Features Agronomy Diseases
Managing scald and resistance breakdown

Researchers are starting to observe changes in the two main barley leaf
disease pathogens, scald and net blotch. These pathogens are not static
and are adapting to the farming system being imposed on individual
fields. This can put stable disease management and genetic resistance
of varieties at risk.

Barley leaf disease resistance can rapidly break down, as with this net blotch infection. (Photo by Bruce Barker)

“One of the main factors that will drive changes
in pathogen populations is the variety grown and the genetics of
resistance incorporated into that variety, explains Dr. Kelly
Turkington, research scientist at the Agriculture and Agri-Food Canada
(AAFC) Research Centre in Lacombe, Alberta. “Although breeders have
developed varieties with resistance, over time selection pressure and
management practices may change the pathogen population so that it can
overcome the resistance that is present in that variety. This is
something that occurs over time, it does not magically appear.”

In some cases, problems with pathogen resistance in a variety may show
up during the registration and assessment process, but for others it
does not appear until the variety has been released. “Once a variety is
released and gains wide commercial acceptance it is at the greatest
risk of seeing selection for the types of pathogens that can overcome
the resistance present in that variety,” says Turkington. “We know from
conversations with farmers and researchers that when a new variety
comes along that performs well and fits into their systems, they may
continue to grow that variety exclusively, which is where problems
begin.”

In Alberta, the most dramatic pathogen shifts in barley have been with
the scald pathogen. A barley field can have a population of millions of
individual scald types, with the predominant population largely made up
of a major race or type adapted to a particular situation. However, in
short barley rotations where the same resistant varieties are grown
several years in a row, there can be selection for members of the scald
pathogen population that have the ability to overcome the resistance
incorporated into that variety. “As a consequence, the more you grow
the same variety, the more you increase the selective advantage for
these adapted races over the resistant races, and they eventually
become the predominant race in that field,” explains Turkington. “You
then see varieties that normally have good resistance become
susceptible to scald during the growing season.”

One of the reasons scald is so challenging is that the pathogen is
extremely variable and the genetics are not well understood. “With
cereal rusts for example, we have a good understanding of genetics and
are able to identify specific resistance genes for rusts. However, with
scald there is a combination of major and minor genes, making it very
difficult to clearly identify specific genetics. Researchers are
working to better understand the differences in genetics between
varieties, to help make it easier for growers to select varieties with
a reasonable difference in genetics,” adds Turkington. For example,
Seebee and Kasota seem to have different genetics, so instead of
growing the same variety every year, growers should try to alternate
varieties in rotation.

On average, yield losses to leaf diseases in barley, depending on the
year and region, can range from five percent to as much as 10 to 15
percent. Systems using tight rotations, growing the same variety every
year and in an environment favourable to disease, may see average yield
losses of 20 to 25 percent and even higher in unusual circumstances.
“The losses go beyond yield, because the disease is also destroying the
green leaf area, which reduces grain filling and kernel plumpness,”
explains Turkington. He cites grain quality losses and the risk of
losing pathogen resistance as additional effects. “In a malt situation,
protein levels may go up above acceptable levels.” 

Integrated management strategies
Variety selection is an important tool, and varieties such as Seebee,
Kasota, Manny and Sundre are good examples of varieties with enhanced
resistance to scald. “Unfortunately scald is very challenging and few
varieties have resistance; many are susceptible or intermediate at
best,” says Turkington. “Using clean, good quality seed is important
because the risk of seed-borne infection is a possibility, particularly
with net blotch. Unfortunately, even though barley producers may be
using an extended rotation for disease control, there may still be a
significant risk if the seed they are planting has increased levels of
net blotch.”

Crop rotation is an important tool, and the best strategy is to follow
barley with a broadleaf crop such as canola or pulses. “Ideally a
one-year break between barley crops helps with scald,” explains
Turkington. “The scald pathogen tends to survive on leaf and leaf
sheath tissue, which are fairly prone to decay.” However, with net
blotch, a two-year rotation is important because the pathogen causes
infection on the leaf and leaf sheath tissues, and on the stems. The
stem tissues can survive from one growing season to the next, so at
minimum a two-year rotation is important. Adjacent fields infected with
net blotch can also be a potential source of infection as the net
blotch pathogen produces wind-dispersed spores that may move from one
field to an adjacent field.

Proper scouting and accurate identification of leaf diseases is
critical. “Know what you are dealing with so you can properly select
varieties, rotations and fungicides,” says Turkington. “Sometimes one
leaf disease is assumed to be the culprit, but once we see leaf samples
and start asking questions, it may turn out to be net blotch for
example, and not scald.”

Plan ahead for fungicide application and rotation. “Although there is
interest in tank-mixing herbicides and fungicides and applying at an
early stage, our research suggests it may be better to wait and apply
fungicides at the flag emergence stage,” explains Turkington.
“Protection at the flag emergence stage protects the most critical leaf
tissue for yield and grain filling.”

If the disease develops early, then using an early application and a
second one at flag leaf emergence may be an option. However, if a
grower is in a short rotation or back-to-back barley and is using the
same chemical for both applications, the concern is that the fungicide
will have been sprayed twice in one growing season, increasing the
potential of having the pathogen in that field adapt to the chemical
used. “Our research group at Lacombe and other colleagues emphasize the
importance of using an integrated approach to pest management including
a variety of tools,” says Turkington. “When you rely exclusively on one
strategy to deal with problems, that is where issues and problems come
up.”

Using a combination of strategies to try to prolong the usefulness of
the sources of resistance and the effectiveness of fungicides is
critical. These strategies need to reduce or eliminate the ability of
pathogens to survive from one growing season to the next, and, more
importantly, their ability to reproduce.