Oat Crown Rust

IUCN Red List

NatureServe

Minnesota

not listed

Widespread. and veru commonWherever wild or cultivated oats occur, except in very dry areas.

Early May to September

common oat, wild oat, common buckthorn

This is a fungus that affects wild and cultivated oats, barley, rye, and a number of other grasses. There are many varieties and all are species specific for their grass host but all use common buckthorn as their alternate host. The variety can only be distinguished on the grass host, and is identified by that host.

On infected grass plants it appears as light orange, pustule-like structures (uredinia) on the upper and lower leaf surfaces. It will occasionally also appear on leaf sheaths, culms (stems), the stalks of inflorescences (peduncles), and awns.

The uredinia are the fruiting structures of the fungus. They are round to oval and up to 3 ⁄16″ long. They contain a dusty mass of thousands of microscopic, orangish-yellow summer spores (urediospores).

On common buckthorn it first appears as swollen, yellowish-green spots on the upper leaf surfaces, leaf stalks (petioles), and twigs. Leaves and twigs are disfigured but the infection is not fatal to the plant.

The life cycle of Crown Rust includes five stages and requires two host species to complete. The five stages are teleoispore, basidiospore, pycniospore, aeciospore, and urediospore.

First stage: Winter spores (teliospores), also called resting spores, overwinter on the remnants of infected wild grasses and on the residue of infected agricultural grasses left in the field.

Second stage: In the early spring, the teliospores germinate and produce summer spores (basidiospores). The basidiospores are wind blown up to a half mile. Some are eventually deposited on the upper leaf surface of young leaves of the alternate host, common buckthorn.

Third stage: In early May, bright yellow spots appear on the upper leaf surface. Each spot contains flask-shaped reproductive structures (pycnia) below and breaking through the upper leaf surface (epidermis) of the leaf. The pycnium is an aerial vegetative body made up of cluster of branching, receptive filaments (hyphae). The pycnium releases spores (pycniospores) in a thick, sweet liquid which attracts insects. The pycniospores are carried by insects or splashed by rain to the receptive hyphae of another nearby pycnium.

Fourth stage: The fertilized pycnia produces a raised, orangish-yellow “cluster-cup” (aecium) on the lower leaf surface. The aecium releases aeciospores which are carried on air currents.

Fifth stage: Aeciospores that land on an alternate host develop into a another kind of spore-producing body (uredium). Within 7 to 10 days asexual spores (urediospores) are formed and released. The urediospores are carried by gravity or wind to other plants and other parts of the same host plant. Generation after generation of urediospores are produced throughout the growing season, continually reinfecting the plant or crop. At the end of the growing season teliospores are produced and the cycle begins again.

Sources: 4, 7, 24, 26, 29, 30.