Rusts and Smuts Rusts

Дата канвертавання27.04.2016
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L.11-G.Biology Mycology D.Ebtihal Muiz

Rusts and Smuts


Rusts are fungi of the order Uredinales (or Pucciniales). Many of these species are plant parasites. Many of the rusts have two or more hosts (heteroecious) and up to five spore stages. However, they most commonly reproduce via asexual spore production. Their spores are airborne and can travel great distances. They mostly cause foliar infections. The group received its common name from the fact that some species have a reddish spore stage, which resembles the corrosion process known as rust. Rust occurs on many species of plant, but in most cases any one species of rust can only infect one species of plant.

Rusts at their greatest complexity produce five different types of spores on two different hosts in two unrelated host families. Such rusts are heteroecious (requiring two hosts) and macrocyclic (producing all five spores types). Wheat stem rust is an example.

By convention, the stages and spore states are numbered by Roman numerals. Typically, basidiospores infect host one and the mycelium forms pycnidia, called spermagonia, which are miniature, flask-shaped, hollow, submicroscopic bodies embedded in host tissue (such as a leaf). This stage, numbered "0," produces single-celled, minute spores that ooze out in a sweet liquid and that act as nonmotile spermatia and also protruding receptive hyphae. Insects and probably other vectors, such as rain, carry the spermatia from spermagonia to spermagonia, cross inoculating the mating types. Neither thallus is male or female. Once crossed, the dikaryons are established and a second spore stage is formed, numbered "I" and called aecia, which form dikaryotic aeciospores in dry chains in inverted cup-shaped bodies embedded in host tissue. These aeciospores then infect the second host genus and cannot infect the host on which they are formed (in macrocyclic rusts). On the second host, a repeating spore stage is formed, numbered "II," the uredospores in dry pustules called uredinia. Urediospores are dikaryotic and can infect the same host that produced them. They repeatedly infect this host over the growing season. At the end of the season, a fourth spore type, the teliospore, is formed. It is thicker-walled and serves to overwinter or to survive other harsh conditions. It does not continue the infection process, rather it remains dormant for a period and then germinates to form basidia (stage "IV"), sometimes called a promycelium.

In the Pucciniales (or Uredinales), the basidia are cylindrical and become 3-septate after meiosis, with each of the four cells bearing one basidiospore each. The basidospores disperse and start the infection process on host one again. Autoecious rusts complete their life cycles on one host instead of two, and microcyclic rusts cut out one or more stages.


Order: Uredinales (Rusts)

This class is composed of a single order of obligate parasites that are pathogenic to ferns, gymnosperms and flowering plants. Basidiocarps are not produced in this order and the life cycle is quite different than that of the Basidiomycetes. In Puccinia graminis (Wheat Rust), the species that will be used as the representative for this order, there are five spore stages that are produced and two hosts are required in the completion ofthe life cyle. The five stages produced are:

  • Stage 0:Spermagonium

  • Stage I: Aecium

  • Stage II: Uredium

  • Stage III: Telium

  • Stage IV: Basidium

As a means of comparing the life cycle of Puccinia graminis with that of the mushroom life cycle, a summary of the life cycle will begin with the basidiospore.

Basidium (Stage IV)

Puccinia graminis is heterothallic and basidia produce basidiospores that are of two mating types, designated as a1 and a2, Basidiospores are capable of only infecting the leaves of Berberis sp. (barberry), the alternative host for this species. Species that require two hosts to complete their life cycles are said to be heteroecious. The cells of the teliospore germinates to produce a short germ tube that will develop into a basidium that is essentially transversely septate.

Figure 1: Basidia and basidiospores are produced from germinating teliospores. The basidium is very similar in appearance to the transversely septate basidia that is present in the Auriculariales of the Basidiomycetes

Spermogonium (Stage 0)

The spermogonium stage (Fig. 2) produces the sex organs in rusts. They are produced on the upper surface of the Berberis   (barberry) leaf. Since the spermogonia are derived from basidiospores, they are of two mating types. They are flasked-shaped and produce spore-like spermatia which ooze out, from the neck, in a sweet-smelling nectar. Also growing from the necks are receptive hyphae. The spermogonia are visited by flies which are attracted by the nectar secretions, and as they visit different spermagonia, spermatia of both mating types, adhere to their bodies and are transferred to receptive hyphae of the other mating types. This begins the dikaryon stage of the life cycle.


