Urediniomycetes - Rusts

Basidiomycota

•      Classes
•      Basidiomycetes – produce basidioma
•      Urediniomycetes – rusts, no basidioma, produce teliospore, obligate plant parasites
•      Ustomycetes – smuts, no basidioma, produce teliospore, facultative plant parasites

Uredinomycetes

•      One order – Uredinales
•      Economically very important as parasites of crop plants
•      Obligate parasites of vascular plants (ferns, gymnosperms, angiosperms)
•      Each species has very narrow host range
•      Generally do not kill host, but weaken and greatly reduce yields of seeds and fruits
•      Produce intercellular hyphae with haustoria
•      Very large group of Basidiomycota - ~5000 spp.
•      Example – Puccinia graminis – stem rust of wheat – tremendous problem in wheat growing areas, Romans made sacrifices to the rust god – Robigus
•      Rust can exhibit the most complex life cycles in fungi

Rust life cycles

•      Long cycle or Macrocyclic rusts produce five different types of reproductive stages (spores and spore bearing structures), may alternate between two hosts
•      Short cycle or microcyclic rusts, one or more of these stages are lacking
•      Examine the different stages and then examine a life cycle

Stage 0 - pycnium

•      Pycnium (spermogonia) produces pycniospores (spermatia)
•      Haploid
•      Stage in which plasmogamy occurs
•      Vary in shape – flask shaped, flat or globose
•      Pycnium produces receptive hyphae – function as female structures
•      Pycniospores function as male gametes
•      Rusts are typically heterothallic
•      Plasmogamy occurs between pycniospore of one mating type and receptive hypha of another

Stage I - Aecium

•      Aecium produces aeciospores
•      Dikaryotic stage
•      Infects different host

Stage II - Uredinium

•      Uredinium produces urediniospores
•      Dikaryotic
•      Serves as the asexual (anamorphic) stage, since it infects same host
•      Form a pustule (acervulus) that breaks through the epidermis of host
•      Urediniospores are generally reddish in color – hence the common name - rusts

Stage III - Telium

•      Telia produce teliospores
•      Begin as dikaryotic cell, site of karyogamy so become diploid
•      Telium is a sorus on host plant
•      Teliospore is overwintering spore
•      Great variation in size, shape and color of teliospores in different species
•      Teliospore functions as probasidium – site of karyogamy and gives rise to basidium
•      Teliospores germinate and give rise to phragmobasidia (metabasidium – site of meiosis) 

Stage IV - Basidium

•      Basidium produces basidiospores

Life cycles

•      A given rust species may produce all 5 stages or only some of these stages
•      Some produce only uredinia –could be classified in Deuteromycota
•      Some species require two hosts – heteroecious rusts
–  Produce pycnia and aecia (Stages 0 & I) on one host, uredinia and telia (Stages III & IV) on second host
–  Hosts are not closely related
•      Some only require one host - autoecious

Puccinia graminis

•      Stem rust of wheat – macrocyclic heteroecious rust
•      Life cycle outlined on handout
•      Teliospores overwinter, germinate in spring (karyogamy and meiosis) to form basida and basidiospores
•      Basidiospores can only infect and grow on alternate host – barberry bush
•      Gives rise to pycnia

Puccinia life cycle

•      Pycnia secrete nectar to attract insects to carry pycniospores
•      Pycniospores fuse with compatible receptive hyphae – plasmogamy
•      Hyphae from pycnia have formed aecial initials, once plasmogamy takes place, aecia develop (n + n)
•      Aeciospores are dispersed by wind, can only infect primary host – wheat
•      Dikaryotic mycelium develops in wheat stem and leaves and gives rise to uredinia with urediniospores
•      Urediniospores are dispersed by wind, continue to infect wheat throughout the summer
•      As wheat begins to ripen, uredinia gradually shift to producing teliospores, become telia (red sori become black sori)
•      Teliospores overwinter, germinate next spring by forming basidia and basidiospores
•      Complex life cycle – took a number of years to work out that the forms on barberry was the same organism that was on wheat
•      In 1600’s, it was observed that barberry plants increased incidence of wheat rust, no one knew why
•      In 1800’s, Anton deBary discovered that all the stages were of the same fungus
•      deBary called father of modern mycology, worked woth plant diseases, rusts and smuts, wrote the first text on mycology

Wheat rust

•      With the discovery of the life cycle of wheat rust, erdication programs were began to get rid of barberry
•      Practical importance of importance of understanding life cycles
•      However, this did not eliminate wheat rust
•      If winters are not too cold, urediniospores can overwinter and establish on wheat in spring
•      In U.S. wheat belt, urediniospores overwinter in Texas and then migrate northward on winds through the great plains states into Canada
•      Also discovered that genetic recombination was not eliminated since rusts have a parasexual cycle operating so that they can carry out genetic recombination in absence of meiosis

Cedar apple rust

•      Gymnosporangium juniperi-virginianae
•      Apple is alternate host – produces pycnia and aecia
•      Junipers are primary host but there is only a telial stage no uredinial stage so that elimination of either host is effective in controlling the disease

White pine blister rust

•      Cronartium ribicola
•      White pine is alternate host – produces pycnial and aecial stages
•      Gooseberries and currents are the primary host – produce uredinial and telial stages
•      Asexual stage is not on economically important host so eliminating it is effective

Ustomycetes

•      Smuts – so called since they form black dusty spore masses on hosts that they infect
•      One order – Ustilaginales - ~1000 spp.
•      Commonly infect the reproductive structures of plants – anthers, overies, embryos
•      Grow in culture as yeast like cells
•      A number of economically important plant pathogens – corn smut, Ustilago maydis, loose smut of oats, Ustilago avenae, bunt and stinking smut of wheat, Tilletia spp.

Smut life cycle

•      Haploid phase is limited
•      Plasmogamy takes place between compatible cells – basidiospores, conidia, yeast cells or combinations
•      Dikaryotic mycelium grows intercellularly in plant host
•      Some species form conidia for asexual reproduction
•      When dikaryotic mycelium reaches sporulation stage, it consist of short dikaryotic cells
•      Cells round up, form thick walls and differentiate into teliospores
•      Teliospores are the overwintering spores
•      Teliospores germinate to produce basidia

Smut teliospores

•      Teliospores may be individual or grouped in spore balls

Smut basidia

•      Ustilago                                        Tilletia

Ustilago life cycle

Tilletia life cycle

Comparison of Rusts & Smuts

•      Urediniomycetes
•      Terminal teliospores
•      4 basidiospores produced on sterigmata
•      Plasmogamy – specialized cells
•      May require 2 hosts
•      Obligate biotrophs
•      Hosts include angiosperms, gymnosperms, ferns

•      Ustomycetes
•      Intercalary teliospores
•      Variable number basidiospores, no sterigmata
•      Plasmogamy – no specialized cells
•      Never require 2 hosts
•      Facultative biotrophs
•      Hosts include angiosperms only