BLACK STEM RUST OF WHEAT Stem rust is

BLACK STEM RUST OF WHEAT Stem rust is caused by Puccinia graminis f. sp. tritici, a heteroecious, long cycle rust fungus with many pathogenic races. The fungus has five distinct stages: the pycnial and aecial stages occur on common barberry (the alternate host), and the uredial (orange or summer) and telial (black or winter) stages occur on wheat. Basidia arise from teliospores and basidiospores infect barberry. Urediospores are orange-red, spiny and oblong with four median germ pores. The brown-black, twocelled teliospores have a terminal cell that is pointed at the top. Puccinia graminis tritici: Principal Host: Triticum aestivum Alternate host Berberis lycium and Berberis vulgaris.

• Symptoms ON WHEAT • Infection can occur on any above-ground plant part, leading to the production of pustules that contain thousands of dry yellow-orange to reddish-brown or black spores. These pustules give the appearance of “rust” on the plant. • Stem rust occurs primarily on stems but can also be found on leaves, sheaths, glumes, awns and even seed. • Symptoms begin as oval to elongate lesions ( Uredial pustules), up to 3 x 10 millimeters that are generally reddish-brown in color. In the late stages of the disease, erumpent pustules produce numerous black sooty spores. Severe infestations with many stem lesions may weaken plant stems and result in lodging. • As the disease pro gresses, tissues around the pustules turn brown and dry resulting in a scorched appearance. • The arrangement of pustules into stripes is an important distinguishing characteristic of this disease. Chlorosis, or yellowing, of leaves can be quite evident with both leaf and stripe rust, and fields with plants displaying severe symptoms may be easily detectable from a distance. • Symptoms are produced on almost all aerial parts of the wheat plant but are most common on stem, leaf sheathes and upper and lower leaf surfaces. • Uredial pustules (or sori) are oval to spindle shaped and dark orange-red (rust) in color. They erupt through the epidermis of the host and are surrounded by tattered host tissue. • The pustules are dusty in appearance due to the vast number of spores produce. Spores are readily released when touched. • Teliospores are produced in the same pustule. The color of the pustule changed from rust color to black as teliospore production progresses.

ON BARBERRY • Symptoms are very different on the woody host. Pycnia (spermagonia) produced on the upper leaf surface appear as raised orange spots. Small amounts of honeydew that attracts insects are produced in this structure. • Aecia, produced on the lower leaf surface, are yellow. They are bellshaped and extend as far as 5 mm from the leaf surface.

Role of Uredospore’s from Plains: Uredospore’s are considered as the possible cause of infection. The infection of wheat rust (uredinal stage) appears in the month of January and February when the plants are one to two feet high. In plains wheat-crop is harvested in the month of March and April. After the harvesting there is the possibility that uredospore’s may survive in the soil to infect the wheat plants next year. But it has been proved that uredospore’s cannot remain viable at 95°F temperature. The temperature in the Indo-gangetic plains even in shady places is 115°F. Therefore, there arises no question of survival of uredospore’s in the summer season in plains and they have nothing to do with the recurrence of rust. Role of Barberry Plant (Alternate Host): It was also considered that the aecidiospores produced on Barbery plant may cause the annual recurrence of the rust disease, but it was observed that eradication of Barberry bushes does not help in controlling the annual recurrence of rust disease. Moreover, the barbery plants are usually found growing on high altitude of 3000 -8000 feet, several thousand kilometers away from the crops grown in the plains. So, there are mush less chances that alternate host play any role in the recurrence of rust disease. (In the hills the alternate host plays an important role in the continuation of the life cycle as both the hosts are found near each other. ) Role of Basidiospores: Basidiospores can infect wheat plants but the experiments show that the spores are totally incapable of infecting wheat and they grow only on Barberry leaves and flowers.

Control Measures of Rust Disease: (1) Cultivation of Resistant Varieties: It is one of the most effective and practical methods to control the rusts of wheat. Some varieties of wheat like N. P. 120, N. P. 52, N. P. 4, N. P. 165 Pb. C. 591 showed good tolerance to rusts Sonara 64 and Lerema Rojo are highly resistant to black rust. (2) Mixed Cropping: In this method the mixed crop of barley and wheat is grown in the field. This method gives a good crop insurance even if the main crop fails. (3) Eradication of Barberry Bushes: The eradication of barberry plants may control the disease by cutting down the life cycle of fungus. It is the most effective method in those countries (e. g. , U. S. A. ) where the pathogen completes its life cycle on alternate host (heteroecious). However, in India the source of primary infection (uredospore’s) lives in the hills and in plains it is brought through winds. Here this method is of no except one way that the absence of barberry bushes will exclude the chances of dikaryotization and new genetic varieties of the fungus. (4) Effect of Fertilizers: Higher dose of the nitrogenous fertilizers makes the crop more susceptible to rusts. The potassium has the opposite effect. Reduction of N in the proportion of NPK ratio helps in reducing the incidence of rust in a susceptible variety. (5) Rotation of Crops: This method is used on hills. In this method the cultivation of barley and wheat plant is replaced by oat. (6) Chemical Control: Spraying of sulphur (13 -6 kg. per acre) over young healthy wheat plants, checks the rust infection to a great extent. Four to five applications of Nabam and Zinc sulphate gave efficient control of wheat rusts. Chemicals such as Actidion, Parzate, dithane liquid plus zinc sulphate, plantavax and RH-124 have given quite encouraging results to control wheat rusts.