Melampsoridium hiratsukanum S. Ito ex Hirats. f., (Alder Rust)

California Pest Rating
Melampsoridium hiratsukanum S. Ito ex Hirats. f.,
(Alder Rust)
Pest Rating:  C


Initiating Event:

The pest rating of Alder Rust has been raised by certain counties and related nurseries due to its increased spread within California.  Currently, Melampsoridium hiratsukanum has a temporary Q rating that is herein reconsidered for a permanent rating.

History & Status:

Background:  Three Melampsoridium species are reported as causing foliar rust on alder (Alnus spp.) namely, M. alni, M. betulinum and M. hiratsukanum. Confusion has existed on the distinct identity of the species as all three are morphologically similar.  This confusion has caused doubt about the authenticity of many older records.  However, since the late 1990s, the use of DNA analysis has confirmed the identities of the three species. Melampsoridium hiratsukanum belongs to the family Pucciniastraceae in the order Pucciniales.  The species was first described on Manchurian alder (A.hirsuta) from Hokkaido in Japan in 1927.

Disease cycle:  Melampsoridium hiratsukanum is a heteroecious rust that produces two different fruiting bodies (uredinia and telia) on the leaves of its main host alder (Alnus spp.) and two different fruiting bodies (spermagonia and aecia) on its alternate host larch (Larix sp.).  On alder, the fungal species forms uredinia which give rise to urediniospores.  These spores are produced repeatedly and form the major stage for reproduction and dispersal.  Urediniospores are the only spores that re-infect their same host plant repeatedly.  Later, the pathogen produces the sexual, overwintering fruiting body structure or telia which give rise to thick-walled teliospores.  During the following spring, teliospores germinate forming a basidium which produces haploid basidiospores.  Basidiospores may infect young needles of larch (Larix spp.) and develop to form spermogonia – which form pycniospores or spermatia and receptive hyphae.  Following fertilization of spermatia and compatible receptive hyphae, dikaryotic mycelium is produced which forms aecia that produce aeciospores. Aeciospores may infect alder but not other larch or Larix spp. On infection, aeciospores produce more dikaryotic mycelium that this time forms uredinia that produce urediniospores thereby completing the life cycle.

The fungal pathogen may also exhibit a shortcut life cycle especially where the alternate aecial host (Larix spp.) is not present.  In such situations, the life cycle of the pathogen is completed on alder alone without the need of the alternate host and is therefore, reproduces and spreads only through the production of urediniospores.  Such may be the situation with the occurrence of the disease on alder plants in California nurseries.  The pathogen has not been reported on an alternate host in California.  It is likely that the urediniospores re-infect fresh alder leaves thereby enabling their survival and perpetuating the disease through a rapid build-up of inoculum.  Such a shortcut life cycle is known for many rust fungi with alternate hosts and is assumed to occur in Austria and Hungary (NOBANIS, 2007; Szabo, 2002).

Dispersal and spread:  Alder may be infected by aeciospores, or as discussed above for California, it may be infected with urediniospores produced in other alder plants.  Also, it may survive as mycelium infecting buds or with urediniospores.  The rust fungus spreads from plant to plant mainly by windblown spores.  Urediniospores are formed repeatedly and in abundance and may be transmitted over several hundred kilometers by strong winds and then washed down by rain on to host plants that are readily infected.  Aeciospores are also capable of long distance dispersal.  Over the past 10 years, the introduction and rapid spread of the pathogen in Europe was most likely due mainly to dry airborne spores, although specific distances have not been reported (Lane et al., 2013).  Infected nursery stock or plantings also provide a means for long distance spread of the pathogen which is capable of surviving in bud scales in dormant buds.

Hosts:  Alder (Alnus spp.), Black, European or common alder (Alnus glutinosa), Manchurian alder (A. hirsuta), Grey or Speckled alder (A. incana), Red alder (A. rubra) (EPPO, 2014), white alder (A. rhombifolia) (Blomquist et al., 2014); Larch (Larix spp.), Dahurian larch (L. gmelinii), Siberian larch (L. russica) (EPPO, 2014) and White or Downy birch (Betula pubescens) (Lane et al., 2013).  Experimental hosts: European larch (L. decidua), Tamarack or American larch (L. laricina) (Lane et al., 2013).

Symptoms and damage potential:  Infected white alder leaves exhibit numerous small orange-yellow uredinia pustules on the lower leaf surfaces, with corresponding yellow to orange spots on the upper surface.  Later, leaves turn dark brown and curl inwards with the production of telia.  Infected leaves often turn yellow, die and sometimes drop prematurely. The disease causes considerable damage to alder foliage in late summer causing them to be easily distinguished from a distance.  Repeated infections can cause defoliation and crown thinning leading to tree death (Lane et al., 2013).

Worldwide Distribution: Records of the detection of M. hiratsukanum prior to 2005 may be dubious due to taxonomic difficulty that existed in distinguishing it from other closely related species, viz. M. alni and M. betulinum without using molecular analysis.  Many records prior to 2005 may have been of M. betulinum (Lane et al., 2013).  Nevertheless, it is probable that M. hiratsukanum originated in Eastern Asia and has spread to temperate regions of Europe and North America.  Reliable records of the global distribution of M. hiratsukanum (EPPO, 2014) include:

Asia: China, Japan, Nepal (Adhikari & Manandha, 1989; EPPO, 2014)

Europe: Austria, Czech Republic, Estonia, Finland, France, Germany, Hungary, Italy, Latvia, Lithuania, Norway, Poland, Romani, Russia, Slovakia, Switzerland, Turkey, Ukraine, UK.

