In this article we will discuss about the life cycle and the spore stage of rust fungi.
Some of the rusts need two different unrelated hosts to complete their life cycles. These are known as Heteroecious (Hetero = different; cion = Home) rusts and the phenomenon exhibited by them is called Heteroecism.
An example of heteroecious rust is Puccinia graminis which requires two hosts to complete its life-cycle. These are wheat (Triticum vulgare)—a monocot and Barberry (Barberis vulgaris)—a dicot.
The host-wheat bearing the teleutospore (teliospore) stage is called the Primary host and the host-Barberry—that bears the spermatial and aecial stages is known as Alternate host.
When some of the spore stages of primary host is formed on a related host, this host is called the Collateral host.
In some other rusts, only one host is required to complete the life cycle. Such rusts are called Autoecious (Auto = me; cion = Home) rusts and the phenomenon exhibited by them is known as Autoecism. Puccinia butlerii is an autoecious rust because it complete its life-cycle on one host, Launaea asplenifolia.
In the life cycle of a rust, the following spore stages are formed:
Stage 0: Pycnium bearing spermatia and receptive hyphae
Stage I: Aecium bearing aeciospores,
Stage II: Uredinium bearing urediniospores.
Stage III: Telium bearing teleospores
Stage IV: Basidiospores
While Basidiospores and pycnium producing spermatia and receptive hyphae are uninucleate, aeciospores, urediniospores and teliospores are binucleate.
When these spore stages were assigned Roman numerals, the function of Pycnia was not known. Hence, these were named as Stage 0. It was Craigie (1927) who discovered that Pycnium is the gametic stage bearing sex cells which are self-sterile.
It is the stage where plasmogamy and dikaryotisation take place when spermatia and receptive hyphae of the compatible mating type come in contact.
Stage I Aecium is produced on the same leaf simultaneously with pycnia. Aecia produce Aeciospores which are called binucleate and cannot infect the host on which they are produced.
Stage II Uredinium (pI. Uredia) is the sorus which produces one celled binucleate repeating spores. These spores are called Urediniospores and are generally produced in the early part of growing season.
Stage III Telium or teleutosorus produces thick walled, two celled binucleate teliospores which serve as the site of Karyogamy and meiosis. These also serve as resting spores.
Teliospores are the characteristic spores on the basis of which the rust genera have been identified. On germination, the teliospores produce Basidiospores which are thin walled, uninucleate and one-celled and constitute the Stage IV.
Modern mycologists have given definitions of these spore stages which are as follows:
Non repeating vegetative spores produced usually after dikaryotisation, which germinate to initiate dikaryotic mycellium, usually associated with pycnia (spermogonia).
Monokaryotic spores produced on basidia usually after meiosis.
Monokaryotic cells behaving as gametes.
Basidia producing spores.
Repeating vegetative spores usually produced on dikaryotic mycellium.
The rust genera have life cycles designated as Macrocyclic, Demicyclic and Microcyclic depending upon the spore stages produced. Macrocyclic rusts are those which produce all the above mentioned five spore stages.
These rusts are also known as long-cycled rusts. These spore stages may be produced in the same host as in case of autoecious rusts or on two different unrelated hosts as in heteroecious rusts.
Puccinia graminis is an example of heteroecous macrocyclic rust while P. helianthi represents the autoecious macrocyclic rust. Demicyclic rusts are those which lack the uredinial stage and may be autoecious or heteroecious.
Gynoconia peckiana is an autoecious demicyclic rust and Gymnosporangium juniperivirginiane is a heteroecious demicyclic rust.
Microcyclic rusts are those which produce only the telial stage. Hemileia is an example of a microcyclic rust.