Brown stripe downy mildew disease affects maize crop severely in many parts of our country. Payak and Renfro (1967) have found that the disease is different from the downy mildew of maize as it is caused by different downy mildew pathogen, and named it ‘brown stripe downy mildew’ disease of maize.
Symptoms of Brown Stripe Downy Mildew Disease:
The stripes caused by the disease on maize leaves are shorter and reddish to purple in the beginning, and turn brown after sometime. They are narrow (3-7 µm wide) and chlorotic with well-defined margins, which extend in parallel fashion between the veins. Necrosis of affected tissues is a prominent feature of the disease. If the development of the chlorotic stripes occurs before flowering, the seeds do not develop and the affected plant dies prematurely.
Although two different types of downy mildews do not commonly affect the same plant population in the same field, brown stripe downy mildew symptoms caused by Sclerophthora rayssiae var. zeae has been reported on the lower leaves of a number of plant of which the upper leaves show the symptoms of Peronosclerospora sacchari, one of the pathogens of downy mildew of maize. It is thought, however, that the symptoms caused by S. rayssiae var. zeae are possibly the result of primary infection, whereas those of P. sacchari are possibly the result of secondary infection.
Causal Organism of Brown Stripe Downy Mildew Disease:
The pathogen is a biotroph. Its hyphae are aseptate characteristic to the class, congregate in the sub-stomatal cavity, and give rise to determinate, un-branched and short sporangiophores that emerge out through stomatal openings. Each sporangiophore produces sporangia sympodially in groups of 2-6. The sporangia are hyaline, ovate, obclavate, elliptical or cylindrical, smooth, having a truncate or rounded apex, and measure 29 – 66.5 x 18.5 x 26 µm in size.
Each sporangium germinates producing 4-8 biflagellate zoospores. The sex-organs are formed in the intercellular spaces. Antheridia are paragynous, whereas the oogonia are subglobose, thin-walled, and hyaline to light straw-coloured. Each oogonium is attached with one to two antheridia. Spherical or sub-spherical oospores, each possessing a prominent oil globule, are centrally located in the oogonia. The oospore wall is confluent with the wall of oogonium.
Brown Stripe Downy Mildew Disease Cycle in Maize:
(i) Perennation:
The pathogen perennates through oospores and collateral host, and their seed-borne nature of survival is also emphasized. The oospores come in soil through infected crop debris and have ability to survive therein even for more than three years.
Some plant pathologists have emphasized the role of Digitaria sanguinalis, a collateral host of the pathogen, in perennation. Presence of S. rayssiae var. zeae in the form of mycelium within the embryo of seeds from diseased plant has also been reported.
(ii) Primary and Secondary Infections:
The perennating structures of the pathogen germinate and establish the primary infection during the growing season of the host. Oospores germinate by germ tubes and form sporangia and the primary infection appears in the form of minute flecks on the leaves which grow to form brown streaks. Sporangia produced on D. sanguinalis also cause primary infection. Secondary infection is brought about by sporangia produced by primary infection streaks. Sporangia formation is considered to be maximum between 12 a.m. and 4 a.m. and they germinate producing zoospores.
Predisposing Factors:
The optimum temperature for production of sporangia of S. rayssiae var. zeae is 22°-25°C and for zoospore formation 20-22°C.
Management of Brown Stripe Downy Mildew Disease:
1. Destruction of infected plants at the earliest reduces the incidence of disease on the crop.
2. Spraying the plants with zinc sulphate (0.5%) and lime reduces the disease considerably.
3. 4 to 6 sprays of Dithane M-45 (0.3%) or Dithane Z-70 at 7-days intervals after 10 days of sowing is highly effective.
4. Ridomil 25 WP (metalaxyl) treated seeds at the rate of 4 g/kg bring 100% control of the disease.
5. Although no resistant varieties to this disease have been developed in our country so far, sowing of resistant varieties is considered the device to control the disease.
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