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Plantwise Technical Factsheet

downy mildew (Sclerophthora macrospora)

Host plants / species affected
Avena sativa (oats)
Avena sterilis subsp. ludoviciana (Winter wild oat)
Bromus inermis (Awnless brome)
Dactyloctenium aegyptium (crowfoot grass)
Digitaria sanguinalis (large crabgrass)
Echinochloa crus-galli (barnyard grass)
Eleusine coracana (finger millet)
Eleusine indica (goose grass)
Elymus repens (quackgrass)
Eragrostis (lovegrasses)
Hordeum vulgare (barley)
Lolium temulentum (darnel)
Oryza sativa (rice)
Panicum capillare (tumble panicgrass)
Pennisetum glaucum (pearl millet)
Poa (meadow grass)
Saccharum officinarum (sugarcane)
Setaria viridis (green foxtail)
Sorghum bicolor (sorghum)
Stenotaphrum secundatum (buffalo grass)
Triticum (wheat)
Triticum aestivum (wheat)
Zea mays (maize)
List of symptoms/signs
Inflorescence  -  abnormal leaves (phyllody)
Inflorescence  -  discoloration panicle
Inflorescence  -  twisting and distortion
Leaves  -  abnormal colours
Leaves  -  abnormal forms
Roots  -  reduced root system
Seeds  -  rot
Stems  -  stunting or rosetting
Stems  -  witches broom
Whole plant  -  dwarfing
Symptoms
Maize

Symptoms vary greatly with time of infection and degree of host colonization by the fungus. Proliferation of the tassel is the most conspicuous symptom of crazy top. Tassel proliferation, which may be partial or complete, results from the replacement of normal floral parts by small leaves: this continues until the tassel resembles a mass of leafy structures called 'crazy top'. Phyllody may also occur in the ears.

As many as 12 well-developed ear shoots have been observed distributed along the stalks of affected plants. Excessive numbers of nodes above the lowest ear and in the ear shank is also a frequent symptom. Occasionally, ear shanks are branched.

Leaves of affected plants are only a third of the width of normal leaves. These strap-like leaves possess a leathery texture and the lower epidermis may be roughened. Longitudinal, translucent streaks of light-green and normal green are evident in the blade. Sheaths of strap-like leaves often demonstrate a warty surface.

Severely affected plants are frequently stunted. Such plants may attain a height of only around 30-40 cm, whereas adjacent healthy plants may reach 2.5-3 m (Ullstrup, 1970). Tillering is often associated with stunting, and as many as 12 tillers have been seen on a single plant. Roots of stunted and tillered plants are poorly developed and the plants can be easily pulled from the soil.

Barrenness is a common symptom of crazy top and well over 95% of affected plants produce no ears. Mildly affected plants may occasionally produce a normal ear, and in rare instances, severely affected plants will produce nubbin ears with a few kernels. Fresh sections of a few kernels from ears of affected plants showed the presence of mycelium in the coleorhiza and scutellum (Ullstrup, 1952).

Wheat

Symptoms on wheat include white chlorotic patches with sparse sporangial growth; the flag leaf is frequently crinkled with malformed ears (Parkash and Sharma, 1987).

Turfgrass

S. macrospora was shown to be the cause of yellowing and proliferation of shoots in several turfgrass species in the USA (Jackson, 1980).
Prevention and control
Cultural Control and Sanitary Methods

It is well established that there is a relationship between wet soil and disease development across a number of plant species: to avoid soil saturation, adequate drainage should be provided (Ullstrup, 1970). In rice, it has been shown that downy mildew disease was reduced after increased mechanization (Ohata, 1981).

Six different nitrogenous fertilizers, applied to a mixture of Chewings fescue [Festuca rubra] cv. Jamestown and velvet bent [Agrostis canina] cv. Kingstown, all reduced the intensity of yellow tuft disease (S. macrospora) symptoms. Water-soluble inorganic fertilizers (e.g. ammonium sulphate and ammonium nitrate) produced better turf quality than slower release sources of nitrogen. The fertilizers masked disease symptoms but did not reduce the number of diseased plants (Dernoeden and Jackson, 1980).

Host-Plant Resistance

Lines that are resistant to S. macrospora have been developed for several hosts. Considerable differences in susceptibility were recognized in 40 medium-season maize hybrids evaluated at Milan, Tennessee in 1979 (Kadian et al., 1988). Only one cultivar was highly resistant and seven were resistant in 27 oat lines exposed to natural infection by S. macrospora under conditions conducive to severe disease development (cool, wet weather) (Paruthi et al., 1992). Stenotaphrum secundatum cv. Gulf Star has shown resistance to Sclerophthora macrospora (Riordan, 1987).

