Leaf rust, rust

Morphology and biology. In its life cycle the fungus P. helianthi has several sporulations or stages of development varying in form and function. They all develop on sunflower. The pathogen is an autoecious species with a full cycle of development. It is known to affect common cocklebur (Xanthium strumarium), which may be an additional source of infection. In the spring, after over-wintering on sunflower, the teliospores produce basidiospores. The latter infect sunflower seedlings. Later, the spermagoniun stage of fungus appears as globular formations on the upper surface of leaves or cotyledons. There are small spores or spermatia. Dikaryotic mycelium occurs, forming aecia. Aecia with numerous spores develop on the lower surface of leaves. Aggregations of cinnamon-brown uredial pustuls with spores appear on the upper surface of leaves infected by aeciospores. In the uredia stage the pathogen forms several generations of spores which promotes mass accumulation of infection and intensive distribution of the disease on sunflower. Urediospores are unicellular, light brown, egg-shaped or spherical, 23-34 x 17-28 ., having high viability. They infect mainly leaves, but also petioles, bracts, back of head, and the upper part of stalks of sunflower. In the second half of sunflower vegetation the dark-brown teliospores occur on affected tissue; the teliospores are positioned on the lengthened colorless pedicel with a constriction in the center. In teliostage the causal agent winters in field, renewing its activity in spring. A line of physiological races have been revealed in the fungus P. helianthi.