Wheat leaf rust

Wheat leaf rust, also known as brown rust, is a serious fungal disease affecting wheat and rye caused by Puccinia triticina. It is the most prevalent of all the wheat rust dieases, occurring in nearly all areas where wheat is grown. It has caused serious epidemics in North America, Mexico and South America.

It is most destructive on winter wheat, probably because this allows the pathogen to overwinter. Infections can lead to a 1-20% yield loss due to the fact that infected leaves die earlier and all the nutrients are directed to the growing fungi. Infection can also cause grain to shrivel.

The pathogen has an asexual and sexual cycle. In North America, South America and Australia the pathogen only undergoes it's asexual cycle. However this does not seem to be a disadvantage to it, and wheat lead rust has many races with different virulence. The sexual lifecycle of wheat leaf rust requires a different host species, Thalictrumn spp.

Wheat Stem Rust spreads via air bourne spores. The germination process requires moisture, and works best at 100% humidity. Optimum temperature for germination is between 15-20 C. Before sporulation, wheat plants appear completely asymptomatic. This is because rust pathogens are biotrophic and require living plant cells to survive.

Lifecycle
P. triticina has an asexual and sexual lifecycle. In order to complete it's sexual lifecycle P. triticina requires a second host Thalictrumn spp. which it will overwinter on. In places where Thalictrum doesn't grow, such as Australia, the pathogen will only undergo its asexual lifecycle and will overwinter as mycelium or uredinia. The germination process requires moisture and temperatures between 15 – 20 °C. After around 10 – 14 days of infection, the fungi will begin to sporulate and the symptoms will become visible on the wheat leaves.

Symptoms
Small brown pustules develop on the leaf blades in a random scatter distribution. They may group into patches in serious cases. Infectious spores are transmitted via the soil. Onset of the disease is slow but accelerated in temperatures above 15°C, making it a disease of the mature cereal plant in summer, usually too late to cause significant damage in temperate areas. Losses of between 5 and 20% are normal but may reach 50% in severe cases.

Control
Varietal resistance is important. Chemical control with triazole fungicides may be useful for control of infections up to ear emergence but is difficult to justify economically in attacks after this stage.