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Bean Rust: The Return of an Old Foe
March 30, 2009

By Sam Markell, Extension Plant Pathologist, North Dakota State University

Rust was a constant disease threat to dry beans grown in North Dakota many years ago.  Although rust epidemics didnt occur every year, when they did, they had teeth.   In 1975, 1980, and 1981 rust epidemics in North Dakota were so severe some fields were not even harvested.   In response to the constant threat of rust, pathologists and breeders made generating rust-resistant cultivars a top priority.  As a result, a very effective rust-resistance gene (called Ur-3) was bred into nearly all the bean varieties grown in our region, and rust epidemics became a thing of the past.  Although rust occasionally showed up sporadically, it always occurred on a variety that wasnt resistant.  Until 2008.

Urediniospores (or summer spores) on bean leaves.

Late in the 2008 season, I started receiving phone calls and disease samples from bean growers and others in the bean industry concerned about fields infected with rust.  Although it was too late in the season for rust to cause much economic damage, rust was occurring on varieties that should have been resistant.   At the same time, Dr. Rubella Goswami, Department of Plant Pathology, NDSU, was coordinating a survey of bean diseases in eastern North Dakota and northwestern Minnesota.  After surveying the growing region, Dr. Goswami confirmed that rust was widespread on resistant varieties in an area concentrated in northern Traill County.

For the next two months, rust collections from seven different fields were evaluated in NDSU greenhouses, and the results all pointed to the same conclusion - a new race had evolved.    

How do races evolve? 

Dusty, cinnamon-brown streaks of rust spores remains when pustules are rubbed.

When the resistance gene Ur-3 was incorporated into cultivars, the pathogen was no longer able to cause disease.  But that didnt stop it from trying.  Rust spores were still around, produced on a susceptible variety, blown in from other areas, or germinated from old infections on a previous years residue.  Eventually, one spore had a genetic change that allowed it to cause disease on varieties with the Ur-3 gene [When a pathogen is able to cause infection on a different resistant gene, it is commonly referred to as a race-change.  Race changes frequently occur through mutation or sexual variation].  Once the race change occurred, that race would have an advantage over the other races, since it can reproduce on beans that other races cant.  Because of this, the new race can increase very fast.  In rusts, an infection from one rust spore can produce thousands of new spores in 10-14 days.  Repeat this cycle a few times, you have an epidemic and the whole population of rust has evolved. 

Although you cant predict when a race change might occur, the bean rust pathogen is ideally suited for race changes.  Rust (technically called Common Bean Rust) is caused by a fungal pathogen (Uromyces appendiculatus) that is genetically complex; it has spore stages that are asexual (where new spores are genetic clones of its parent spore) and sexual (where where new spores are genetically mixed between two parent spores).  Additionally, the pathogen is robust.  It can survive North Dakota winters, it can produce millions of new spores in a very short period of time in a very small field, and spores can blow many miles and still cause infections.  Being robust and genetically variable are two factors that likely made this race change inevitable.

What are the implications? 

NDSU bean pathologists and bean breeder have evaluated some of the most common varieties grown in our region against the new race of rust.  Unfortunately, all 26 varieties tested were susceptible to the new race (Table 1).  That doesnt mean that every bean cultivar grown in the region is susceptible, but it would be unwise to expect that any cultivar is resistant.  Because the new race is able to cause disease on most (if not all) bean varieties, we expect the new race will likely become established quickly and our rust problems in the future will be similar to those in the 1970s and 1980s.

However, it is important to note that the new race is not widespread yet.  We found it in a fairly small area centered in northern Traill County.  I feel comfortable saying that it will overwinter, and may likely spread in 2009, but how far it spreads is unpredictable.  Historically, even when rust was the top disease concern for bean growers, the disease was not severe every year.  Conditions were typically too warm for disease to be much of a problem until late in the season.  It was the unusual years that were cool and wet when rust was widespread.  Whether severe or mild, the new race is likely to be detected first in that area, and spread from there.

Additionally, breeders and pathologists have been breeding other resistance genes into beans for some time already, in anticipation of a race change.  NDSUs dry bean breeding program has at least one gene effective against the new race in advanced lines already.  Although it will still be a few years, resistance beans will be available again in the future.

Disease cycle and symptoms 

Rust survives the winter in bean residue as black teliospores (often called winter spores).  Teliospores germinate in the spring, the pathogen goes through several often unseen spore stages, and finally it produces pustules full of dusty cinnamon-brown urediniospores (often called summer spores) on bean leaves These pustules are diagnostic of the disease, and the first symptom of infection.  In cool and wet conditions (60-75 F; heavy dews), urediniospores will infect beans and produce new pustules in approximately ten days.   

Management for 2009 

Unlike in the 1970s and 1980s, very effective fungicides on rust are labeled for beans.  However, before considering fungicide use on your varieties, it is important to remember that other races occur in ND/MN, and growers still sometimes grow varieties susceptible to the old races.  If your neighbor is growing one of the varieties that is susceptible to the old races of rust and gets rust early in the season, he/she should spray a fungicide to protect their crop.  But that doesnt mean your crop is in danger. The infection could be caused by one of the old races.  Although all of the varieties in Table 1 are susceptible to the new race, many are resistant to the old ones. 

Disease severity and yield loss depend on favorable weather and the time of disease onset, so the first step to effective management is to scout for rust.  If you find it, take note of how many pustules you find and what growth stage your beans are at.  Early infections can lead to dramatic yield reductions, whereas late infections may look bad, but do not damage yield.  Table 2 uses growth stages and rust severity to help make general fungicide recommendations.  Dont get lost in the detail.  The general interpretation of the table suggests 1) the earlier you find rust, the more likely you are to benefit from a fungicide application, 2) the closer you get to striping, the less likely you are to encounter yield loss, and 3) after striping, spraying is not beneficial. 
Although limited fungicide trials have been done recently on bean rust, fungicide trials on rusts in other crops provides us with good information.  Tebuconazole (Folicur® and generics) is an excellent fungicide on rusts, and is often my recommendation if you already have rust on your plants.  Prothioconazole (Proline®) is relatively new, but in tests on sunflower rust in ND in 2008, prothioconazole gave similar levels of control to tebuconazole, and with white mold efficacy it may be the logical choice if rust shows up during flowering.  Pyraclostrobin (Headline®) has some effect on rusts after they occur (although somewhat less than tebuconazole or prothioconazole), but is most effective as a preventative, or in the early stages of an epidemic.   Data from sunflower rust trials indicate pyraclostrobin at 9 fl oz conferred rust-control for several weeks when applied immediately before pustules were first observed.  Azoxystrobin (Quadris®) works similar to pyraclostrobin, but in some rust trials azoxystrobin conferred less control.  Other preventative products are labeled, but the above four are likely the most effective.  NDSU plant pathologists are working with Northarvest and chemical companies to set up fungicide trials in 2009. 

Lastly, let us help you.  If you find rust please let us know.  I recommend letting your county agent know so we can spread the word.  Please remember to tell us the variety.  Good luck in 2009.

Markell is currently an Extension Plant Pathologist with responsibilities in Diseases of Broadleaf Crops.


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