Questions of whether old reliable wheat varieties are losing their resistance to common diseases continue to pop up as growers in the upper Southeast begin getting ready for wheat planting.
It has been widely reported that sister varieties Tribute and McCormick have had increasing problems with powdery mildew over the past two years. Whether this is a true loss of resistance, a shift in disease race, or a permanent problem continues as growers make decisions on which varieties to plant for the 2009-2010 growing season.
Carl Griffey, a Virginia Tech Professor and leader of one of the top small grain breeding programs in the country says the explanation is complicated.
While Tribute's mildew resistance has broken down in some states and areas, this variety still offers producers a useful variety as it remains effective against leaf rust in most regions and has moderate resistance to fusarium head blight.
Griffey points out that Tribute has one of the highest test weights among soft red wheat varieties. And, growers know how to control powdery mildew with seed and/or foliar fungicides, he says.
It now appears, that resistance to powdery mildew in Tribute and McCormick (sister lines) is primarily controlled by the race specific gene Pm17. In the grand scheme, race specific genes are generally effective for only a very short period of time.
Once cultivars possessing such genes become widely grown they tend to gradually lose effective resistance qualities.
Gene Pm17 actually has remained effective for a longer time than most race specific genes. Unusually high levels of powdery mildew were initially found on Tribute at Eastern Shore Agriculture Research and Extension Center in Painter, Va., in 2008 and on McCormick at the Eastern Virginia Agricultural Research and Extension Center in Warsaw, Va., in 2008 and became more prevalent among growers in Virginia and other states in 2009.
Griffey says, when discussing resistance and race changes, one must consider the type of resistance, the type of pathogen, and the source of initial disease inoculum.
In diseases such as powdery mildew and leaf rust distinct races exists. These diseases vary in their virulence for specific resistance genes, many of which give complete control and others no control, depending on the race(s) present.
In contrast, other diseases such as fusarium head blight, the pathogen does not have distinct races, resistance is only partial, but should remain effective and durable over time.
There are two basic types of resistance: (1) race specific resistance which generally gives a very high level of control, near immunity in many cases and (2) race non-specific, generally adult plant resistance which gives moderate yet acceptable levels of disease control.
Race specific resistance places significant pressure on the pathogen to mutate in order to survive, which is why this type of resistance is often not durable and effective only for a brief period.
Because adult plant resistance does not completely inhibit the pathogen, it does not place extensive pressure on the pathogen to mutate and generally is durable. The adult plant resistance to powdery mildew identified in 'Massey' (also present in USG 3209, USG 3555, and Shirley) has remained effective for nearly 30 years.
Changes in prevalent races and the impact on a cultivar's resistance (race specific) also depend on where the initial source of inoculum comes from each year. In the case of the rusts (leaf, stripe, and stem), which generally do not over-winter in cooler environments, the primary source of inoculum comes from Mexico and the southern U.S.
“Thus, the question of whether a cultivar having a specific race specific gene will be resistant or susceptible depends on the predominant races present at a given time and where they migrated from (wind blown spores),” Griffey explains.
“In the case of powdery mildew, the initial source of inoculum is from the local area from spores over-wintering on straw from the previous wheat crop. While race changes in the rust pathogens primarily occur via mutation, race changes in powdery mildew pathogens occur via both sexual recombination and mutations and, therefore, the powdery mildew pathogen consists of many different "races" each year,” Griffey adds.
The predominant race(s) will be those that are most "genetically fit, aggressive, and capable of surviving on the existing wheat varieties.
In most cases, when a race specific gene "breaks down", or to be more technically correct, when virulent races develop, it will generally remain ineffective as long as cultivars having the gene are grown because the pathogen population will continue to consist of races capable of overcoming the gene.
For growers in the upper Southeast who are planning to plant Tribute or McCormick for the 2009/2010 season, Griffey suggests growers either consider use of a seed treatment, such as Baytan for growers who previously grew either of these varieties and noted significant powdery mildew development and/or to follow the standard recommendation of scouting fields early and often and apply an effective foliar fungicide when disease thresholds are met.
While Tribute’s resistance to leaf rust has remained effective in most areas, McCormick is very susceptible to leaf rust. In contrast, Tribute should not be planted in fields known to carry soilborne mosaic virus as it is very susceptible to this disease.