Peronosclerospora sorghi (Sorghum downy mildew)
P. maydis (Java downy mildew)
P. philippinensis (Philippine downy mildew)
P. sacchari (Sugarcane downy mildew)
Scleropthora rayssiae var. zeae (Brown stripe downy mildew)
Sclerospora graminicola (Graminicola downy mildew or green ear)
Sclerophthora macrospora (crazy top)

Downy mildews are important maize diseases in many tropical regions of the world.  They are particularly destructive in many regions of tropical Asia where losses in excess of 70% have been documented. Downy mildews are caused by up to ten different species of oomycete fungi in the genera Peronosclerospora, Scleropthora and Sclerospora. Downy mildews originated in the Old World although they have since been introduced to many regions of the New World.

Symptoms

Symptoms of downy mildew on maize caused by the various pathogenic species are similar, although symptoms can vary depending on plant age, prevailing climatic conditions, and host germplasm.  Infection of maize plants at the seedling stage (less than 4 weeks old) results in stunted and chlorotic plants and premature plant death.

Leaves on older plants display characteristic symptoms of downy mildews which include mottling, chlorotic streaking and lesions, and white striped leaves that eventually shred.  ‘Downy’ growth is often observed on both leaf surfaces, but is more common on the lower leaf surface.  Infected plants have leaves that are narrower and more erect compared to healthy leaves. Infected plants are often stunted, tiller excessively and have malformed reproductive organs (tassels and ears).  Infected plants may not seed, while tassels may exhibit ‘bushy’ growth.

Confirmation

The causal agent of downy mildew can be determined by detailed examination of spore-bearing structures (conidiophores or sporangiophores) and spores (conidia or sporangia).  Geographic location can be used as a preliminary diagnostic. White stripe symptoms caused by downy mildew are not limited by veins and can therefore be distinguished from signs of iron deficiency. 

Details of species morphology, geographic distribution, alternate hosts and yield losses are shown in Tables 1 and 2.

Table 1. Morphological characteristics of various downy mildew pathogens of maize.

Table 2. Geographic distribution, alternate hosts, and yield losses of various downy mildews of maize.

Life cycle

Depending on the pathogen species (Table 3), the initial source of disease inoculum can be oospores that overwinter in the soil or conidia produced in infected, overwintering crop debris and infected neighboring plants.  Some species that cause downy mildew can also be seedborne, although this is largely restricted to seed that is fresh and has a high moisture content (Table 3).

At the onset of the growing season, at soil temperatures above 20°C, oospores in the soil germinate in response to root exudates from susceptible maize seedlings. The germ tube infects the underground sections of maize plants leading to characteristic symptoms of systemic infection including extensive chlorosis and stunted growth.  When oospores initiate infection, the first leaf generally remains disease free as it is able to outgrow the fungi. However, the whole plant will show disease symptoms if the pathogen was seedborne.  Oospores are reported to survive in nature for up to 10 years.

Once the fungi has colonised host tissue, sporangiophores (conidiophores) emerge from stomata and produce sporangia (conidia) which are wind and rain splash disseminated and initiate secondary infections. Depending on the species, sporangia germinate directly or release zoospores that initiate infection (Table 3).  Sporangia are always produced in the night.  They are fragile and can not be disseminated more than a few hundred metres and do not remain viable for more than a few hours.

Germination of sporangia is dependent on the availability of free water on the leaf surface. If sufficient water is available, sporangia germinate and infect the plant through stomata on the leaf, sheaths, or stems in a couple of hours. Initial symptoms of disease (chlorotic specks and streaks that elongate parallel to veins) occur in 3 days.  Conidia are produced profusely during the growing season. As the crop approaches senescence, oospores are produced in large numbers. 

Table 3. Infection characteristics of various causal pathoges of downy mildew in maize.

Factors favoring disease

Downy mildews are very significant maize diseases in tropical regions of Africa and Asia, where prolonged periods of leaf wetness and cultivation of alternate hosts are prevalent during the growing season.  Cool, wet and humid conditions are optimal for disease development.  In favorable conditions, disease cycles are rapid, leading to severe infection and spread of disease.

Perenation of infected crop debris serves as a source of inoculum in subsequent seasons. Cultivation of alternate hosts in rotation or simultaneously will build pathogen pressure. Moist soils favor oospore germination and therefore damp soil as a result of irrigation or reduced tillage techniques will encourage disease development.

Damage

• Mechanism of damage:Systemic infection of young seedlings leads to stunted growth, chlorosis and premature death, limiting yields. Under severe infection, tassels are malformed and ears are barren, leading to extensive yield loss.
• When damage is important: Damage is critical when systemic infection of young seedlings occurs, which can result in premature death of the plant and barren ears.

Management principles

• Use resistant varieties.
• Do not rotate or simultaneously cultivate maize with alternate hosts of downy mildew.
• Remove infected crop debris if possible.
• Treat seed or crop with systemic fungicide.
• Plant when soil temp is below 20°C which is unfavorable for oospore germination.
• Ensure seed has low moisture content (below 9%) before planting.
• Control weeds to increase aeration within the crop and reduce moisture levels in the soil.
• Reduce crop density to increase aeration.

References

Adenle, V.O. and K.F. Cardwell. 2000. Seed transmission of Peronosclerospora sorghi, causal agent of maize downy mildew in Nigeria. Plant Pathology 49:628-35.

Boude, M.R. 1982. Epidemiology of downy mildew disease of maize, sorghum and pearl millet. Tropical Pest Management 28:220-23.

Frederiksen, R.A. and B.L. Renfro. 1977. Global status of maize downy mildew. Annual Review of Phytopathology 15:249-275.

Jeger, M.J., E. Gilijamse., C.H. Bock and H.D. Frinking. 1998. The epidemiology, variability and control of the downy mildews of pearl millet and sorghum, with particular reference to Africa. Plant Pathology 47: 544-69.

Smith, D.R. and B.L. Renfro. 1999. Downy Mildews in Compendium of Corn Diseases. Edited by Donald G. White. St. Paul: The American Phytopathology Society. Pp25-32.

Thakur, R.P. and K. Mathur. 2002. Downy mildews of India. Crop Protection 21:333-45.

Contributors: Biswanath Das