leaf rust problem

Leaf rust -

Leaf rust is one of the most common and most important wheat diseases in wheat growing areas of the world. Leaf rust is caused by a parasitic fungus called Puccinia recondita f. sp. tritici.

Leaf rust causes very small (about 1/32 inch long by 1/64 inch wide), orange pustules that erupt through the leaf surface. In some cases, pustules are surrounded by a narrow yellow or white halo. The pustules contain masses of powdery orange spores of the rust fungus. Spores may spill out of pustules and form a grainy orange dust on the leaf surface around the pustule.

As leaves age, pustules begin to produce dark black spores instead of orange spores. These black pustules look like tar spots and are most easily seen on the lower leaf surface and leaf sheaths. Although leaf rust may initiate tiny orange spots on culms and heads, it does not form large, open pustules on these organs. This helps distinguish leaf rust from stem rust. Stem rust is uncommon and usually only occurs late in the season because it requires warm temperatures. Leaf rust pustules occur randomly across the leaf; this distinguishes leaf rust from stripe rust, which has narrow yellow stripes of pustules. Stripe rust is rare in warm areas because it requires very cool weather.

DISEASE CYCLE

Alternative hosts are not important in the leaf rust life cycle. Although some strains of the leaf rust fungus can survive on jointed goatgrass or wheatgrass, they appear to be different from the strains that attack cultivated wheat. Many rusts have special alternate hosts for completion of the sexual cycle. Meadow rue (Thalictrum spp.) is the alternate host for wheat leaf rust in Eurasia. However, the sexual cycle on meadow rue apparently does not occur in North America. Therefore, the leaf rust population in the U.S. is composed of distinct races that do not cross with each other. This slows the development of new races because mutation is the only means of genetic change.

The leaf rust fungus can only survive in living leaf tissue. It is not soilborne or borne in crop residue. In the summer, it survives on volunteer wheat. In the fall, spores blow to newly planted wheat. Early planted wheat sometimes sustains heavy rust infection and may turn yellow in the fall. This does not seem to cause winterkill of the wheat. Leaf rust can survive the winter as latent infections if green leaves survive the winter. In the early spring, pustules erupt and fresh spores blow to new leaves. If rust does not survive through the winter in Kansas, spores eventually blow up from Oklahoma or Texas. However, the delay often reduces the final severity of the disease. The rust fungus moves back to volunteer wheat around harvest time.

Leaf rust epidemic severity increases exponentially over time. That's why rust epidemics appear to suddenly "explode" during favorable weather. Rust development in the spring is favored by daytime temperatures between 60 and 75F. The infection process requires moisture, which can be provided by rain or dew. Heavy rain is unfavorable for rust because it tends to wash the spores off the leaves. Infection can occur in as little as four hours during favorable weather. Dispersal of spores to upper leaves and between fields is favored by dry, windy conditions.

Description of infection types and symptoms

0 Low No uredinia or other macroscopic sign of infectiton
0; Low Few faint flecks
; Low No uredinia, but hypersensitive necrotic or chlorotic flecks present
1 Low Small uredinia often surrounded by a necrosis
2 Low Small to medium uredinia often surrounded by chlorosis
Y Low Ordered distibution of variable-sized uredinia with largest at leaf tip
X Low Random distibution of variable-sized uredinia
3 High Medium-sized uredinia without chlorosis or necrosis
4 High Large uredinia without chlorosis or necrosis

For nomenclature of genes refer to -

D.L. Long and J.A. Kolmer..A North American System of Nomenclature for Puccinia triticina. Phytopathology 79:525-529

CONTROL

Resistant varieties possess one or more special leaf rust resistance genes called Lr genes. Currently there are more than thirty different Lr genes available, but most varieties have only a few Lr genes. In order to be virulent on a given variety, the leaf rust fungus must be able to defeat all the Lr genes in the variety. Different races of leaf rust can defeat different combinations of Lr genes in the wheat. The prevalence of different rust races is always changing in response to the popularity of different wheat varieties with different Lr genes.