Melampsoridium hiratsukanum – invasive rust species in Lithuania , and its co-occurrence with eriophylid mite

An invasive East Asian rust fungus Melampsoridium hiratsukanum, obligate biotroph belonging to Pucciniastraceae, Pucciniales (Basidiomycota) is found widely spread on leaves of Alnus incana in eastern, central and southern parts of Lithuania. On Alnus glutinosa this fungus is rare, sometimes occurring with an alder leaf pest, a microscopic eriophyid gall mite Acalitus brevitarsus. Information on the distribution patterns, ecological and morphological characters of this neomycete is given.


INTRODUCTION
Majority of pathogenic neomycetes are classified as having considerable negative ecological and economic impact (Pimentel et al. 2001;Desprez-Loustau et al. 2007, 2010;NOBANIS 2007).Alien fungal diseases of woody plants represent a serious risk to forest ecosystems and growing plantations.In the mid-1990ies epidemic of a new foliar rust fungus affecting alder trees appeared in the Baltic region, in Estonia and Finland (Põldmaa 1997;Kurkela et al. 1999) and started to spread rapidly in Europe (Hantula, Scholler 2006).The agent was identified as Melampsoridium hiratsukanum S. Ito ex Hirats.based on morphological characterization and later was confirmed by molecular data (Hantula et al. 2009).
The detailed comparison of urediospore morphology and r-DNR sequence analysis of the ITS region showed that European population of M. hiratsukanum is conspecific with population of this fungus from Eastern Asia and that both populations belong to a single palearctic population (Hantula et al. 2009).In Europe this aggressive rust commonly cause considerable damage to foliage of grey alder, but may also infect more resistant black alder and cause serious problems in forest stands mixed with larch (Kurkela et al. 1999;Hantula et al. 2009).Introduction of this pathogen may be a result of an unintentional human activity such as transporting propagative plant material and seeds around the world or because of natural climatic factors.Natural migration of M. hiratsukanum could be explained by continuity of its main (Alnus spp.) and alternate (Larix spp.) host distribution from the Far East, where it inhabits native for Asia Manchurian alder (Hiratsuka et al. 1992), to Siberia (Kuprevicz, Tranzschel 1957), Fennoscandia, Baltic region and further to Europe, where it occurrs on Eurasian grey and native for Europe black alders as well as on various (Dahurian, Siberian, European) larches (Põldmaa 1997;Kurkela et al. 1999;Hantula et al. 2009;Lilja et al. 2011).Changing climate with milder winters and increased precipitation probably also stimulated invasion and rapid spread of this neomycete in the new regions on new hosts during the last decades.
The main aim of present study was to indicate the occurrence of Melampsoridium hiratsukanum on Alnus spp. in Lithuania, to discuss its ecology and distribution patterns.

MATERIAL AND METHODS
Samples of the rust infected leaves of Alnus incana (L.) Moench and A. glutinosa (L.) Gaertn.were collected from various regions of Lithuania during the autumn of 2009-2012.Observation of disease symptoms (Figs 1, 2) and severity continued until late October each year concerned.For comparison morphologically similar rusts, infected leaves of nearby growing Betula pendula Roth, which may occur on alder leaves as well, were collected.Specimens with characteristic rust symptoms collected during present study and all specimens collected earlier by other mycologists available at the herbarium of the Nature Research Centre, Institute of Botany (BILAS), were microscopically examined using the standard microscopic techniques with a Nikon stereo microscope at the magnifications up to 40x and with an Olympus CX 41 microscope at the magnifications up to 400-1000x.The fungus was identified according to morphological characters using (Kuprevich, Tranzschel 1957;Kaneko, Hiratsuka 1981;Roll-Hansen, Roll-Hansen 1981;Hiratsuka et al. 1992;Hantula et al. 2009).
Description and illustrations were made from fresh preparations in distilled water.Dried voucher specimens are preserved in the BILAS Herbarium.

