixed infection. We also confirm that a rust on A. rubra in Western Canada is identical to M. hiratsukanum present in East Asia and Europe.     

High‐resolution melting analysis allows rapid and accurate differentiation of the three Melampsoridium species occurring on Alnus

Alder (Alnus) leaves can be infected by three species of Melampsoridium rust: M. alni, M. betulinum and M. hiratsukanum. The latter is an invasive species in Europe that can be differentiated from the other two on the basis of morphological and molecular characters. This study presents a rapid, one‐step, closed‐tube and inexpensive method to differentiate species of Melampsoridium using two primer sets and high‐resolution melting (HRM) analysis of alder leaf material.     

New Picea hosts for Chrysomyxa ledi and Thekopsora areolata

A severe epidemic of Chrysomyxa ledi needle rust occurred over large areas in northern Finland in 2015. In late summer, young current‐year cones and needles of 7–8 Picea species were sampled in a northern botanical garden, and the frequencies of cone rusts, Thekopsora areolata, Chrysomyxa pirolata and C. ledi, were investigated in cones, and C. ledi was investigated in needles. In addition, the frequency of C. ledi was estimated visually in current‐year needles of ca. 400 trees of 17 species and varieties of Picea in the garden. Chrysomyxa pirolata was absent in cones of all Picea. No cone rusts were observed in cones of P. rubens and Picea mariana. Aecia of Thekopsora areolata occurred moderately in cones of P. abies (26% of cones) and rarely in P. engelmannii (3%) and P. omorika (6%). Chrysomyxa ledi was the most frequent rust in cones; aecia occurred abundantly in cones of P. abies (100%), P. engelmannii (69%), P. glauca (45%) and P. omorika (41%), and occasionally on P. pungens (13%). Thekopsora areolata from cones of the three Picea species showed 100% similarity in ITS2 sequence to one another and to samples in the GenBank. Samples of Chrysomyxa from needles and cones of Picea and leaves of Ledum palustre had ITS2 sequences 99.9%–100% similar to one another and 99.8%–100% similar to the closest sequence of C. ledi and C. rhododendri in the GenBank. Thekopsora areolata was recorded for the first time in cones of P. omorika and C. ledi in cones of P. engelmannii and P. pungens. In current‐year needles of Picea, C. ledi was abundant on P. abies, P. obovata, P. engelmannii, P. jezoensis, P. glauca and P. sitchensis, the rust was scarce on P. asperata var. nobilis, P. mariana, P. pungens and P. omorika, and the rust was absent on P. glehnii and P. rubens.     

A new species of Melampsora rust on Salix elbursensis from Iran

A rust fungus was found causing stem cankers on 1‐ to 5‐year‐old stems of Salix elbursensis in the north west of Iran. The rust also forms uredinia on leaves and flowers of the host willow. Light and scanning electron microscopy revealed that the new rust is morphologically distinct from several Melampsora species occurring on the willows taxonomically close to S. elbursensis, but indistinguishable from Melampsora larici‐epitea. Examination of the internal transcribed spacer (ITS) region of the ribosomal DNA suggested that the rust fungus is phylogenetically close to Melampsora allii‐populina and Melampsora pruinosae on Populus spp. Based on both the morphological characteristics and the ITS sequence data, the rust is described as a new species –Melampsora iranica sp. nov.     

Fruiting and sporulation of Thekopsora and Chrysomyxa cone rusts in Picea cones and Prunus leaves

Natural fruiting and sporulation of cone rusts were investigated in cones of Picea spp. and leaves of Prunus spp. in a botanical garden in northern Finland in 2007–2012. Thekopsora areolata was the most frequent cone rust in Picea abies cones, where it colonizes the host tissues and hinders normal seed development. Aecia of T. areolata were also common in cones of Picea engelmannii and occasionally in cones of P. glauca. Aecia of T. areolata sporulated in cones that were at least one year old. Chrysomyxa pirolata, another pathogenic cone rust, fruited and sporulated annually but infrequently in current‐year cones of P. abies. The spruce needle rust, Chrysomyxa ledi, fruited and sporulated commonly in current‐year cone scales of P. abies, P. omorika and P. glauca, while P. rubens, P. mariana and P. pungens appeared to be resistant during the study period. Chrysomyxa ledi did not affect seed development in infected cones. Uredinia of T. areolata frequently occurred on leaves of 41 Finnish and Russian cultivars, varieties or subspecies of Prunus padus L. ssp. badus and ssp. borealis and Pr. virginiana both in the botanical garden and in the field, while 13 exotic Prunus spp. lacked rust fruitbodies. All the Pr. padus cultivars were highly susceptible to T. areolata, thus, spreading the rust efficiently to surroundings. This is the first report of aecia of T. areolata in cones of P. engelmannii and P. glauca, and those of C. ledi in cones of P. omorika and P. glauca. Molecular identification confirmed the presence of T. areolata and C. pirolata on all hosts, and all samples of C. ledi belonged to the C. ledi‐rhododendri complex.     

