Kingdom: Fungi
Phylum: Basidiomycota
Class: Pucciniomycetes
Order: Pucciniales
Family: Pucciniaceae
Genus: Puccinia
(according to the latest classification system)
P. graminis is an obligate parasite, and heteroecious rust.
P. graminis causes black rust of wheat
Heteroecious fungus:
Life cycle is completed on two different hosts. The wheat plant is called the primary host where dikaryophase is completed and the barberry plant is secondary or alternate host where haplophase is completed. Both hosts are required to complete the life cycle.
Puccinia life cycle summarized in five stages
Life cycle in Wheat
• Wheat is the primary host upon which dikaryophase of the pathogen is completed
• This phase consists of well developed branched, septate, dikaryotic vegetative mycelium and two spore stages namely, uridenial stage and telial stage.
Stage I in Wheat: Aecidiospores germinates on wheat; later dikaryotic hyphae form Uredia bearing uredospores.
• In wheat, primary infection starts with the germination of aecidiospore formed on the aecidial cups on the lower surface of infected barberry leaves.
• Aecidiospores are binucleate and on germination form primary hypha which enters the host through stomata. It forms haustoria for absorbing nutrients. This binucleate hypha or dikaryotic hyphae produces two kinds of spores uredopores and teleutospores
Uredenial Stage
• A few days after infection, dikaryotic mycelium form a specialized pustules or sori on the surface of the host tissue called uredosorus. Uredospores are formed on uredosorus.
• Uredospores are produced on long stalks and each spore is bi-nucleate, red or orange coloured and oval to round in outline.
• Large number of uredospores are formed in uredosorous and the pressure exerted by the developing uredospores breaks host epidermis to expose the uredospores.
• A uredospore can infect only a wheat plant. After falling on a suitable host under favorable condition it germinates by forming germ tube. Later it forms an elongated vesicle called appresorium on reaching stoma. Appresorium give rise to infection hyphae producing uredosorus and uredospores again within 10-12 days after germination. Thus, these spores cause several successive secondary infections during the season, and spread the fungus and the disease from field to field.
• The uredospore infections are seen as reddish brown pustules on leaves and stem of wheat plant. Therefore, uredineal stage also called as “red stage” or “summer stage”
Stage 2 in Wheat: Seasonal change triggers the formation of Telia or teleutosorus bearing teleutospores in the place of uredosorus.
Telial stage
• Towards the end of the growing season of wheat crop, when the conditions are unfavorable for uredospores, uredosori produce another kind of spores called teleutospores. Now the red stage is gradually replaced by “black stage” or causes black rust of wheat
• At first, teleutospores are formed on uredosorus, later teleutospores are formed in specialized pustules called teleutosori and the stage is called telial stage.
• Teleutospores are produced on long stalks and each spore is two celled, binucleated cells, spindle shaped and dark brown or black in colours with thick, smooth wall.
• Large number of teleutospores is formed in teleutosorous and the pressure of growing breaks host epidermis to expose the teleutospores.
• The two nuclei in each cell of teleutospores fuse to form a diploid nucleus at maturity (karyotype). This diploid spores undergoes a resting period to tide over winter.
Stage 3 in Wheat: Teleutospores produce promycelia bearing basidiospores on basidium.
Basidial stage on Wheat
• In the following spring, teleutospores germinate by forming promycelium. Promycelium comes out of the germ pore of each cell. The diploid nucleus enters promycelium and undergoes meiosis forming four haploid nuclei.
• These nucleus are separated by the formation of a cross wall. This four celled structure is called basidium.
• Each haploid cell of the basidium produces a slender, short, lateral, tube-like structure known as sterigma. Later, basidiospores is produced at the end of each sterigma. Basidiospore represent the beginning of haploid phase.
• Thus, from a single cell of teleutospore four haploid, unicellular, uninucleate basidiospores are formed. Two of the basidiospore produced belong to the ‘+’ strain and the other two ‘–’ strain.
• Basidiospores are released by an explosive mechanism and carried away by wind.
• The haplophase consists of a haploid vegetative mycelium and a spore stages, the pycnidial stage. Dikaryophase starts with aecidiospore.
Stage 4 in Barberry: Basidiospores germinate forming extensive hyphae with Spermogonia or Pycnia bearing spermatia and receptive hyphae (pycnidial stage).
Basidial stage on Barberry
• Basidiospores cannot germinate on wheat. It germinates only upon falling on the alternate host, the barberry.
• On the leaf of Barberry plant, basidiospores germinates forming germ tube and grows extensively forming haploid, septate, uninucleate mycelium.
• Mycelia of both strains co exist in the same leaf. They produce pycnidial stage on the upper surface and aecidial stage on the lower surface.
Pycnidial stage or Spermagonial stage
• The haplomycelium forms dense mats of hyphae on upper epidermis which later organize to form pycnidium of both strains.
• Spermagonium or pycnidium is a small flask shaped structure that opens to the outside by a small pore called ostiole. The ostiole is guarded at the edge by the long, delicate, sterile hyphae known as periphysis.
• There are two kinds of hyphae in a pycnidium. Long delicate receptive hyphae that extends beyond the ostiole and slender, short, vertical, uninucleate hyphae which arise form the base of the spermagonium called spermatial hyphae. It bears spermatia or pycniopores at the tip.
• The spermatia are unicellular, small, oval thin walled cells. A pycnidium gives rise to only + or ‘–‘ spermatium
• The transfer of spermatium of one strain to the pycnidium containing receptive hyphae of opposite strain is affected by insects.
• The spermatia of one strain when comes in contact with the tip of the receptive hyphae of opposite strain lead to the dissolution of intervening wall at the point of contact resulting in the formation of a dikaryon (two nuclei in common protoplasm without fusion). This pair of nuclei of opposite strains is called a dikaryon and this process is called dikaryotization.
Stage 5 in Barberry: Aecidia bearing aecidiospores produced in Barberry which infects wheat again.
Aecidial stage on Barberry
• From this dikaryotic cell, dikaryotic mycelium is formed which later organize to form the aecidia in the mesophyll of barberry leaf.
• Aecidia are cup shaped structures on which aecidiospores are produced. A mature aecidiospores are unicellular, thick walled, bi-nucleate and orange yellow coloured with many germ pores.
• The aeciospores are disseminated by wind. They are incapable of germination in barberry plant. It germinates when falling on wheat plant, the primary host. Thus the life cycle is completed.
