Wheat leaf rust (brown rust)
Leaf rust is caused by P. triticina, a basidiomycete belonging to the uredinales. It is a macrocyclic, heteroecious fungus, with five distinct spore stages. Like most rust fungi it requires two taxonomically diverse hosts to complete its life cycle.
The disease is recognized by yellowish-brown to cinnamon-brown pustules scattered on the upper leaf surface. Small secondary pustules may develop in circles around older pustules on susceptible host cultivars. A halo of pale green or yellow appears around the pustules when host resistance is incomplete. It occurs mainly on the leaf blades, although leaf sheaths can also be infected under favorable conditions, high inoculum densities, and extremely susceptible cultivars. When the temperature increases, some pustules turn black due to the production of teliospores. Telia remain covered by the host epidermis and are blackish-brown in color. On the alternate host plant plants e.g. meadow rue (Thalictrum spp.speciosissimum) the pycnia are clustered in small groups on slightly swollen yellowish to reddish-brown areas on the upper leaf surface. Aecia are usually in clusters on gall-like areas on the abaxial surface of the leaf.
Aerial dispersal occurs as urediniospores are passively released from uredinia on wheat leaves. The urediniospores become airborne and may be transported up to several hundred kilometres in an air mass before being deposited by gravity or washed out of the atmosphere by rain. Infections may also occur from urediniospores that survive between wheat crops or during the dormancy stage of winter wheat on volunteer wheat or native grasses. Urediniospores are spread from these areas by wind to cause annual recurrence of the disease in areas a few hundred kilometres away.