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Impact
This disease is found in most maize-growing areas of the world with temperate to warm-temperate climates. It is widespread in the USA, but of no economic importance, although individual fields occasionally experience severe losses (Ullstrup and Sun, 1969; Ullstrup, 1970; Jons, 1980). Infection was reported on 75% of the plants in a field in Indiana in 1949 (Ullstrup, 1952). It has been reported in other countries but is considered to be far less important than some of the other downy mildews (Peronosclerospora philippinensis, P. sacchari, Sclerophthora rayssiae var. zeae) (Ullstrup, 1970). The situation is similar in wheat and other cereal grains, in that isolated fields may show significant incidence of the disease. In June 1985, Lloyd durum wheat and Azure barley plants showing stunting, leathery leaves, excessive tillering and deformed heads were observed in wet areas of two fields near Casselton, North Dakota (20-50% incidence). Similar symptoms were observed in wet areas in oat breeding nurseries in Fargo and Prosper; this is the first report of downy mildew on these cereals in North Dakota (Jons et al., 1986), despite the fact that the pathogen is widespread on grasses in the Dakotas region (Semeniuk and Mankin, 1964).

Yellow tuft caused by S. macrospora was observed in Kentucky bluegrass [Poa pratensis] sod fields in Illinois, USA in 1992. The sod was 18 months old and growing on soil used continuously for sod production during the previous 5 years. Tufts (5-15 cm diameter) of diseased turf were observed at three field locations that collectively represented about 3 ha; about 10% of that turf was affected. Excessive soil moisture for 4 weeks and daily air temperatures ranging from 15 to 26┬░C had preceded disease development (Wilkinson and Pedersen, 1993). In another study, internodal lengths were reduced in 10 of 12 cultivars and accessions of infected St Augustinegrass [Stenotaphrum secundatum] compared with healthy control plants. Lengths of leaves and width of leaves and internodes were also reduced in many of the diseased plants. Plant height and fresh shoot weight of infected cv. Scott 1081 increased an average of 60 and 48%, respectively, when the fungus was eradicated with metalaxyl (Grisham et al., 1985).

In rice, seedlings inoculated with S. macrospora were planted together with healthy ones. The relation between rice yield and incidence of diseased seedlings in the field showed a logarithmic curve (Katsube, 1973). In another report, the incidence of downy mildew was highest in rice seedlings sown directly in the field and lowest in those grown in a flooded nursery. A close negative correlation between grain yield and infected tillers in plants infected before maximum tillering was found (Katsube, 1977). A decrease in downy mildew of rice and an increase in other diseases has been associated with increased use of mechanization in production practices (Ohata, 1981).

A high incidence of downy mildew disease of broomcorn (Sorghum sp.) was associated with infection by S. macrospora rather than Peronosclerospora sorghi, which had been reported on sorghum in Iran in 1977 (Izadpanah and Afsharifar, 1990)
Related treatment support
Plantwise Factsheets for Farmers
Pakistan, Directorate General Agriculture (Ext. & A. R.); CABI, 2014, English language
Vietnam, Son La Plant Protection Sub-Department; CABI, Vietnamese language
Pakistan, Directorate General Agriculture (Ext. & A. R.); CABI, 2014, Urdu language
Raman, H.; CABI, 2013, English language
Raman, H.; CABI, 2013, Dari language
 
Pest Management Decision Guides
Iqbal, M.; Khan, Y. S.; CABI, 2013, English language
Ndungu, J.; Njoroge, C.; Otipa, M.; CABI, 2015, English language
Iqbal, M.; Khan, Y. S.; CABI, 2013, Urdu language
Asghar, M.; CABI, 2014, English language
 
External factsheets
CIMMYT Plant Pest and Disease Factsheets, Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT) (International Maize and Wheat Improvement Center), English language
CIMMYT Plant Pest and Disease Factsheets, Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT) (International Maize and Wheat Improvement Center), English language
PANNAR Seed Factsheets, PANNAR Seed (Pty) Ltd, 2009, English language
University of Illinois Extension Factsheets, University of Illinois, 1988, English language
PlantVillage disease guide, PlantVillage, English language
Video factsheets
Agropedia ICRISAT PPT-Videos, IIT, Kanpur, 2014, English language
Agropedia ICRISAT PPT-Videos, IIT, Kanpur, 2014, English language
Agropedia ICRISAT PPT-Videos, IIT, Kanpur, 2014, English language
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