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Downy mildew of maize
• Characteristic symptom on maize is the development of long, rather broad, chlorotic stripes along
most of the entire length of the leaf.
• The yellowish white colour of the stripes persists for more than a month or at least for three weeks
before becoming darker and only in the very late stages browning of the tissues is noticed.
• Puckering of young leaves bearing the chlorotic areas is common.
• Downy growth of the fungus can be seen on both surfaces of the leaf which is more prominent in
young leaves than on older leaves.
• Plants exhibit a stunted and bushy appearance due to shortening of the internodes.
• Downy growth also occurs on bracts of green unopened male flowers in the tassel.
• Small to large leaves are noticed in the tassel. Proliferation of auxiliary buds on the stalk of tassel
and the cobs is common (Crazy top).
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• Sorghum DM - Peronosclerospora sorghi / P. philippinensis
• Crazy top DM – Sclerophthora macrospora
• Brown stripe DM – Sclerophthora rayssiae var zeae
• Sugarcane DM – Peronosclerospora sacchari.
• Downy growth on both surface of the leaves, consisting
of sporangiophores and sporangia.
• Sporangiophores are quite short and stout, branch profusely into series of
pointed sterigmata which bear a single Sporangium.
• Sporangia are thin walled, hyaline, elliptical or ovate oblong with rounded
apex.
• Oospore on maize leaves are typically round, thin walled.
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Disease cycle
• The primary source of infection is through oospores in soil and also
dormant mycelium present in the infected maize seeds.
• Oospores are reported to survive in nature for up to 10 years.
• Alternate hosts also plays an important role in survival of pathogen. Kans
grass, (Saccharum spontaneum), Sorghum bicolor, S. halepens.
• Secondary spread is through airborne conidia.
• 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.
• Once the fungus has colonised host tissue, sporangiophores (conidiophores)
emerge from stomata and produce sporangia (conidia) which are disseminated by
wind and rain splash and initiate secondary infections.
• Sporangia are always produced in the night.
• They are fragile and can not be disseminated more than a few hundred meters 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.
• Conidia are produced profusely during the growing season. As the crop approaches
senescence, oospores are produced in large numbers.
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Management
• Deep ploughing.
• Crop rotation with pulses.
• Rogue out infected plants.
• Treat the seeds with metalaxyl at 6g/kg.
• Spray the crop with Metalaxyl + Mancozeb @
1kg on 20th day after sowing.
• Grow resistant varieties and hybrids viz. CO1,
COH1and COH2.
Brown spot of maize
• Early symptoms are water soaked lesions which are light
green at first but reddish brown and finally brown.
• Initially the spots are oval in shape which later on coalesce
to form brown blotches.
• These spot and blotches are more concentrated at the basal
portion of the leaf blade.
• Such symptoms also occur on leaf sheath and stems.
• When the stem is infected the tissues are weakened and the
stem breaks at the infected point causing severe damage.
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• Pysoderma zea maydis
• Hyphae – coenocytic
• Sporangia – oval to circular, smooth and brown in
colour.
• On germination, the sporangia open through a lid,
liberating unicelate hyaline thin walled zoospores.
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Disease cycle
• This fungus produces survival structures (sporangia) that
overwinter in corn residue and soil.
• During the next season the sporangia spread as airborne
spores, releasing zoospores when sufficient moisture is
present.
• The Zoospore becomes attached to the young leaves and
germinate to produce infection hyphae which later enter the
host tissue to cause the characteristic spots.
• High temperature and abundant moisture during the early
growth period of the host favour the disease development.
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Management
• Field sanitation practices reduces the inoculum
potential.
• Promoting residue decomposition through tillage
or other methods and rotating away from corn
will reduce the amount of the fungus available to
infect future corn plantings
• Foliar fungicides that list Physoderma brown spot
as a target disease on their labels are available,
but efficacy data are currently limited.