North America: Canada, USA (California and Oregon:  Bloomquist et al., 2014; Pscheidt & Ocamb, 2013).  Most likely, it is widespread in nurseries of western US coastal states (Blomquist 2014: pers. comm.)

Official Control: Currently, there are no reports of official control imposed against Melampsoridium hiratsukanum.

California DistributionMelampsoridium hiratsukanum is most likely widespread on alders in California nurseries (Blomquist 2014: pers. comm.). The pathogen has been in California since 1931 and a first published report was issued in 2014 of its detection in white alder from a nursery in Santa Cruz County (Blomquist et al., 2014).  However, disease symptoms on trees caused by the pathogen have not been found in California wild lands – mainly due to the absence to high moisture conditions necessary for the perpetuation of the disease in its natural habitat.

California Interceptions:  The pathogen has been in California since in the 1931 as evident by a sample that was submitted to the Federal Herbarium in Beltsville, MD.  However, this sample had been misidentified, and later, when it was correctly identified the revised identification was never published.  The pathogen has also been found in several locations in California Central Valley.

The risk Alder Rust would pose to California is evaluated below.

 Consequences of Introduction: 

 1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.

– Medium (2) may be able to establish in a larger but limited part of California.

High (3) likely to establish a widespread distribution in California.

Risk is High (3)Melampsoridium hiratsukanum is already established and suspected to be widely distributed wherever alder is grown in California nurseries.  The pathogen has not been found in California wild lands.

2) Known Pest Host Range: Evaluate the host range of the pest. Score:

Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

– High (3) has a wide host range.

Risk is Low (1): The host range is mainly limited to alder and larch trees. Only a few records of its occurrence on birch are from Europe (U.K.).  In California, the pathogen has only been found on alder.     

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score:

– Low (1) does not have high reproductive or dispersal potential.

– Medium (2) has either high reproductive or dispersal potential.

– High (3) has both high reproduction and dispersal potential.

Risk is High (3): Alder Rust has high reproduction and dispersal potential via windblown spores that are capable of being transmitted by strong winds over distances of several hundred kilometers.  Also they may be spread over long distances via infected nursery stock

4) Economic Impact: Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts.

Medium (2) causes 2 of these impacts.

– High (3) causes 3 or more of these impacts.

Risk is Medium (2): Trees infected with Melampsoridium hiratsukanum could cause significant loss of foliage, thereby generally reducing their yield and value in nursery productions.

5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

– Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

– High (3) causes two or more of the above to occur.

Risk is Medium (2): Alder rust has been in California since 1931, although the pathogen was misidentified.  Although it is difficult to find nursery alder trees that are not infected with the pathogen, according to Dr. Cheryl Blomquist (CDFA, pers. comm.), this rust pathogen has not been found in the wild lands, and no significant impact to California’s environment has been reported.  Nevertheless, it may cause significant concern for home/urban gardens or ornamental settings.

Consequences of Introduction to California for Alder Rust:

Add up the total score and include it here. (Score)

-Low = 5-8 points

Medium = 9-12 points

-High = 13-15 points

Total points obtained on evaluation of consequences of introduction to California = 11 (Medium).

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

-Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation is Medium (-2)Melampsoridium hiratsukanum is widespread in California nurseries, but has not been detected in wild lands.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

Final Score:  Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 8.


 While Melampsoridium hiratsukanum has been in California since 1931, it was not possible to distinguish it from M. alni and M. betulinum based on morphology alone.  The latter two species have been reported earlier from California.   It may well be those earlier detections are, in fact, M. hiratsukanum which have yet to be molecularly differentiated.  If this is determined to be so, then it will only strengthen the proposed C rating for this pathogen, as also will its detection in California’s wild land habitats.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Alder Rust is C.


Agrios, G. N.  2005.  Plant Pathology Fifth Edition.  Elsevier Academic Press, USA.  922 p.

Blomquist, C. L., H. J. Scheck, J. Haynes, P. W. Woods and J. Bischoff.  2014.  First published report of rust on white alder caused by Melampsoridium hiratsukanum in the United States.

EPPO.   2014.  Melampsoridium hiratsukanum (MELDHI).  PQR database.  Paris, France: European and Mediterranean Plant Protection Organization.

Lane, C., S. Matthews Berry and H. Anderson.  2013.  Rapid pest risk analysis for Melamsoridium hiratsukanum.  The Food & Environment Research Agency, Version 3, March, 2013.

NOBANIS.  2007.  Melampsoridium hiratsukanum.  NOBANIS – Invasive Alien Species Fact Sheet.

Pscheidt, J. W. and C. M. Ocamb.  2013.  Pacific Northwest Plant Disease Management Handbook.  URL:

Szabo, I.  2002.  First report of Melampsoridium hiratsukanum on common alder in Hungary.  Plant Pathology, 51:804.

Responsible Party:

John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110,[@]

Comment Period:

The 45-day comment period opened on Monday , October 19, 2015 and closed on December 3, 2015.

 Pest Rating:  C

Posted by ls