Basidiomycota Rusts & Smuts

Pucciniomycotina- Rusts 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

Rusts Very large group of Basidiomycota - ~8400 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

Pycnia 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

Uredinia Form a pustule (acervulus) that breaks through the epidermis of host Urediniospores are generally reddish in color – hence the common name - rusts

Uredinia

Stage III - Telium Telia produce teliospores Begin as dikaryotic cell, site of karyogamy so become diploid Telium is a sorus on host plant

Telium Teliospore is overwintering spore

Teliospores Great variation in size, shape and color of teliospores in different species

Teliospore 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 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)

Puccinia life cycle 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

Puccinia life cycle 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)

Puccina life cycle 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

Puccinia life cycle

Puccinia life cycle 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 with 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

Wheat rust 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

Ustilaginomycotina - Smuts Smuts – so called since they form black dusty spore masses on hosts that they infect ~1200 spp. Commonly infect the reproductive structures of non woody angiosperms (grasses) – anthers, ovaries, embryos Grow in culture as yeast like cells

Smuts 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

Smut life cycle 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