The fungus, U. transversalis, like many other members of family Pucciniaceae is an obligate plant parasite.4 Therefore it derives its nutrition from its host plant for the duration of its lifecycle. Typically found on both wild and domesticated individuals of the Iridaceae family, U. transversalis is found in tropical or sub-tropical climates. (Note: extension to more temperate latitudes is anthropogenic. See next section, Geographic Distribution.) It is the causal agent of a rust disease called “Gladiolus Rust”. This is a problematic pathogen for a number of ornamental and cultivated cut flowers.
Primarily, U. transversalis infects Gladiolus hybrids, more commonly known as sword lilies.1 However it has been documented as a pathogen on the leaves of other tropical members of the Iridaceae family.5 Some specific primary host species include: Gladiolus dalenii(sword-lily),Gladiolus x hortulanus,G. psittacinus(dragon's-head-lily),and G. saundersii; while secondary species include: Anomatheca laxa(false freesia),Crocosmia aurea,C. aureavar. aurea,Melasphaerula ramosa,Tritonia lineata,T. securigera,T. qualida, otherTritoniaspp., andWatsoniaspp., W. angusta, W. densiflora, andW. meriana(bulbil Watsonia).9
The pathogen is most infective at temperatures that range between 10 and 20°C, while the leaves retain moisture for a minimum of 12 hours.4 Incubation length seems to be temperature dependent requiring around 22 days at 10°C and only 8-10 days at 25°C. However, the incubation period can be shortened with higher temperatures. Germination of urediniospores primarily occurs at 5-20°C, with the optimum temperature being between 15 and 20°C.9 With regard to alternate hosts and a secondary lifecycle, the details remain cryptic.
Some early indicators of the presence of this fungus include typical chlorosis, or leaf yellowing and lesions. As the pathogen continues to thrive and symptoms of the rust progress, the lesions will turn from dark brown to a black color (as appropriate for the transition from uredinia to telia takes place).9 The ecological significance of this fungus along with its relevance can be comprehended by monitoring and understanding its biology.
Useful pictures of spore and examples of lesions on plants can be found at: http://www.invasive.org/browse/subinfo.cfm?sub=4776.
Although not a culturally significant fungus in human history, U. transversalis poses a very real threat to the commercial horticulture and floral industries. When left unmanaged or untreated with fungicide, this disease can spur a total loss of gladiolus crops and is classified as a national quarantine pest.4 For example, U. transversalis has been deemed as an intense aggressive pathogen of commercial gladioli in western France and England since the 1960s.1
While Gladiolus Rust is not widely established in the United States, it was detected in commercial nurseries of both California and Florida.3 These recent discoveries warrant great concern for many, especially those that would be affected by such a decline in floral health. As a result eradication and control efforts for this fungus are presently underway and have been a major topic of research. More specifically, the Center for Plant Health Science and Technology (affiliated with the USDA APHIS) has assembled a technical committee to generate an appropriate management plan focused on the infested areas of FL and CA.1
For additional detection and management strategies of this fungus and the associated disease, be sure to review each of the appendices provided by USDA APHIS in Gladiolus Rust (Uromyces transversalis): A National Management Plan for Exclusion and Eradication at: http://www.aphis.usda.gov/plant_health/plant_pest_info/gladiolus_rust/downloads/grust-ntlmgntplan.pdf
From a morphologic perspective, U. transversalis can be easily mistaken at first glance for other rusts due to its powdery, yellowish-orange uredinia that are up to 3mm on both sides of the infected leaves. Sori run transverse across leaf surface (hence, species epithet transversalis) and may be aggregated or dispersed.3U. transversalis can be distinguished from other species such as U. gladioli and U. nyikensis, by thinner walled urediniospores and teliospores, along with smaller telia.7 Furthermore, other species lack the signature transverse sori and produce telia without paraphyses.6
Scientists have yet to discover spermogonia or aecia on this species.5 Therefore the complete life cycle of this fungus remains unknown. On the other hand, much has been documented with respect to the other spore-producing structures. Hernandez (2004) provides a very detailed account of the uredinia, telia, and their associated spores, and his description has been referenced by many other papers. His work summarized the characteristics of uredinia to beamphigenous, yellowish-orange in color, with a round to oblong or irregular shape that is typically transverse (horizontally) spanning 0.5-1.5 mm long and 0.5 (-2) mm broad. Corresponding urediniospores are variable in both form and size. Each uredinia produces between 6 and 8 scattered urediniospores. They are usually ovate, ellipsoid, or oblong, and measure at 14-26 × 13-25 µm. The cell wall is translucent and roughly 1.5(-2) µm thick with tightly arranged outgrowths (verruculose). Germ pores are difficult to see.5
