In our country, out of the major two rusts infesting crop, wheat, leaf rust and stripe rust, the former is more harmful.
The three types of host related with this disease are as follows:
(i) Alternate Host:
This host is required to complete the life-cycle e.g. Barberis is the alternate host of stem rust whereas Thalictrum is the alternate host of leaf rust. In India however alternate hosts have no importance in the life-cycle of rust fungus. Yellow rust has no alternate host in India.
(ii) Collateral Host:
Besides agricultural crops the uredial and conidial stages of the rust pathogen survive on Grass hosts e.g. Bromus sp and Agropyron are the collateral host of yellow rust. Brome grasses serve as collateral host for leaf rust and similarly Bromus sp (In Northern India) and Brija minor (in Southern India) are collateral hosts for Black rust.
(iii) Primary Host:
The host on which the rust pathogen produces its telial & resting stages is the Primary host. Dr. K.C. Mehta and his team have identified three hosts on which uredial spores are produced.
(a) Continued available host.
(b) Self growing wheat plants.
(c) Grass host e.g. Bromus etc.
Mehta and his associates undertook research surveys from the foothills to the higher attitudes of Himalayas and concluded that Rust fungus perpetuates in hills of Northern India. L.M. Joshi, a student of Dr. Mehta reported that although the rust pathogen perpetuates in Northern India but its main source was the coastal areas of Indian Ocean and Bay of Bengal.
According to Dr. K.C. Mehta wheat crop occupies maximum time of the year in hilly areas of Sindh. The short period of 2-3 months when the wheat crop is not available in the fields, the pathogen survives through its uredial stages on wild wheat and other grassy hosts.
Predisposing Factors:
(a) Nearness of hills.
(b) Speed and direction of wind.
(c) Amount of moisture and sunlight.
(d) Suitable hosts.
There are four stages in the life cycle of rust organism. These have been represented by roman digits in the given table. The functions of these four stages had been well understood before 1927.
Craigie in 1927 discovered the function of Pycnial stage and reported that plasmogamy occurs in this stage. Craigie for the first time reported Heterothallism in Rust fungus. Among these five stages only uredial (II) and telial (III) are found on wheat. Pycnial (O) and Aecial (I) stages are present on Berberis and Mahonia whereas Besidial stage is produced on the inactive substratum on which teleutospores are produced.
Physiological Specialization:
Erikson (1893) reported that Puccinia graminis was not a composite pathogen. He classified it into five formae specialis or varieties. These varieties differed from each other with respect to the shape of uredospore and other morphological characters such as colour and roughness. The term f. sp. has been placed between gaminis and tritici which indicates that triticum is a special form of Triticum allies. This very property of the organism is called Physiological specialization.
Later Stakman (1915) reported that even puccinia graminis f. sp. tritici was not a composite organism. He concluded that the special form of Puccinia graminis tritici was again made up of several forms which he called ‘Biotypes’. He again observed that even a Biotype was not a composite organism and was composed of several ‘races’.
Biotypes – represented by Arabic numerals i.e. 1, 2, 3, 4
Races – represented by roman letter a, b, c, d,
Example- the complete name of fungus causing Black rust/stem rust is as follows:
Puccinia graminis f. sp. tritici 16d.