Panhandle Perspectives - Aug. 17, 2016

Stripe Rust, Part 2 – Biology and Life Cycle

 

Robert M. Harveson, Extension Plant Pathologist
Panhandle R&E Center, Scottsbluff

Author’s note:  This is the second article in a four-part series on stripe rust and describes its biology and life cycle. Other parts in the series describe history and geographic distribution; reasons for its recent emergence in wheat production throughout this region; and management strategies to attempt to limit economic losses in the future.

Introduction

Rust diseases on cereals are not a new phenomenon.  Awareness of their effects goes back thousands of years. Rusts were one of the major “blights” of ancient history mentioned in the Bible and various writings of the Greeks and Romans that periodically led to famine and increased fears and superstitions when the nature of the cause could not be determined. 

Life Cycle for Wheat Rust Diseases

Panhandle Perspectives
Bob Harveson

There are three primary rust diseases affecting wheat: stem rust, leaf rust, and stripe rust, although early records do not distinguish between them. Each is distinct, caused by one of three different pathogens; however, they are all similar in their biological life histories. The life cycles of wheat rust fungi are very complex, involving five different spore stages (each producing a distinct spore type) that appear in a regular sequence, requiring two unrelated hosts, also known as alternate hosts, to complete their life cycles.

The spore type that causes yield reductions on wheat is called the urediniospore, and it is also the one that is the most easily noticed. These spores are reddish-brown to yellow or orange in color, depending on the disease, which also is the source of the name “rust.”  Stripe rust spores tend to be more yellow while those of leaf rust are orange.

The spores are produced in blister-like lesions called pustules on wheat leaves. The urediniospore stage is also called the repeating stage because it is the only one with the ability to cause multiple infections on the wheat crop within the same year. 

Disease Spread and Epidemiology

Wheat rust pathogens are particularly prone to rapid, large-scale spread because of their capacity to produce spores. Rust pustules can produce 10,000 urediniospores per day, each of which could theoretically then produce another new pustule within 10 days. Fortunately, not all spores cause infection (generally about 10 percent), but this example illustrates the astronomically high reproductive potential of these pathogens under optimal environmental conditions.

The repeating urediniospore is furthermore well-suited for wind transportation and can efficiently and rapidly be transported over hundreds or thousands of miles. In fact, by the use of airplanes in the 1930s it was found that the rust spores from grasses and grains were detected on petroleum jelly-coated microscope slides from elevations as high as 16,000 feet.  Furthermore, in North America rust infections have been documented as migrating from northern Mexico and south Texas through the Great Plains to North Dakota within six months.

Stripe Rust – Conditions Favoring Disease

Stripe rust of wheat, caused by Puccinia striiformis f. sp. tritici, has been long been regarded as a low-temperature disease and primarily problematic in cool, wet conditions. Optimal spore germination has been documented to be 50-54 degrees Fahrenheit with an optimal temperature for disease development (latent period) of 55-60 degrees Fahrenheit. The latent period is the time between infection and new spore formation.

This is 10-15 degrees Fahrenheit lower than the average for the leaf and stem rust diseases, which have historically been more problematic for Nebraska production. Yet in the last decade, the disease has emerged to consistently induce severe economic losses in Nebraska under very warm conditions previously thought to be unlikely.

Next: Potential reasons for recent severe epidemics

Editor’s note: This article is based on material from these sources:

Chen, W., Wellings, C., Chen, X., Kang, Z., and Liu, T.  2014.  Wheat stripe (yellow) rust caused by Puccinia striiformis f. sp. tritici.  Mol. Plant Pathol. 15: 433-446.

De Wolf, E.  2010.  Wheat stripe rust. EP-157, Kansas State University Agricultural Experiment Station and Cooperative Extension Service.

Line, R. F.  2002.  Stripe rust of wheat and barley in North America: a retrospective historical review.  Ann. Rev. Phytopathol. 40: 75-118.

Wegulo, S. N., and Byamukama, E.  2012.  Rust diseases of wheat. NebGuide GS180, Nebraska Cooperative Extension.