Sclerospora graminicola (downy mildew of pearl millet)

There is considerable variation in the symptoms, which almost always develop as a result of systemic infection. Symptoms vary according to host, time of expression and ambient conditions; under field conditions systemic infection has been observed as early as 6 days after sowing. Systemic symptoms generally appear on the second leaf, and once these appear, all the subsequent leaves and panicles also develop symptoms, except in cases of recovery resistance where plants outgrow the disease (Singh and King, 1988). The disease can also appear on the first leaf under conditions of severe disease development. Leaf symptoms begin as chlorosis at the base of the leaf lamina and successive new leaves show a progression of greater leaf coverage by symptoms. 'Half-leaf' symptoms, characterized by the diseased basal portion of the leaf and non-diseased areas towards the tip, occur in pearl millet genotypes. Under conditions of high humidity and moderate temperature, the infected chlorotic leaf area supports a massive amount of asexual sporulation, generally on the abaxial surface of the leaves, giving them a downy appearance. If symptoms start early, plants are severely stunted and chlorotic and may die; if symptoms are delayed, dwarfing may yet occur; some shoots may escape disease.Severely infected plants are generally stunted and do not produce panicles. The name 'green ear' stems from the appearance of green panicles due to transformation of floral parts into leafy structures, which can be total or partial. This is sometimes referred to as virescence (Arya and Sharma, 1962). These leafy structures can also be chlorotic, and sometimes support sporulation. In certain cases, green ear is the only manifestation of the disease. There is a report of artificially induced localized green-ear symptoms resulting from panicle inoculation under greenhouse conditions (Semisi and Ball, 1989). Symptoms are rarely seen as local lesions or isolated spots on leaf blades (Saccas, 1954; Girard, 1975). Spots vary in shape and size and are at first chlorotic and produce sporangia, and later become necrotic.

Seed Sanitation

Seed contamination with oospores is a primary source of over-wintering inoculum and is also important in dissemination of the pathogen from one region to another. To prevent the spread, seed treatment procedures have been developed.

Some observations indicate 6% loss in pearl millet due to S. graminicola in east China (Porter, 1926), 45% in Allahabad, India (Mitter and Tandon, 1930), and up to 27% loss from 1962 to 1964 in Rajasthan state, India (Mathur and Dalela, 1971). According to reports from Africa, there was 60% loss in Mozambique (De Carvalho, 1949), 10% loss in Nigeria (King and Webster, 1970), and 0-50% in other West African countries (Saccas, 1954; Selvaraj, 1979; CILSS, 1986, 1987).The yield-reducing potential of downy mildew is high, as dramatically demonstrated in HB3, a popular hybrid in India, when pearl millet grain production was reduced from 8 million tonnes in 1970-71 to 5.3 million tonnes in 1971-72 (AICMIP, 1971/72). This reduction was to a large extent due to a downy mildew epidemic, in which yields in some fields were reduced by 60-70%. Subsequent to this epidemic, grain yield losses continue to occur quite frequently due to downy mildew epidemics in India (Singh et al., 1987). It has been demonstrated that the losses in yield can be directly related to disease severity (Williams and Singh, 1981).In two field experiments with pearl millet cv. HB 3, plant height and test weight were found to be inversely related to infection level (Gupta and Singh, 1996a). Reduction in grain yield varied from 9 to 97% depending on disease incidence. At 62% disease incidence, the estimated yield loss was 34% over the healthy crop. The yield reduction was 57% when the yield of the unprotected crop was compared with that of the protected crop.Effect on mineral nutritionThe nutritional quality of pearl millet is important when it is used as green fodder and stover for animal feed. Studies on the mineral composition of healthy and S. graminicola-infected leaves of pearl millet cultivars revealed an increased accumulation of N, P, Zn, Na, K, Mg, Pb and Cu, and decreased levels of Cu, Fe, Ca, Mn and Ni in diseased leaves compared to healthy leaves (Gupta and Singh, 1999a; Yadav and Thakur, 2000).