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All above-ground parts of the plant are susceptible, particularly young, actively growing, or meristematic tissues. Smut galls may form on leaves, stalks, ears, or tassels of infected plants.
Galls are semi-fleshy, initially firm and light in colour, then turn black. Galls are initially covered with a glistening, greenish-white to silvery-white tissue. The interior of these galls darkens and turns into masses of powdery, dark olive-brown to black spores, except for galls on the leaves. When the membrane ruptures, dry, brown powdery spores are released.
Mature galls may reach 15 cm in diameter. Galls on leaves usually remain small (0.6-1.2 cm diameter), become hard and dry and do not rupture. Severely infected plants may be stunted. Plants with galls on the lower stalks may be barren or produce several small ears; such maize plants appear reddish during the autumn. Early infection may kill young plants, but this is rare.
In the course of 3 years' observation of 25 hybrids, the tendency for symptoms to appear only in the stem, leaves, tassels or ears was slight in most of the hybrids. In those in which it was strongest, the tassel was the most preferred organ and the stem the next. A greater or lesser tendency for the disease to affect the ear among other organs was also identifiable in some varieties (Dracea, 1979).
Cultural Control and Sanitary Methods
Removing maize plants infected with head smut (before the smuts rupture) is a suggested control practice in China (Zhu et al., 1981) and Taiwan (Tseng, 1988). In former Yugoslavia, more common smut occurred in non-irrigated than irrigated maize (Osvald and Osvald, 1991). In Germany, the average incidence of U. maydis was 50% higher in maize undersown with legumes (Lathyrus, lupins, faba beans and soyabeans) (Hegewald, 1984). Higher plant densities have been associated with increased U. maydis infection (Kostandi and Soliman, 1991). Incidence of common smut has been seen to rise with increasing nitrogenous fertilization (Hegewald, 1984; Cross, 1991; Kostandi and Soliman, 1991).
In Germany, 2-year field tests showed that disease incidence was significantly increased by effective weed control with different herbicides. Eliminating weeds facilitated the passage of the fungus from the soil to the seedling leaves. Ustilospore germination and basidiosporel budding were not affected by herbicides at field concentrations; herbicides stimulated the disease in inoculation tests and increased the percentage of severely diseased seedlings, probably due to a temporary retardation in growth. Reduced competition by weeds and an improved water supply increased disease incidence after inoculation (Pfister and Grossmann, 1979).
It has long been known that maize lines vary in susceptibility to U. maydis. Resistance is regarded as the most feasible and practical control method (Christensen, 1963; Ainsworth, 1965). Work continues on producing lines that are resistant to the pathogen (Cross, 1991; Bogachev, 1994). Little is known about the inheritance of resistance to common smut. Christensen (1963) stated that smut resistance appears to be associated with functional, physiological and morphological characters of maize. No major race-specific loci conferring resistance to U.maydis have been discovered. Studies using quantitative trait mapping (for example, Lubberstedt et al., 1998; Kerns et al., 1999) indicate that resistance to U. maydis involves many genes, each contributing a relatively small amount to overall resistance. Tests for resistance have shown that maize ears are most susceptible to U. maydis, and that young reproductive and vegetative organs are more susceptible than older ones. Testing for resistance was performed on the basis of the response of the ears to infection. Combinations of the greatest number of favourable characters in breeding material correlated with resistance promoted long-term maintenance of the resistance trait in new hybrids (Fed'ko et al., 1990).
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:
Maize losses ranging from 5 to 10% were frequently reported in the USA in the 1920s (Christensen, 1963). Recent losses are usually less than 1%, although this may be higher in sweetcorn (McGee, 1988). In a survey of 12 Minnesota counties in 1977, plant infection averaged 3.3 to 16.6% (Kommedahl and Windels, 1977). U. maydis is a minor disease in Australia (Jones, 1982), the UK (Cook, 1977) and India (Handoo and Aulakh, 1982). A crop-loss study in Germany spanning 1982-84 showed that relative yield was highest (99%) for resistant hybrids and lowest (82%) for susceptible hybrids. Galls were graded in severity on a scale of 1 (healthy plants) to 9 (dead plants). Small galls (grades 2 and 3) did not appreciably reduce yield, but grades 4-8 caused losses of 11-95%. Yield losses associated with smut galls on ear, stem + ear, stem + tassel, stem and tassel were 57.1, 48.9, 38.6, 24.3 and 9.3%, respectively (Kostandi and Geisler, 1989)
Fruiting bodies of U. maydis are a seasonal delicacy in Mexico, where they are known as 'huitlacoche'. In the USA, the susceptible maize cultivar Silver Queen is cultivated in the Pennsylvania region; the immature galls on the cobs are harvested when they are still silvery and fresh, approximately 5-7 days before standard harvesting. However, because of uneven maturation of Silver Queen plants, premium smutted ears still appear at harvest for the fresh market. Galls are removed from these cobs, vacuum-sealed and blast-frozen for marketing; the shelf life when frozen is 6 months to 1 year. The texture is not as crunchy in frozen galls as in fresh galls but the flavour is not affected. The galls deteriorate rapidly through dehydration and collapse under their own weight when removed from the cob (Arnold, 1992).