Original content by John Watkins, emeritus plant pathologist. Content edited and approved by Stephen Wegulo, UNL Extension Plant Pathologist
Puccinia triticina is an obligate parasite on wheat. The uredinia and telia occur on Triticum; pycnia and aecia are rare occurring on meadow rue (Thalictrum spp.). Unlike those of stem rust, uredinia are with out conspicuously torn epidermal tissues at their margins. They are round to ovoid, orange-red, and erumpent. Urediniospores are subgloboid, red-brown with thick echinulated walls; teliospores are two-celled and round at the apex.
Leaf rust is generally found on the leaves, but occasionally is found on glumes and awns. Symptoms are circular or oval, orange pustules on the upper surface of infected leaves. The spores within these pustules are easily dislodged and cover hands and clothing with an orange dust. As the wheat ripens, this orange stage turns into a new stage of the fungus, which is black, and often a mixture or orange and black stages will be intermingled on a leaf.
Favorable Weather Conditions
The leaf rust fungus is an airborne pathogen whose spores are spread by wind over long distances. In the central Great Plains, rust spores blow north in spring from over wintering sites in Texas and northern Mexico usually reaching Nebraska in mid-May. Once established in an area, the spores spread locally within fields as well as to nearby wheat fields. About every 7-10 days new rust pustules new rust pustules are formed. Weather plays a key role in the development of leaf rust. Moderate nights and warm (60°-80° F) days that create long dew periods in the wheat canopy are ideal for rust development. Under the right conditions, severe rusting of susceptible varieties occurs 30 to 40 days after initial rust development.
Growing resistant varieties is the best and most economical method of minimizing rust losses. Leaf rust is more of a threat to dryland wheat in southeast and south central Nebraska than it is to wheat in the west central and Panhandle. Irrigated wheat can be infected wherever it is grown in Nebraska.
Leaf rust is effectively controlled by the timely application of foliar fungicides. Fungicides currently registered for leaf rust control on wheat in Nebraska include Quadris (azoxystrobin): Stratego (propiconazole + trifloxystrobin); PropiMax EC and Tilt (propiconazole) and the mancozeb-based products Manzate, Dithane, and Penncozeb. Fungicide application may not always be cost effective if rust severity levels are low or if rust is slow to develop. Monitoring rust development in Oklahoma and Kansas in May usually gives a good indication of the potential for rust development in Nebraska. Fungicide treatment should be considered only for susceptible varieties.
For additional information see these UNL Extension NebGuides:
- Leaf, Stem and Stripe Rust Diseases of Wheat (G1245)
- Rust Diseases of Wheat (G2180)
- Foliar Diseases of Winter Wheat: Management with Fungicides (G1979)