Maize is attacked by different downy mildew causing fungal pathogens belonging to genus Peronosclerospora (Sclerospora) throughout the world. P. philippinensis (S. philippinensis) and P. maydis (S. maydis) cause disease on maize, P. sorghi (S. sorghi) on maize as well as jowar (Sorghum; Sorghum vulgare), and P. sacchari (S. sacchari) on maize as well as sugarcane (Saccharum officinarum). All these downy mildew diseases occur in our country and, of these, the most common and dangerous are those caused by P. sorghi, P. philippinensis, and P. sacchari.
Downy mildew disease of maize caused by P. sacchari has been reported causing widespread incidence in the tarai area of Uttar Pradesh and areas adjacent to Delhi. However, a downy mildew disease called ‘brown stripe downy mildew’ on maize has been reported by Payak and Renfro in 1967 occurring in many parts of India and caused by Sclerophthora rayssiae var. zeae.
The major damage done by the downy mildew diseases is either a complete loss of the plant during its early stage of growth or varying degrees of grain loss if cobs are formed. If normal looking cobs do develop they rarely reach maturity or bear mature grains.
Symptoms of Downy Mildew Disease:
The symptoms produced by downy mildew pathogens are almost similar on maize leaves. The most characteristic symptoms that develop are the long, rather broad, chlorotic stripes along almost the entire length of the leaf. Sometimes, the stripes developed as a result of secondary infections are short and narrow in the beginning and coalesce later resulting in the formation of irregular patches and loss of margins. Stripes are yellowing white in the beginning, continue as such, and turn darker quite late.
Although leaf shredding in observed in very severely affected leaves in the late stages but is not common. Symptoms are very common on upper leaves in comparison to lower ones due to rapid secondary infections. The lower mature leaves may also show considerable quantity of symptoms only if the plants are systemically infected. However, the pathogen can be observed on both surfaces of the leaf in the form of white hazy downy growth. The symptoms are also seen on bracts of green unopened male flowers in the tassel.
The chlorotic stripes on leaves affected by P. philippinensis become more incense in comparison to those developed by P. sacchari and P. sorghi because of the downy growth of the pathogen followed by yellow discolouration, browning and necrosis of cells, and stunting of the plant.
Causal Organism of Downy Mildew Disease:
Massive and wedge-shaped, short, stout conidiophores emerge through stomatal openings single or in usually 2 (rarely 3). They widen gradually upwards and dichotomously branch near apex 2 to 3 times (sometimes 4). Their growth is determinate and the length varies from 132 to 261 µm (with average of 184.8 to 187.4 µm) in case of P. sacchari and P. sorghi, while from 140-420 µm in case of P. philippinensis.
The branches are stout and sometimes swollen in the middle. The conidiophores produce thin-walled, hyaline, ellipsoidal, cylindrical or oblong-ovate conidia each on the fine tip of a long sterigma. The conidia measure 24 – 46.6 x 12 – 20 (mostly 26 – 36 x 12 – 18) µm in size. Thin-walled, rounded, reddish-brown oospores of average 25 pm diameter are produced on the leaves of the host. Both conidia and oospores germinate by germ tube, which are slender, aseptate, and 3.8 µm in width.
Downy Mildew Disease Cycle in Maize:
(i) Perennation:
The pathogen mostly perennates through oospores present on maize crop debris and on various alternate hosts. P. sacchari produces oospores only on sugarcane and the same alternate between sugarcane and maize. There are many collateral hosts (e.g., Saccharum spontaneum commonly called ‘Kass grass’, Sorghum bicolor, and S. halepense) on which the pathogens survive.
Many plant pathologists have pointed the possibility of seed-borne nature of downy mildew pathogens of maize. P. maydis has been reported internally seed-borne. The mycelium of P. philippeninsis has been traced up to the endosperm of the maize seed. Chang (1970) from Tiawan and Samengoen (1970) from Indonesia reported the presence of mycelium of P. sacchari and P. maydis in maize seeds. Presence of mycelium in the embryo of seeds from diseased plants has been found in Punjab.
Safeeulla (1976) and M. Rao and co-workers (1984, 85) have reported that P. sorghi is internally seed-borne in South India. Besides mycelium, however, 80% embryo of seeds from diseased plants has been found to contain oospores. Presence of oospores in the seed appears to be an important aspect of seed-borne nature of the downy mildew pathogen of maize. Normally, when seeds are dried to a moisture content necessary for storage the mycelium is inactivated but oospores survive.
(ii) Primary Infection:
Primary infection takes place during the growing season by means of the perennating structures. Oospores germinate by germ tubes that enter inside the host and, as a result, the mycelia invade and establish. Conidia produced by the pathogen on collateral hosts fall on the host leaves after dissemination, and germinate by germ tube. The germ tubes are hyaline, slender, aseptate and 3.8 µm wide. They develop from one or both ends of the conidium or from its lateral side.
(iii) Secondary Infection:
Secondary infections during the growing season take place by the conidia produced as a result of primary infection. The conidia are produced abundantly under conditions of high humidity and moderate temperatures. They are disseminated mainly by air currents, fall on leaves and germinate thereupon. Conidia also infect stigma and style and their germ tubes invade style and ovary wall, and may reach upto embryo.
Secondary infections may take place even after pollination and withering of stigma. One or usually more secondary infection cycles take place during the same growing season resulting in huge amount of secondary inoculum (conidia) formation. Oospores are produced late in the season within reddish-brown and irregularly elliptical oogonia.
Predisposing Factors of Downy Mildew Disease:
Since high humidity and moderate temperature conditions are required for conidia production, the latter are produced mostly during night. The optimum temperature for both production and germination of conidia is about 25°C; they are not produced at or below 13°C and at or above 31°C.
Free water on leaf surface is essential for formation and germination of conidia. However, conidial germination and growth of germ tubes in early morning hours is so rapid due to heavy dew that the host surface is normally fully covered with light weft of germ tubes.
Besides this, the age of plant and the availability of some minerals also influence disease incidence. Fewer conidia are produced on mature leaves; the deficiency of zinc and other micronutrients make the plants more susceptible to pathogen attack. It has been reported that the crops planted in July have very high disease incidence in comparison to May and June planted ones in terai belt of Uttar Pradesh. In Punjab, the maximum infection takes place in July-August planted crops.
Management of Downy Mildew Disease:
Use of cultural practices, fungicides, and resistant varieties in an integrated manner are recommended to manage the downy mildew disease of maize.
1. Preventive cultural practices such as destruction of collateral hosts and diseased crop debris and the long rotation of crops are most important.
2. Four to six sprays, starting from 10 days after sowing at an interval of a week, of 0.3% Dithane M-45 have been found effective.
3. Sowing of Demosan treated seeds followed by rouging of diseased young plants 20, days after sowing and one foliar spray of margosa (neem) oil controls the disease most effectively. 100% disease suppression has been claimed by treating seeds with Ridomil 25 WP (metalaxyl) at the rate of 4 g/kg seed. It has been found that seed-treatment by Apron 35 WS proves better than Ridomil if compared at the level of dose of the fungicide required to achieve same level of control.
4. Although none of the commonly cultivated varieties of maize are resistant, the variety Phil. DMR1 proves resistant to the pathogen.
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