Smut Diseases of Cereal Crops (DPI Vic)

September, 1995 Smut diseases of cereal crops AG0401 Denis Ballinger and Trevor Bretag, Horsham ISSN 1329-8062

Smut diseases are caused by fungi which produce large machine should be slow enough to ensure thorough masses of black spores in the plants they attack. These coverage of all seeds. spores germinate to produce the next generation of the An Agriculture Note listing the various registered seed fungus. dressings is available. Each of the cereal smuts is caused by a distinct species of fungus specific to a particular cereal. A smut which attacks wheat does not attack barley or oats. The main types of smuts which attack wheat, barley and oats are the head smuts - covered smut and loose smut.

Covered smuts replace the grain with a ball of spores Spores spread at harvest enclosed in a membrane, but the glumes or "chaff" are not affected. Loose smuts replace both the grain and the chaff Smut spores on in the affected head with a loose mass of spores which seed blows away in the wind, leaving behind only the bare Infected head backbone. Covered smuts of wheat and barley Covered smut in wheat is known as bunt, ball smut or stinking smut. Infected wheat heads remain green longer Shoot infected than normal, the glumes are opened wider than normal by the rounder smut balls and there is a distinctive putrid fishy odour. Figure 1. Infection cycle of the covered smuts Covered smut in barley is easier to see in the field than bunt in wheat because the smut ball is not covered by chaff. Even if the membrane ruptures, the sticky spores are not easily dispersed by wind. During harvest, the covered smut balls break open and the spores cover the surface of healthy grains. Harvest equipment is also contaminated, so grain from healthy crops is at risk if harvested after an infected crop. Infection inside seed Grain contaminated by covered smuts is not suitable for use by the normal markets. Smut inside seed When infected grain is sown, the spores germinate and Spores blow to penetrate the shoot from the germinating grain before it healthy flowers emerges from the soil. The fungus then grows inside the plant and replaces the developing grain with covered smut balls. Control Shoot infected from inside seed Seed should not be saved from infected crops. Figure 2. Infection cycle of the loose smuts All seed should be dressed with a fungicide registered for the control of covered smut. The flow of grain through the

surface. The alternative is to obtain seed from a crop Loose smuts of wheat and barley known to be free of loose smut. Loose smuts of wheat and barley are also known as flying or blow smuts. Oat smuts The loose mass of spores emerges at the same time as the Both loose smut and covered smut attack oats. These healthy crop ears. The spores blow away when the healthy diseases show similar field symptoms, especially if crop is flowering. Some of them settle in open healthy covered smut has weathered. flowers and if conditions are suitable the spore germinates Loose smut is much more common in Victoria than and infects the embryo of the developing seed. covered smut of oats. Loose smut spores infecting oats do The infected seed develops normally, but contains the not penetrate the embryo like the loose smuts of wheat and fungus inside the embryo. If an infected seed is sown, the barley. The spores remain in the outer parts of the grain fungus grows inside the plant and replaces the developing and infect the young seedling. head with a loose mass of spores. Some covered smut spores lodge under the husk of oat Grain which is infected by loose smut is not discoloured grains and therefore complete control requires the use of and is acceptable for normal uses. Dockages can be caused more active fungicides than some of those which control by loose smut when part of the spore mass remains in the covered smuts in wheat and barley. flag leaf. These spores remain as a dried lump and may Control enter the sample. Oat smuts can be controlled by dressing seed with a The black spore masses are so obvious among the green fungicide registered for the control of oat smuts, or by ears emerging from the crop that the disease looks selection of seed from crops known to be free of infection. alarming at very low levels of infection. One infected head in 1000 is easily seen and a 1% infection appears Flag smut in wheat devastating, even though yield is not seriously affected. Flag smut of wheat is rarely important because of the The level of loose smut infection in a crop varies widespread use of resistant varieties. All the currently according to weather conditions during the flowering recommended wheat varieties for Victoria (except Rosella) period of the crop which produced the seed used. Cool, have good resistance to flag smut. This smut attacks the moist conditions extend the flowering period, giving loose leaves of the wheat plant and produces stripes parallel to smut spores more chance to infect the open flowers. Seed the leaf veins which rupture to expose the spores. produced in cool wet seasons is likely to carry increased infection levels, whereas hot dry conditions at flowering The affected plant is usually stunted and produces little or time may reduce the infection level. no grain. Most spores fall to the ground but some may be gathered at harvest to contaminate the grain. Control The major source of flag smut infection is from soil-borne Seed crops should be inspected at flowering and the spores since these survive in the soil as well as on grain. infection level counted. If infection levels are serious and the weather is cool and moist, the seed produced should be Control dressed with a fungicide registered for the control of loose Stubble of infected crops should be burnt and three or four smut. These chemicals are absorbed into the young years allowed before planting the next wheat crop. seedling and kill the fungus without harming the plant. Some seed dressings are registered for the control of seed- They also control covered smut spores on the grain borne flag smut.

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