Source: http://vasat.icrisat.org/?q=node/338

Distribution of Downy Mildew

Downy mildew of pearl millet, some times referred to as ‘green ear’ is the most destructive disease of pearl millet. This disease is widely distributed in temperate and tropical areas of the world, and is especially important in India.

               Distribution of Downy Mildew

In India, the disease is present in all the states where pearl millet is cultivated. In India, downy mildew epidemics caused substantial yield losses during 1970s and 1980s. Grain yield losses of 10 to 60% have been reported.

Economic importance of Downy Mildew

The yield reducing potential of downy mildew is very high, and this was dramatically recorded in HB 3, a popular hybrid, when pearl millet grain production in India was reduced from 8 million tons in 1970-71 to 5.3 million tons in 1971-72. This reduction was due to a downy mildew epidemic, in which yields in some fields were reduced by 60 to 70%.

Symptoms of Downy Mildew

There is considerable variation in the symptoms, which almost always develop as a result of systemic infection.Systemic symptoms generally appear on the second leaf. Once these symptoms appear, all the subsequent leaves and panicles also develop symptoms. The disease can appear on the first leaf also under conditions for severe disease development .

Leaf symptoms begin as chlorosis (yellowing) at the base of the leaf lamina, and successively top leaves show greater leaf coverage by symptoms.

Under conditions of high humidity and moderate temperature, the infection could be very severe. Severely infected plants are generally stunted and do not produce earheads.

When earhead is infected, the floral parts are transformed into leaf like structures, which can be total or partial, hence the name 'green ear' .

    

Disease-free and malformed earheads (green ears). Note the different types of malformation that  occur.

Management of Downy Mildew

Management practices should aim at reducing the movement of primary soil and seed-bearing fungal inoculum, and the secondary spread of the fungus within and between pearl millet fields.

This can be achieved by the possible combination of the 3 disease management practices:

• Cultural,
• chemical, and
• host-plant resistance.

A. Cultural Methods

All cultural methods are aimed at manipulating the environment to the advantage of the host and disadvantage of the disease causing fungus. Five such methods have been suggested to manage the pearl millet downy mildew.

Sanitation

Use disease-free seed and effective removal of disease infested plant material in the field after harvest of the crop are essential to reduce the primary inoculum in the soil.Downy mildew-infected plant material should be burnt, or if feasible the field should be plowed deeply to bury infected the plant material.

Early Sowing

A pearl millet crop sown very early in the season generally has less downy mildew than that sown late in the season. This can be practiced only if sowing is possible with sufficient rains during very early in the season.

Transplanting of pearl millet

A transplanted crop of pearl millet suffers significantly less from downy mildew than a direct sown crop, both in the rainy and post-rainy seasons.Hence, this method can be followed to reduce the downy mildew problem where irrigation facility available.

Roguing

Removal and destruction of infected plants reduces the spread of disease during the same season and also reduces the disease buildup of epidemics during the following seasons. To practice this method farmers should be able to detect infected plants at an early stage. They would have to be convinced that the return from reducing the disease would be worth the extra effort.

Diversification of Cultivars

Growing one hybrid for several years over a large area should be avoided. Instead, if hybrids are to be grown, several of them should be cultivated at the same time within a given area. Growing open pollinated varieties provide another opportunity to keep the disease under control.

B. Chemical methods

The systemic fungicide Metalaxyl was used successfully to control downy mildew in pearl millet. Seed treatment with Metalaxyl 35% at 6 g. Per kg seed controlled the disease excellently for about the first 35 days after sowing.

Foliar application of the Ridouril HZ 72 at 3g  per liter arrests further development of the disease in systemically infected plants. If sprayed before floral initiation, disease-free heads are produced.

Management of Downy Mildew

Plots grown without and with seed treatment with Metalaxyl. The untreated hardly produced any heads.

C. Host –plant Resistance

Use of resistant cultivars is the most cost-effective method for the control of downy mildew. Four open pollinated varieties, WC-C75, ICMS 7703, ICTP 8203, and ICMV 155 released in India are resistant to downy mildew.