The rust fungi :
Fungi belonging to the order uridinales commonly are reffered to as rust fungi . There are 5000 spicies of this order belonging to 140 to 150 different genera. Economically spreading, these are among the most important of all basidiomycota spicies. All are parasitic on plants, often causing great losses to many cultivated crops.In the past, Romans celebrated the festivals to please “Rust god” “Robigus” and festival name was “Robigalia”. Rust are obligate parasite i.e; only survive on living tissues.Rusts can attack various crops and may produce various types of symptoms like, tissues information, sunken necrotic leisons, abnormal branche (witch’s_brooms) , and may effect flowers.In effected flowers, infertile pollens are produce these flowers are also called “pseudo flowers”.
The most common symptom of rust is, the production of rusty powder over the foliage portion of the plants.Rust affected leaves reduce photosynthetic rate and chlorophyll levels.Rusts adversely affect the transpiration of the foliar portion of the plant.
Rust fungi may produce as many as different stages in their life cycles. Each spore stage of life cycle is given roman numerals.The stages of life cycle of rust are as follows;
Stage0: spermatogonia, bearing spermatia (n) and receptive hyphae (n)
Stage 1: Aecia bearing aceispores (n+n)
Stage 2: Uredia bearing urediospores(n+n)
Stage 3: Telia bearing teliospores(n+n=2n)
Stage 4: Basidia bearing basidiospore (n)
Based on life cycle pattern , spicies of uredinales may be divided into three categories;
Sikandar Hayat s/n Saleh Muhammad
Department : Plant Pathology ( Mycology of plant) (COA&UOS)
College of Agriculture, University of Sargodha, Lahore-Khushab bypass road Sargodha
1; Macrocyclic Forms:
Have all stages of life cycle
2; Demicyclic Forms:
Lack uredinial stage but have all rest of four stages.
3; Microcyclic Forms:
Lack uredinial and aceispores stage, but have rest of three.
Rust complete their life cycle on two different hosts, this condition is known as “Heteroecism”. While there are rusts which complete their life cycle on the single host, this condition is known as “Autoecism”. In “heteroecism” one host is called primary host while other is called alternate host.
Rusts produce Spermogonia which comprise of spermatia and receptive hyphae. Spermatia works as male sex organ cells while receptive hyphae behave as female sex organ cells.Spermagonia are produced from primary, homokaryotic mycelia growing in the tissues of their h Spermogonia are produced after four drops of infection.
1_Aecia and aeciospores:
Aecia and aeciospores form next in the rust life cycle. The term aecium comes from aikia , which means injury and refers to the blisters on the host resulting from the formation of the aecia by the parasite. An aecium consist of a group of typically eukaryotic hyphal cells with in the parasitized host that give rise to the chains of dikaryotic aeciospores.
2_Uredinia and urediniospores:
Urediniospores consist of so called repeating stage of the rust since several “Crops” of spores may be produce from one growing season.They are borne in structures resembling acervuli but that are called Urenidia (sing.Urenidium;L.urere =burn) because of their reddish colour.The uredinial cells are formed sub epidermally from dikaryotic mycelium originating from the germination of an aeciospore or an earlier urediospore.
3_ Telia and teliospores:
Telia (sing.telium) are groups of binucleate cells that give rise to special thick walled cells called teliospore also called as teleutospores. In many rust fungi the old uredinia actually are converted to telia. Typically, large number of teliospores protude from the surface of the infected host. Teliospores which may be unicellular or composed of two or more cells, are formed from the tips of binucleate cells of telium.
In teliospores consisting of more than one cell, each cell is capable of germinating and giving rise to a promycellium bearing basidiospores. External Basidia are , however, not protuce by teliospores of all spicies. In some spicies, meosis is completed in teliospore, and basidia are produced internally as a result of the division of teliospore cells into four cells. Each of these cell’s then germinates and produces a basidiospore outside the teliospore Teliospore are smooth, some spiny and others variously sculptured.
4_ Basidia and basidiospores:
The basidiospores is typically unicellular, Haploid structure. As already noted, basidiospores usually receive a single nucleus. From the basidium, although in some instances, two nuclei may move into the same spore. Rust teliospore is a pro basidium. When favourable conditions for germination arrive, in most spicies the promycellium grows out from each cell of the teliospore. Next the diploid nucleus migrates into the promycellium, after that, meosis takes place.The four haploid nuclei become distributed at more or less equal distances from each other. In the promycellium and septa develop between themselves to divide the promycellium into four uni nucleate cells. Each of these cells than produces a strigmata at the tip of which a typically pear or kidney shaped basidiospores develop. Mature basidiospores possess a thick wall and lack surface markings.
Rust fungi are self sterile therefore known as heterothallic fungi.
Stages 0 and 1 are produce on Barberry while 2 and 3 are produced on wheat.
Stage 4 i.e; don’t cause infection on primary and alternate hosts.
The spores which produce rusts or infect urediniospores .
However, when their colour become black then their teliospores, not urediniospores.
CONCEPT OF FAMAE SPECIALES:
Formal speciales are named after the host plants. They attack and are designated by adding the abbreviation “f.sp” and a third name to the latin binomial. These may be defined as “ fungi that are either very similar or identical morphologically but differ in their abilities to parasitize a host species or a group of host spicies.Forma speciales often are composed of populations of individuals of the same genotype referred to as “biotypes”. A group of biotypes with a similar virulence _ Avirulence pattern on a particular group of plants is called “race”.Races attack on a cultivar (variety).
E.g; Puccinia graminis f.sp. tritia