Features Agronomy Diseases
Changes on horizon for crown rust resistance ratings

by Pam de Rocquigny, provincial cereal crops specialist; and Holly Derksen, field crops pathologist

Crown rust is caused by a fungus, Puccinia coronata f.sp. avenae. The crown rust fungus is specific to cultivated oat, wild oat, and a few other wild grasses, but will not infect wheat, barley or rye. Crown rust reduces oat yield and causes thin kernels with low test weight - factors which greatly reduce milling quality. Losses on susceptible varieties due to disease can be substantial if infection is early, and if weather conditions are favourable for the development of fungal spores and their spread.

Symptoms of crown rust include orange pustules of spores on the upper and lower surface of leaves. Each pustule contains thousands of spores that can be spread to neighbouring plants and produce a new pustule in only seven to 10 days under ideal conditions. In severe cases, these pustules may also be observed on the sheaths and glumes.

Source of crown rust in Manitoba
The main source of crown rust in Manitoba is urediniospores blown in on the "Puccinia Pathway" from the United States. Normally the first inoculum starts arriving at the beginning of June. However, this can vary depending on how much inoculum is present in the United States and when the winds blow from the south. During the growing season, reports on the current rust situation and how it is progressing in the United States are available in the USDA's Cereal Rust Bulletins.

Crown rust can also overwinter on stubble and related grasses in Manitoba. The teliospores survive by overwintering and then germinate to form basidiospores which will only infect buckthorn. However, the fungus will undergo sexual reproduction on buckthorn and the products of this sexual reproduction (aeciospores) can then infect nearby oat crops. Buckthorn can be found in Manitoba, especially near rivers, but it is not common.

Races in the population always changing
Within the rust fungal population, there are a number of different "races" that develop and are specific to oat varieties carrying certain rust resistant genes. The breakdown of genetic resistance may occur over several years depending on the aggressiveness of the new race, favourable environmental conditions for increase of the race, and the presence of susceptible cultivars or wild oats or buckthorn. Disease surveys are an important component of monitoring if there are changes within the population. From field surveys, infected plant material is collected and the frequency of the various rust races is determined.

Developing crown rust resistant varieties is an ongoing battle for cereal breeders. As soon as new varieties are developed with specific genes for resistance, it puts selection pressure on the rust population which develops new races that overcome this resistance. The goal of cereal breeders is to "pyramid" genes for resistance, meaning that several genes are incorporated into one variety to extend the durability of resistance.

The crown rust resistance gene Pc91 was a source of effective resistance, but a relatively recent change in the crown rust pathogen population in Manitoba has seen virulence developed on varieties that carry the Pc91 gene. It has been noted that the Pc91 gene lost its resistance in the United States as well; that has resulted in some resistance ratings changes in various state's variety guides.

The virulence change may impact the current Resistant (R) rating of the varieties Souris, Stainless, HiFi, AAC Justice and CDC Morrison which contain the Pc91 gene. However, more data is being collected by Agriculture and Agri-Food Canada (AAFC) and Manitoba Agriculture, Food and Rural Development (MAFRD) through field surveys and trial testing (done in coordination with MCVET) that will provide the additional information needed to update the ratings for the Seed Manitoba 2016 guide.

Control options for crown rust
Seeding resistant varieties is normally step one when developing an integrated disease management plan for management of crown rust. However, there are other management strategies available including early seeding, which may be a viable option in 2015. With an early seeding date, it is hoped the crop is advanced enough by the time rust is blown in from the United States. Typically with late infections the crop will not suffer significant yield or quality loss.

Another option is the use of a foliar-applied fungicide. There are a number of fungicides registered for control of crown rust in oats. Refer to the Plant Disease Control section of the Guide to Crop Protection for additional information. Scouting will also be important as the crop matures and advances towards flag leaf. More intensive scouting may be necessary if precipitation is in the forecast.

May 5, 2015  By Manitoba Agriculture Food and Rural Development