The impact of cronartium flaccidum on the growth of pinus sylvestris

The blister rust of two‐needle pines in Europe is caused by the rust fungus, Cronartium flaccidum (Alb. Schw.) Wint. There are two races of the fungus: One host‐alternating and another pine‐to‐pine race. The latter race is considered to be more common in northern Sweden. The impact of this rust on growth of Scots pine (Pinus sylvestris L.) was investigated in three selected stands in northern Sweden. Radial stem increment was reduced 40–70% by severe attacks and 20–40% by minor attacks. The reduction of stem volume growth was probably greater. The faster growing trees seemd to be more severely attacked. Trees growing on poor soils appeared to lose almost as much growth capacity from a minor attack as from a severe attack.     

Absence of residual effects of a defeated resistance gene in poplar

In a few plant pathosystems, defeated major genes have been shown to contribute to partial resistance to disease. This hypothesis has never been tested before in a forest tree, but pathogenic variation associated with recent hybridization in poplar rust in the Pacific northwest provided an opportunity. An F₂ progeny of 256 poplar clones in the field near Corvallis, Oregon, USA, has been monitored for rust severity and infection type since the advent of the new hybrid rust, Melampsora × columbiana, in the mid‐1990s. All 256 clones displayed a susceptible infection type in 1997 and again in 2000, and yet variation in uredinial density (i.e. partial resistance) was still observed. To determine which clones possessed a defeated resistance gene, a greenhouse inoculation was performed with an isolate of M. medusae, one of the parents of M. × columbiana. Clones that would have been resistant to M. medusae, prior to the advent of M. × columbiana, were thus identified. The inoculation resulted in a 1 : 1 segregation (χ²=0.772; p=0.38) for resistance, indicating the presence of a major gene. However, the F₂ clones possessing the defeated resistance gene displayed the same level of partial resistance in the field in both 1997 and 2000 as their full siblings lacking the gene.     

Fusarium species in declining wild apple forests on the northern slope of the Tian Shan Mountains in north‐western China

Wild apple forests in the Tian Shan Mountains in north‐western China have been adversely affected by an unknown disease in recent years. Symptoms attributed to this disease that affects wild apple trees include xylem browning and dieback which are suggestive of infection by Fusarium species. Therefore, the research team conducted the first survey for Fusarium in the afflicted wild apple forests. Twig samples with symptoms of xylem browning and dieback were collected in the Xinyuan, Gongliu, Yining and Huocheng Counties of Xinjiang Uighur Autonomous Region in China. Based on phylogenetic analyses and morphological observation, sixty strains of Fusarium accounted for 48% of the total number of fungi isolated from samples were subsequently classified into six species including twenty‐four F. avenaceum, seventeen F. solani, ten F. tricinctum, five F. proliferatum, two F. sporotrichioides and two unfamiliar Fusarium sp. 1. The five previously known species of Fusarium were then tested for pathogenicity to leaves and twigs in vitro. The results indicated that all of the species, except for F. tricinctum, can cause obvious lesions on the leaves of host plants and on the twigs of Fuji and wild apple. This is the first report of Fusarium species pathogenicity in Xinjiang wild apple forests, confirming a new host for these pathogens in this study.     

