Rust are the most deleterious fungal disease of the cereals especially of wheat crops worldwide. This is due to their wide range of distribution, high capability of mutation to form new races that are capable to attacking the previously resistant varieties, have the capability to move long distance with the wind and air and have the potential to complete more rapidly their life cycle under the optimal conditions. Wheat rust fungi are obligate parasites, required living organism for the completion of their life cycle because they cannot be grown on the artificial media. The life cycle of the rust disease of the wheat is very complicated, required one alternate host organism or their life cycle and diverse types of the spores are need for the completion of their life cycle under the suitable condition. Leaf rust, stripe rust and stem rust of wheat are the three most deleterious disease of the wheat. In Pakistan, leaf rust of wheat is the most common diseases as compared to other two rust diseases of the wheat. Although stripe rust of wheat is also treacherous diseases but less dangerous as compared to the leaf rust of wheat. The prevalence of stem rust of wheat is in traces due to unsuitable environmental conditions. But a new race of the stem rust of wheat is detected known as Ug99 in East Africa which overcome the resistant source in the wheat cultivars worldwide. Until seven new races of the Ug99 are known and it also spread to the various countries of the world and affect the quality and quantity of the wheat. Leaf rust disease cause more damage when upper leaves of the wheat are severely attacked by the rust spores before the flowering stage. Rust reduce the size of the infected leaves, which reduce the period of grain filling as a results size of kernel remain as small. Leaf rust of wheat produce orange-brown like urediospores on the upper surface of the infected leaves and under the most suitable condition, urediospores can cover the entire upper surface of the leaves in the susceptible cultivars. Many urediospores are formed in the single uredia. With the passage of time as the wheat plants goes to the maturity stage, Uredinial stage of leaf rust convert into another stage known as telial stage, (telial stage is similar with the uredinial stage in size) but they are black in color and develop on the lower surface of the leaves. Telia produce the spores known as teliospores which are brown-black in the color. Teliospores are not always produced particularly if the wheat infection occurs late in the growing season.
Occurrence of leaf rust of wheat
The prevalence of wheat rust diseases in cultivated areas of the world have significantly affect the human culture and their civilization. It was mentioned in the Israel’s excavations that urediospores of the wheat rust have been discussed in the 1300 B.C. It was thought that patriarch Jacob family face the problem of rust epidemic in 1870 B.C., and suggested that rust affect the Roman and Greek in about 500 B.C, they all peoples thought that their rust God is angry from their behaviors, so they face the problem of crop failure.
First report of leaf rust of wheat
Firstly, it was general concept that wheat crop is attacked by the mildew, blasting and blight, with the advancement of agriculture knowledge, conformed that rust is due to the fungal attack. Aristotle (384-322 B.C.) demonstrated that “warm vapors” are the main cause of rust and cause highly losses in the epidemic form in any country. Many scientists discuss the rust in unique way, Theophrastus reported that cereals crops were severely attacked the rust as compared to the legumes. First of all, the person who named Felice Fontana in 1767 demonstrated that rust diseases of the wheat caused by the fungi. But up to 19th century, there was not clear difference between the rust and smut diseases of the wheat crop. In the first report, it was not clear differences between leaf and stem rust of the wheat. De Candolle (1815) was the first person who described that wheat leaf rust was due a fungal pathogen and given name as Uredo rubigo-vera. Anton De Berry in the 1860s described the process of heteroecism of stem rust of wheat (Puccinia graminis) on different cereals and barberry crops as well. In some publication, it was given the concept to demonstrate that “blast” and “mildew” are the rust type fungal pathogens which may have the potential to affect the various cereals crops. The focus was directed to identification of wheat rust and their proper and integrated management in the 1880s, because both of these have basic science to control the highly losses, led Large (1940) to observe that “the greatest single undertaking in the history of plant pathology was to be the attack on rust in cereals”.
Rust status in Pakistan
In Pakistan, many epidemics reports of stripe (Puccinia striiformis) and leaf (Puccinia recondita) have been reported in 1980s and 1970s respectively. Depending upon the susceptibility level of wheat cultivar, 10 to 100% yield losses have been recorded. During annual survey of wheat rust, stripe rust firstly appears in the farmer fields of central Punjab (Faisalabad) in mid-February which spreads to other areas of the Punjab on the highly susceptible cultivar due to moving of urediospores by the wind and air in the mid- March.
Comparison of leaf rust of wheat
Brown or Leaf rust (Puccinia recondita) of wheat is most deleterious as compared to other stripe and stem rust under the natural conditions. Leaf rust first appear on the leaf blade, but under the presence of high amount of inoculum concentration, suitable environment and susceptible cultivar leaf sheaths also affected. Puccinia recondita is demonstrated by the uredinial stage producing urediospores which are round to ovoid, erumpent and having the 1.5 mm diameter with orange to brown shade that are freckled on upper and lower surface of the leaves of the wheat crop. Uredinia produced urediniospores having 20 microns in diameter orange to brown in color, sub-globoid, with the eight spores which are freckled with the wall of urediospores.
Suitable environmental condition for leaf rust
It is demonstrated that brown rust (Puccinia recondita) cause a significant hazard to wheat production in the large area of the wheat production under the natural filed condition. Rust fungi are the obligate parasite and need a living host for their proper growth and completion the life cycle. Existence in the off season, the rust in the form of teliospores survive on the barberry plants or either voluntary plants (self-own) become dormant and appear in the crop season and cause highly losses. High irrigation and incidence of the cool and rainy condition in the February – March month promote the inoculum to spread and cause severe diseases losses. Furthermore, suitable temperature, high level of humidity, aggressive pathogen and susceptible cultivars are main component of the diseases spreading.
But selection of resistant cultivars is the most effective method to control the rust disease of the wheat. But pathogens have the ability to change their race and effects the crops. Therefore, horizontal resistant should be introduced for the proper management of rust diseases.
Biological management of leaf rust of wheat
The bacterial strain, Pseudomonas putida have the capability to produce several types of antibiotics, siderophores and slight quantity of hydrogen cyanide (HCN), which suppress the Puccinia recondita growth in vitro and vivo. Some strains of the bacteria produce only hydrogen cyanide (HCN), which suppress the symptoms produced by the Puccinia recondita in the field as well as in lab trials. Different bio-control agents were evaluated against leaf rust of wheat with their combined effect and lonely. The bio agents like V. lecanii, B. bassiana, P. fumosoroseus, M. anisopliae and C. cladosporiodes were used against Puccinia recondita in the field and lab conditions. The results showed that B. bassiana and V. lecanii while P. fumosoroseus and V. lecanii showed the best results against the suppression of pustule size.
This article is collectively authored by Ghulam Muhae-Ud-Din*, Muhammad Amjad Ali, Usman Arshad, Amjad Abbas, Abdul Jabbar and Muhammad Jabran Department of Plant Pathology, University of Agriculture, Faisalabad-Pakistan.