Leaf rust
Leaf rust is caused by the fungus Puccinia triticinia (previously called Puccinia recondite f. sp. tritici). The disease can also infect rye.
Leaf rust produces reddish-orange to reddish-brown spores which occur in small, 1.5 mm, oval pustules. These are found on the top surface of the leaves, distinguishing leaf rust from stem rust which is found on both surfaces of the leaf.
Queensland wheat varieties mostly have reasonable resistance (rating of MR-MS - 5 or higher) and so leaf rust is currently not of major concern to Queensland wheat growers.
Leaf rust develops at temperatures from 15-20ºC. The spores require free moisture (dew/rain/irrigation) on the leaves and close to 100 % humidity to successfully infect wheat. The first signs of the disease (sporulation) occur 10-14 days after infection.
Inoculum must be present for the disease to develop. Practising crop hygiene by removing volunteer wheat, which forms a green bridge for the fungus through the summer, can eliminate or delay the onset of leaf rust.
Stem rust (Black rust)
Stem rust is caused by the fungus Puccinia graminis f. sp. tritici. In addition to wheat it can also attack barley, oats and rye.
Stem rust initially produces reddish-brown spore masses in oval or elliptical pustules with torn margins, 3×10 mm on the stems, the glumes and unlike leaf rust, on both sides of the leaves. Towards the end of the season the pustules become black as a different spore type is produced.
Queensland wheat varieties have reasonable resistance to stem rust (rating 5 or higher). However in the past, stem rust has had the ability to cause significant economic damage (50-100 % of yield). This has happened when conditions are conducive for the disease and susceptible varieties are used, or a new stem rust pathotype has developed, which has overcome the wheat’s resistance.
Stem rust develops at higher temperatures than the other wheat rusts with 20°C as optimum and a range of 15-40°C. Spores require free moisture (dew, rain or irrigation) and take up to six hours to infect the plant. New infections produce their own spores 10-20 days later.
Inoculum must be present for the disease to develop. Practising crop hygiene, by removing volunteer wheat, which forms a green bridge for the fungus through the summer, can eliminate or delay the onset of stem rust.
As with all crop diseases, rusts continuously change, producing new pathotypes. These are detected when disease is found on a previously resistant wheat variety. Should stem rust be seen on a variety which was previously resistant please contact us on 13 25 23.