Figure 2. The spermogonium stage of Puccinia graminis forms on the upper surface of the Berberis leaf. Structure on the lower surface of the leaf is the aecium, the next spore stage in the life cycle. This spore stage does not form unless spermatia of one mating strain fertilize the receptive hyphae of a different mating strain.

Figure 3: A high magnification of the spermogonium stage. The spermogonium is flask-shaped. The spermatia are borne at the base of the spermogonium. The upright hyphae are the receptive hyphae.

Aecium (Stage I)

The aecium stage is directly linked to the spermagonium stage. When spermatia are transfered  to compatible receptive hyphae, this begins the dikaryotic stage of the life cycle and directly produces the aecium on the lower surface of the barberry leaf. The aecium is an upside-down,sac-shaped structure (Fig. 4)in which chains of aeciospores are formed.The aeciospores burst through the lower surface of the leaf and are dispersed by wind.


Figure 4. Aecium stage emerging from the lower surface of the Berberis leaf. The aeciospores are borne in chains and are held together by little rectangular cells called disjunctors.

Uredium (Stage II)

The aeciospores cannot reinfect the barberry host. Instead infection can only occur on the primary host, Triticum aestivum (wheat), where a new dikaryotic infection occurs. When two hosts are required in the completion of a rust life cycle, the rust is said to be heteroecious. The wheat is said to be the primary host while barberry is said to be the alternate host. The dikaryons infect the wheat stems and leaves and will form uredia that contain orange-brown urediospores (Fig. 5). This order is commonly called the rusts because of the orange-brown (rusty) colored pustules that form on the wheat plant after the urediospores have broken through the epidermal surface. The urediospores are comparable to conidia in that they will reinfect wheat plants and produce more uredia and ureiospores. This stage begins during summer and continues until late summer in North America.


Figure 5. A prepared slide showing the rust spores as they break the host epidermis. This is the repeating stage of the life cycle and continually reinfects wheat from early spring to summer.

Telium (Stage III)

Towards the end of summer, the uredium begins to produce teliospores (Fig. 6), a dark, thick-walled, two celled spore. Teliospores do not produce telia! It is the uredium that gradually becomes a telium by producing more and more teliospores. Because of the color of the teliospores,the telium is black. Following karyogamy, the teliospore overwinters. Meiosis takes place in each cell of the teliospore, in spring,and germinates to form the promycelium (=basidium). The promycelium becomes transversely septate, forming four cells. Each cell produce a sterigma and a basidiospore, and this now completes the life cycle.


Figure 6. Teliospores, on the wheat plant. The teliospores are dark, two celled, thick walled spores. These spores overwinter before producing the basidiospore stage. 

Comparison of the Puccinia graminis and mushroom life cycle

The monokaryon derived from the basidiospore, in the rusts, are heterothallic as in the mushroom life cycle. However, the dikaryon phase and its formation, in the life cycle of the rusts, is extended and more complex in comparison:

  • In the rusts, the monokaryons, derived from germination of basidiospore, produce sex organs, i.e. spermogonium with spermatia and receptive hyphae.The monokaryons in the mushroom life cycle does not produce sex organs and fusion takes place between hyphal cells of compatible monokaryons.

  • The dikaryon of the rusts produces three spore stages: aeciospore,urediospore and teliospore, with a change in host from barberry to wheat following the aecium stage. The dikaryon in the mushroom life cycle does not produce additional spore stages prior to basidia and basidiospore formation unless an asexual stage is present, in which case conidia are produced.

  • Basidiocarp formation is absent in the rusts.


Smuts (mostly Ustilaginomycetes of the class Teliomycetae) cause plant disease, and commonly affect grasses, notably including cereal crops such as maize. They initially attack the plant's reproductive system, forming galls which darken and burst, releasing fungal spores which infect other plants nearby.