RESULTS AND DISCUSSION
On alders, three morphologically similar Melampsoridium rusts may occur: M. betulinum (Pers.)Kleb , M. hiratsukanum S. Ito ex Hirats.and M. alni (Thüm.)Dietel, but only two of them -M.betulinum and M. hiratsukanum are known in Europe so far (Hantula et al. 2009).In Lithuania, M. betulinum is widely spread on birches, but was not yet recorded on alders (Minkevičius, Ignatavičiūtė 1991).Microscopical examination of Lithuanian material showed that the rust developing on leaves of Alnus incana and on leaves of A. glutinosa produced regularly echinulate yelloworange, ovoid to ellipsoidal urediniospores, commonly 24-32 × 12-15 μm in size with 4-6 bizonate germ pores and by size and morphology differs both from native rust M. betulinum and other Asian alder rust, M. alni (Tab.1).Following morphological characters of uredinia and urediospores the rust obtained on Alnus leaves in Lithuania was identified as M. hiratsukanum (Figs 3-4).Uredinia of M. hiratsukanum produced longer, resembling sharp spines, ostiolar cells (up to 50 μm), comparing to morphologically similar M. betulinum.Uredinia of M. betulinum collected during present study were characterized by shorter ostiolar cells (up to 35 μm long) and by smooth, rounded upper end of urediniospores, which were slightly larger in size, commonly about 26-48 × 12-17 μm.M. alni is very similar to M. betulinum by size of urediniospores and lack of echinulation at the apex but its urediniospores have two germ pores, one at each end of the spore (Hiratsuka 1927;Kuprevich, Tranzschel 1957;Hantula et al. 2009).
During present investigation, Larix spp.trees were checked in various parks and forest plantations as well, but aecial stage of M. hiratsukanum was not found.It is possible that the fungus can reproduce and spread only by its urediniospores (Hantula, Scholler 2006).Apparently, reduced fungus life cycle restricted to the telial and uredinal host (Alnus spp.) prevails in Lithuania.
ExaminEd spEcimEns: Melampsoridium hiratsukanum: on Alnus incana, Gružu forest, Ukmergė distr., 10 Sep., 1997, leg. A.Treigienė, det. Ignatavičiūtė, BILAS 31446;on A. incana, Vilainiai forest Kėdainiai distr., 16 Sep., 1998, leg. A.Treigienė, det. Ignatavičiūtė;BILAS 31445;on A. incana, Plateliai forest, Plungė distr., 3 Sep., 2003, leg. A.Treigienė, det  B. pendula, Juodkrantė environs, Neringa, Curonian Spit, 5 Sep.2012, leg./det.S. Markovskaja, BILAS 50319.distribution in Lithuania.After inspecting mixed forest stands, parks and other habitats in various regions of Lithuania, it was assessed that M. hiratsukanum is already widely spread in the country on Alnus incana (leaves' infection in some cases reached 20 to 80%) while on A. glutinosa it has appeared recently and does not cause strong damage of leaves (about 10%).Most strongly infected (up to 60-80% of leaves) and defoliated already in September were juvenile grey alder trees growing along roads and water bodies.From 2009 till 2012, M. hiratsukanum was recorded on Alnus incana in 22 localities of Biržai, Kaunas Kėdainiai, Klaipėda, Kretinga, Molėtai, Ignalina, Panevežys, Plungė, Šalčininkai, Širvintai, Švenčionys, Ukmergė, Trakai, and Vilnius districts, while on Alnus glutinosa it was found only twice, for the first time in 2010 at the edge of the great cormorant (Phalacrocorax carbo sinensis) colony (Curonian Spit, environs of Juodkrantė) and later, in 2012, in the environs of Lake Tabaliukai, Trakai district (Fig. 5).Notable, that in Curonian Spit, in the forest stand affected by great cormorant colony, M. hiratsukanum infected leaves of both A. incana and A. glutinosa trees growing nearby and in some cases M. hiratsukanum infected Alnus glutinosa leaves together with an alder pest, microscopic mite Acalitus brevitarsus Fockeu.Symptoms caused on leaves by A. brevitarsus resemble rust fungus infection.This small arthropode, eriophyid mite (Eriophyidae, Acari, Arthropoda) inhabit exclusively Alnus glutinosa leaves (Figs 6-9).Like many gall-inducing mites, individuals of A. brevitarsus cause deformation of leaves or blister-like galls and elicit growth of clusters of hairs on upper surface of leaf (called an erineum) among which the mites live (Figs 6, 7).The hairs have several near-horizontal branches at the tip (Fig. 8), so each group of hairs is like a forest with a closed canopy.Both the pest A. brevitarsus and the rust fungus M. hiratsukanum were recorded together on the same host leaves only in the stand affected by cormorants (Curonian Spit).In the other parts of Curonian Spit and in the continental part of Lithuania they have never been found to occur together.Evidently, the trees affected by cormorant excrements became less resistant to various pathogens and pests.Successive infections of both agents can cause considerable damage to alder foliage and thus negatively influence the tree health and the functioning of the whole forest ecosystem.

CONCLUSION
Comparison of morphological data indicated that alders and birches in Lithuania are infected by different rust species.Leaves of Alnus incana and A. glutinosa are damaged by invasive Asian rust M. hiratsukanum, while native M. betulinum inhabits birch leaves.From its first record in 1997 till now M. hiratsukanum has widely spread in Lithuania on A. incana.On A. glutinosa M. hiratsukanum was recorded for the first time in 2010, co-occurring with eriophyid mite Acalitus brevitarsus.It is hypothesized that the successive colonization and co-occurrence of new pathogen (M.hiratsukanum) and pest (Acalitus brevitarsus) on back alder was a result to the changes (decrease of trees resistance) caused by a strong negative impact of great cormorant colony, namely by forest hypertrophication from the bird excrements.