Additionally, Hernandez (2004) describes teliaas minute structures, black in color. They are initially covered by the epidermis, found in small or large clumps, which may be scattered (though oftentimes grouped and loculate). These groupings are separated by a zone of amber-colored paraphyses that are 50-112.5 µm diameter and 55 -75 µm deep with spores in 3-4 closely packed rows.5 Teliospore shape ranges from oval to pear-shaped (pyriform). Less frequently globose teliospores may be present. Their color can be considered a warm light brown or a darker brown near the apex. Said apex is rounded and appears truncate compared to the rest of the spore body, with a reduced base that is less frequently rounded. Teliospore measurement ranges from (17.5-) 20-25 (-34) × (14-) 15-17.5 (-21) µm. They have a smooth cell wall, in contrast to the textured urediniospores, that is 2 µm thick and 4-6 (-8) µm at the apex. The spore pedicel is semipersistent with a translucent sheen or sometimes slight tint at the apex. It is usually around 45 µm long and 3 µm thick.5
Kingdom: Fungi
Phylum: Basidiomycota
Subphylum: Pucciniomycotina
Class: Urediniomycetes
Subclass: Urediniomycetidae
Order: Uredinales
Family: Pucciniaceae
Genus: Uromyces
Species: U. transversalis
As supported by Species Fungorum and MycoBank, the presently accepted Latin name for this fungus is Uromyces transversalis (Thüm.) G. Winter (1884).8,11 Only two widely accepted synonyms exist for this species, one is the original classification by Thümen in 1876 as Uredo transversalis, and the other is Caeomurus transversalis (Thüm.) Kuntze 1898.1 The citation for its first description can be considered the following: Uromyces transversalis(Thüm.) G. Winter.Flora, Jena.42: 263 (1884).8,11
In general terms, U. transversalis is commonly called “Gladiolus Rust”. As the common name and its classification suggests, U. transversalis is a member of the fungal group of rusts, with close relatives described as U. freesiae Bubák 1900 and U. watsoniae Syd. & P. Syd. 1910. Another species, U. gladioli is often associated with this fungus as well.1
Originally native to southern Africa, U. transversalis has a few scattered discoveries in South America (Argentina, Brazil, Colombia), Martinique, Australia, and New Zealand.5 In 1966, it was reported from southern France and northern Italy, where it spread to Malta in 1969 as well as Morocco and southern Italy in 1977.1 Taking a leap into the Pacific, the pathogen was discovered lying dormant in Melbourne, Australia in the late winter of 1994.7 Then in October 2005, several gladiolus plants in production fields of eastern Michoacán, Mexico exhibited the typical rust lesions associated with Gladiolus Rust. Nearly 100% disease incidence was present in this field.10
It was not until April 2006, that U. transversalis was first detected and confirmed in the United States, where it is a more popular concern and topic of research. The fungus was found on a small gladiolus farm in Manatee County, FL. Shortly thereafter in May 2006, Gladiolus Rust caused by this fungus was detected at four different locations in San Diego, CA.1 As of today, the northernmost reach of U. transversalis stemmed from the delivery of infected cut gladiolus flowers from Florida to Minnesota in April 2008. Those leaves were promptly collected and disposed of as regulated by the Minnesota Department of Agriculture. Continuing its voyage to the Caribbean, rust lesions were once more observed on leaves of hybrid gladiolus plants in both a private garden and commercial flower production field in Santiago de Cuba City and Güira de Melena, respectively.6
The rapid and widespread presence of this fungus is augmented by the international trade of corms and cut flowers (not to mention any products that are smuggled and unregulated). Aerial dispersion of urediniospores via wind or mechanical dispersion via clothing and equipment are also contributing factors to the movement of U. transversalis throughout the globe.
The map at this link, depicts the time period from 2006 to the present, during which the progress of U. transversalis has been monitored and recorded: National Agricultural Pest Information System (NAPIS). Purdue University. "Survey Status of Gladiolus Rust -Uromyces transversalis(All years)." Published: 09/24/2013. http://pest.ceris.purdue.edu/map.php?code=FJAAUBU&year=alltime. Accessed: 09/26/2013.
