Stripe rust (wheat yellow rust)
Puccinia striiformis, a basidiomycete belonging to the uredinales, is the cause of stripe rust on cereal crops and grasses. Several formae speciales of P. striiformis West. var. striiformis have been successively named on the basis of physiological specialization: P. striiformis f.sp. tritici collected from wheat. Like other cereal rusts, P. striiformis forms races which are usually identified with a differential set of wheat cultivars.
Symptoms occur on all aerial parts of the plant, but are most frequently seen on the leaves. They appear about 1 week after infection, and sporulation starts about 2 weeks after infection, under optimum temperature conditions. The pustules, which are tiny, yellow- to orange-colored, are often arranged into conspicuous stripes. Lines of bright yellow new urediospores give the typical striped appearance on the leaves from which the specific name striiformis and common name stripe rust are derived. In Europe the yellow color of the spores caused it to be given the common name yellow rust. During the summer, stripe rust infection of wheat spikes may occur, resulting in the formation of masses of spores between the glume and the lemma. At the end of the season, black telia may form in patches of tissue that have been killed by stripe rust uredia.
The pathogen spreads by means of airborne urediospores. After landing on wheat plants they germinate in high humidity, usually at temperatures of less than 15°C, and the germ tubes enter the leaves or other parts of the plant via the stomata. P. striiformis hyphae are thread-like and spread between intercellular spaces of their host. Hyphae are dikaryotic and often multinucleate. Once inside the leaf, haustoria are inserted into the mesophyll cells and the mycelium spreads along the leaf. In mature leaves it spreads longitudinally between the veins of the leaf. Two kinds of spores are produced by P. striiformis: urediniospores and teliospores. Urediniospores are 20-30 µm in diameter, yellow-orange and spherical. They have thick, spiny walls and 6-12 scattered germ pores. Teliospores are two-celled, ellipsoid to clavate, orange to brown, and 12-20 x 36-68 µm. The teliospores are flattened at their apex and have smooth, thick walls with a slight constriction at the septum. It is an obligate parasite and damage to plant is caused by extracting nutrients via the haustoria and by disruption of the epidermis, which reduces the water retention capacity of the leaves.