The characteristic part of the life cycle of smuts is the thick-walled, often darkly pigmented, ornate, teliospore, which serves to survive harsh conditions such as overwintering and also serves to help disperse the fungus as dry diaspores. The teliospores are initially dikaryotic but become diploid via karyogamy. Meiosis takes place at the time of germination. A promycelim is formed that consists to a short hypha (equated to a basidium).

In some smuts, such as Ustilago maydis, the nuclei migrate into the promycelium that becomes septate, and haploid yeast-like conidia/basidiospores (sometimes called sporidia) bud off laterally from each cell. In various smuts, the yeast phase may proliferate, or they may fuse, or they may infect plant tissue and become hyphal. In other smuts, such as Tilletia caries, the elongated haploid basidiospores form apically, often in compatible pairs that fuse centrally resulting in "H"-shaped diaspores, which are by then dikaryotic. Dikaryotic conidia may then form. Eventually, the host is infected by infectious hyphae. Teliospores form in host tissue. Many variations on these general themes occur.

Smuts with both a yeast phase and an infectious hyphal stage are examples of dimorphic Basidiomycota. In plant parasitic taxa, the saprotrophic phase is normally the yeast, while the infectious stage is hyphal. However, there are examples of animal and human parasites where the species are dimorphic but it is the yeast-like state that is infectious. The genus Filobasidiella forms basidia on hyphae, but the main infectious stage is more commonly known by the anamorphic yeast name Cryptococcus (e.g., Cryptococcus neoformans and Cryptococcus gattii).

The dimorphic Basidiomycota with yeast stages and the pleiomorphic rusts are examples of fungi with anamorphs, which are the asexual stages. Some Basidiomycota are only known as anamorphs. Many are yeasts, collectively called basidiomycetous yeasts to differentiate them from ascomycetous yeasts in the Ascomycota. Aside from yeast anamorphs, and uredinia, aecia, and pycnidia, some Basidiomycota form other distinctive anamorphs as parts of their life cycles. Examples are Collybia tuberosa, with its apple-seed-shaped and colored sclerotium; Dendrocollybia racemosa, with its sclerotium and its Tilachlidiopsis racemosa conidia; Armillaria, with their rhizomorphs; Hohenbuehelia, with their Nematoctonus nematode infectious, state; and the coffee leaf parasite, Mycena citricolor and its Decapitatus flavidus propagules called gemmae


Order: Ustilaginales (Smuts)

This class is composed of several orders, but we will only consider the order Ustilaginales. Species in this order are composed entirely of fungi that are parasitic on flowering plants. However, unlike the rusts, the smuts are not obligate parasites throughout its entire life cycle. Only the dikaryon stage is obligately parasitic. The monokaryon stage, which is yeast, is saprobic. Thus, they are dimorphic. There are not many species of smuts that occur in Hawaii. Two species that are relatively common are: Ustilago cynadontis, Bermuda Grass Smut (Fig. 1) and U. maydis, Corn Smut (Fig. 2).

Figure 1: Ustilago cynadontis, Bermuda Grass Smut. The black powdery spores are the teliospores It is this dirty, powdery stage that led to this group of fungi being called smut.

Figure 2: Ustilago maydis, the Corn Smut. This species can be found where corn is grown in Hawaii.

The Ustilaginales is similar to the rusts in two significant characteristics: both lack a basidiocarp, during sexual reproduction and produce their basidia from germinating teliospores. However, the smuts do not have sex organs nor do they produce multiple spore stages as is the case of the rusts.

The life cycle is simple and more similar to the Basidiomycetes. We will describe the life cycle of Ustilago maydis, the corn smut, as an explane of a smut life cycle. There are, it should be noted that there are several variations of life cycle.

Following dispersal of the basidiospores, the spores typically germinate by budding to produce yeast cells, which are saprobic. When the yeast cells contact a compatible mating strains, dikaryon formation will occur. Once the dikaryon has formed, it becomes an obligate parasite and must have a host. Once in the host, the mycelial cells become rounded and break apart to give rise to the thick-walled teliospore stage.

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