Loose smut of wheat is caused by the fungus Ustilago tritic ( U. nuda ). In Victoria, this disease is rarely seen because the regular use of seed treatments controls this disease.
Loose smut symptoms do not become apparent until ear emergence. At this time ears of infected plants emerge earlier, have a darker colour and are slightly taller than the ears of healthy plants. On infected ears the florets are full of a mass of black spores. These spores are initially held by a thin membrane, which soon ruptures releasing spores. Eventually all that remains of the head is the bare stalk, (Figure 4.4).
Figure 4.4 Wheat ears infected with loose smut. The ear on the right indicates all that remains after the loose smut has blown free
Ears of infected plants emerge early. The spores released from the infected heads land on the later emerging florets and infect the developing seed. Infection during flowering is favoured by frequent rain showers, high humidity and temperatures of between 16-22°C.
There are no visible signs of infection as the fungus survives as dormant hyphae in the embryo of the infected seed. When infected seed germinates the fungus grows within the plant. As the plant elongates the fungus proliferates within the developing spike and spores develop instead of healthy grain. Eventually the wheat head is replaced by a mass of spores, ready to infect healthy plants, (Figure 4.5).
Figure 4.5 Disease cycle of loose smut in cereals. Illustration by Kylie Fowler
Grain Trade Australia’s commodity standards allow a maximum of 3 pieces of loose smut infected ear per half litre for all grades.
BUNT OF WHEAT
FLAG SMUT OF WHEAT