Susceptibility and lesion development in Scots pine saplings infected with Peridermium pini in northern Finland

Susceptibility of Pinus sylvestris provenances from northern Finland to Peridermium pini and lesion development caused by the rust were investigated by inoculating 768 saplings. Resin‐top disease symptoms on the stem including stem swelling, occurrence of spermatial fluid and rust aecia, were recorded over a period of 9 years. The disease incidence was low (3.6%), varying within the range of 0–9.4% among pine provenances and 3.4–3.9% among spore sources in 1994–2002. Aecia were produced for the first time 2–4 years after inoculation, appearing mainly in early June, 2–9 years after inoculation. The aecia ruptured in late July to early August, and the overall sporulation lasted for 1–8 years among the pine provenances. Spermatial fluid was observed regularly in the year prior to aecia development 2–7 years after inoculation. Annual fresh swelling was observed for 1–6 years after inoculation. No statistically significant differences occurred neither between the pine provenances, the spore sources nor the years with regard to all the major disease variables. The results suggest that Scots pine provenances from northern Finland are highly resistant to P. pini regardless of the pine provenance or spore source.     

Responses of swamp bay,Persea palustris,and avocado,Persea americana,to various concentrations of the laurel wilt pathogen,Raffaelea lauricola

Laurel wilt, caused by the fungus Raffaelea lauricola and transmitted by the exotic ambrosia beetle Xyleborus glabratus, has killed members of the Lauraceae plant family throughout the southeast United States. A series of experiments were conducted to examine the effects of inoculum concentration on the development of laurel wilt in swamp bay, Persea palustris, and avocado, Persea americana. In each experiment, host plants were inoculated with aqueous suspensions of 10², 10³, 10⁴ or 10⁵ conidia of R. lauricola, and plants were rated periodically for external symptom development (wilting and foliar dieback). At the end of experiments, plants were rated for internal symptoms of the disease (discoloration of sapwood) and assayed for R. lauricola on a semi‐selective medium. Symptom severity in swamp bay was significantly lower for the 10² treatment than at higher (10³–10⁵) concentrations, whereas 10² and 10³ conidia caused less disease than 10⁴ and 10⁵ conidia in avocado. At the lowest inoculum concentration, 67% of the swamp bay plants and 20% of the avocados died by the time the respective experiments were terminated. The pathogen was recovered from a high proportion of the symptomatic sapwood of swamp bay (100%) and avocado (94%), and sapwood discoloration and recovery of R. lauricola from inoculated stems of swamp bay were highly correlated with recovery of the pathogen and symptom development in roots. Clearly, swamp bay and avocado are very sensitive to R. lauricola. The ability of only 100 conidia of this pathogen to kill these hosts suggests that few individuals of X. glabratus or other ambrosia beetles that carry low levels of the pathogen would be sufficient to transmit conidia that infect and lead to disease development. The results are also relevant to the development of disease‐tolerant host selections, as they indicate levels of the pathogen appropriate for use in screening plants for disease resistance.     

Climatic favourability zones for Eucalyptus rust in Brazil

Brazilian forest‐based industries are supported by more than 5.5 million hectares of Eucalyptus growing under different climatic conditions with different degrees of favourability for rust Puccinia psidii, including both traditional and expanding areas, where such disease is of major concern for the industry, foresters and scientists. The main objective of this study was to define favourable climatic zones for Eucalyptus rust in Brazil with the following aims: (i) to develop a spatial method for estimating the mean night‐time temperature (T_ng); (ii) to assess and validate a Eucalyptus rust model; and (iii) to map Eucalyptus rust favourability zones in Brazil based on the proposed model. A straightforward method, based only on latitude, day of the year, maximum and minimum air temperatures, was developed to estimate T_ng, which is a key variable for a proper application of Ruiz rust model. Based on 37 field experiments with natural rust occurrence, it was observed that climatic conditions are determining factors for disease severity. Significant correlations between disease severity observed in the field and climatic conditions were found: a negative correlation with temperature (r = .50, p < .01) and a positive correlation with relative humidity (r = .89, p < .0001). A significant correlation (r = .81, p < .0001) between the normalized infection index, produced from Ruiz model, and the average rust score was also observed. Once the model was validated under field conditions, it was applied using historical average data of air temperature and leaf wetness duration to obtain monthly Eucalyptus rust favourability maps for the whole country. These final maps show that favourable climatic zones for Eucalyptus rust are extremely dynamic, with high temporal and spatial variability in Brazil and that climatic conditions should be considered for expansion of Eucalyptus to new areas, in breeding programmes, and for defining the most suitable seasons for forest establishment in each climate of the country. These results provide forestry managers with practical tools to reduce uncertainty about the expected severity of Eucalyptus rust in Brazil.