Entyloma ageratinae, commonly known as the mist flower smut, is a leaf smut fungus and plant pathogen widely employed as a biological herbicide in the control of the invasive plant Ageratina riparia. The pathogen was first identified in Jamaica in 1974 and was isolated as a distinct species in 1988.
E. ageratinae has been the most effective and widely implemented biological control of A. riparia. The fungus was first identified by scientists searching for a naturally occurring control agent in Jamaica. Specimens were then transported to Hawaii, where they were examined and trialled as a control agent. After the species' proven success at weakening A. riparia populations, it was implemented in wide reaching projects in Hawaii and New Zealand. The pathogen is now endemic to populations in Hawaii, New Zealand, Australia, and to a lesser extent, in South Africa.
Taxonomy
The fungus was initially considered a species within the Cercosporella genus, and has formerly been referred to as Cercosporella argeratina (nomen nudum). The species was later named Entyloma compositarum. In 1988, Entyloma ageratinae was isolated as a separate species from E. compositarum by R.W. Barreto and H.C. Evans. Other authorities on the field, however, argue it should instead be classified as the forma specialis E. compostarum f. sp. Ageratinae. Description The fungus grows almost exclusively on A. riparia plants. The only other known species to develop symptoms of the disease is Ageratina adenophora. Though the two species are closely related, the disease's effects on A. adenophora are comparatively benign; it does not form spores and produces only small lesions. Contraction by A. adenophora is rare, and has only been documented in laboratory trials.E. ageratinae is both a biotrophic and necrotropic pathogen, consuming both living and dead matter. The fungus initially forms small, circular chloratic lesions on the upper surface of living A. riparia leaves. These lesions expand, darken and turn reddish-brown as the pathogen becomes necrotic. White, aseptate, holoblastic spores form on the underside of leaves 7–10 days after infection. Infected leaves eventually wilt and fall off the main plant prematurely, stunting its growth. The fungus may also cause shoots to die off.
The species has a short life cycle and spreads most quickly in windy environments with heavy rainfall. Temperatures above 20 °C are fatal to it.
Biological control agent
Hawaii A. riparia was first introduced to Hawaii as an ornamental plant around 1925 and is locally known as Hamakua pa-makani. The plant was first documented as naturalised near Hilo in 1926. It thrived in the moist climate of Hawaii, preferring ditches and low pastures. By 1930, it had spread to much of the northwest of the island. Mid-19th century methods to control the plant on the island included spraying affected areas with a solution of arsenic or calcium chlorate. These early methods were not only toxic to the native ecosystem, but proved ineffective at limiting the spread of the plant. By 1973, A. riparia had infested an estimated 62,500 ha of the island of Hawaii.
A $10,000 grant provided by Barbara Cox Anthony funded a total of three trips in search of naturally occurring biological control agents for the A. riparia. The first two trips to the species' center of origin in Mexico in 1973 and 1974 were ultimately unsuccessful. A rust fungus was identified in El Mirador, but limited biological information meant that it could not be transported back to Hawaii because of pathogenic quarantine regulations. A third expedition made to Jamaica in 1974 successfully discovered diseased plants in the Blue Mountains. Importantly, none of the adjacent crops or flora in Jamaica showed symptoms of infection by the pathogen. Because the fungus appeared highly specific to the A. riparia, it was permitted for live transportation to Hawaii by the Department of Agriculture and the Federal Animal and Plant Health Inspection Service. Diseased branches of the plant were collected and placed in iced plastic bags. On arrival to Hawaii 48 hours later, five diseased leaves had survived the trip. These five leaves were used as the initial sources of inoculation for Hawaiian A. riparia plants in a quarantine terrarium in Manoa.