Features Agronomy Genetics/Traits
Rotate barley varieties to reduce leaf disease

Much has been written about the importance of managing net blotch and scald
leaf diseases in barley through crop rotations. As part of the foundation for
minimizing the build up of these diseases, crop rotation remains a key tool
for growers seeking to avoid barley leaf diseases. However, some farmers often
grow barley continuously on the same field for several years, due to land availability
and the need for feed barley production to supply their feedlots. To address
that concern, researchers at Agriculture and Agri-Food Canada (AAFC), Alberta
Agriculture, Food and Rural Development (AAFRD), and the University of Alberta
conducted a three year study to see if barley variety rotations could be used
to reduce the impact of leaf diseases while maintaining crop productivity.

"Barley cultivar rotation can be used as a short-term strategy to help
reduce leaf disease levels and maintain crop productivity for Alberta barley
producers when rotational options are limited because of livestock feed requirements
or market factors," explains AAFC plant pathologist Kelly Turkington, who
led the group of researchers. "However, it must be viewed as a short-term
solution."

In a survey conducted by Murray McLelland of Alberta Agriculture, Food and
Rural Development in 1992, an average of 36 percent of the farmers grew barley
on barley, with the proportion increasing to 75 percent in western parts of
central and southern Alberta. For these growers who have difficulty rotating
between barley and other annual forage crops such as triticale or oats, Turkington's
research shows that at the very least, rotating between barley varieties can
temporarily help reduce disease impact.

The field trial included 22 rotations with barley including two barley varieties
with resistant (Kasota) and intermediate (AC Lacombe) reactions to scald; a
previously resistant barley, CDC Earl; and a susceptible check, 'Harrington',
and Wapiti triticale. Barley and triticale were direct seeded at a rate of 220
seeds/m2 into summerfallow in the first year and into
standing residues remaining from the previous growing season in the next two
years. Planting dates were May 13 to 20 each year

In late July of each year, scald and net blotch severity were rated for each
rotational treatment. Yield, kernel weight and test weight were measured at
harvest.

In the second year of trials (the first to show rotational effects), Turkington
saw significant rotation differences for scald and net blotch severity. In general,
disease severity was highest and yield was lowest when a barley variety was
grown on its own residue, especially for varieties other than the resistant
Kasota. Yield and kernel weight were lower, while scald and net blotch levels
were higher for barley variety monoculture (growing the same variety year after
year) than for barley variety rotations.

In the third year of trials, when a barley variety was grown on its own residue,
scald severity was usually higher compared with barley variety rotation. A similar
trend was also observed for net blotch, except for the varieties Kasota and
CDC Earl. The analysis indicated that higher levels of scald and net blotch,
and decreased kernel and test weights, occurred for barley variety monoculture
compared with barley variety rotation.

In both the second and third years, barley planted on triticale residue generally
had higher yield, higher kernel weight and test weight, and lower disease levels
compared with barley planted on barley residue.

More information needed on barley parentage
There are 20 or so barley varieties currently available with varying levels
of resistance to net blotch and scald. If using a barley variety rotation for
short-term disease management, selecting varieties based on genetic resistance
would be useful. Unfortunately, that information is not well understood.

"There isn't a lot of information available on where the resistance comes
from in the barley cultivars, so that makes it hard to rotate on the basis of
genetic resistance," explains Turkington.

Initially, the researchers suggested selecting barley varieties from different
breeding institutions to help rotate genetic resistance. For example, a barley
like AC Lacombe (developed at AAFC Lacombe, Alberta) could be rotated with a
variety like CDC Dolly (from Crop Development Centre at Saskatoon, Saskatchewan).
However, this is not a guarantee that the resistance comes from different parentage,
although it is one of the best strategies available when rotating barley varieties.

To further refine those recommendations, Turkington has initiated several more
studies to determine variety reaction to scald and net blotch. In the first
trial, he is growing 30 to 40 lines of barley inoculated with specific isolates
of the scald pathogen taken from particular barley varieties. If, for example,
a variety like Kasota suffers less due to isolates taken from certain other
varieties, then these other varieties might be candidates for barley variety
rotation with Kasota in a short-term monoculture production system.

The other trial with colleagues George Clayton and Neil Harker at Lacombe is
looking at barley variety rotations in a monoculture situation versus barley
variety rotation or rotation with triticale or oats. The variety Seebe is grown
continuously on the same land, and is being compared to a CDC Helgason/AC Harper/Seebe
rotation or rotation with oats or triticale. "We're seeing some benefit
of that rotation for net blotch management, as well, although the more diverse
rotations that include oats and triticale tend to be the best."

Eventually, Turkington hopes that the trials will help provide more information
on how barley varieties react to scald and net blotch. He suggests that farmers
talk to their seed company representatives to find the parentage of the varieties,
and to ask if the breeders know which lines the resistance comes from. That
information would help them rotate between barley varieties with different sources
of genetic resistance.

However, continuous production of barley, even utilizing variety rotation,
will probably not provide long-term sustainable leaf disease management, especially
for scald, says Turkington. Research conducted at Lacombe by Kequan Xi with
AAFRD Lacombe has shown that there is a tremendous amount of diversity in the
scald pathogen and therefore potential for rapid changes in the prevalent genotypes
of the pathogen, with some scald races having the ability to attack, either
individually or in combination, several known sources of resistance. In addition,
producers using barley variety rotation for scald management will also need
to monitor other diseases and choose varieties with satisfactory levels of resistance
to avoid diseases such as net blotch from increasing in severity.

Researcher Andy Tekauz at AAFC Winnipeg found extensive variability in the
net blotch pathogen compared with previous studies, and demonstrated the occurrence
of isolates virulent on several sources of resistance. As a result, he suggests
that, "Future cultivars with improved net blotch resistance may select
for and increase these more virulent pathotypes," meaning that continual
use of these varieties will likely lead to the eventual loss of that source
of resistance.

Turkington cautions that sustainable leaf disease management using rotations
will need to include non-host crops such as triticale and oats that can provide
a suitable alternative to barley for feed grain and silage. Other strategies
to add diversity, such as mixtures of barley cultivars with potentially different
resistance genes, or intercropping of barley with suitable alternatives, such
as triticale or oats, may also result in reduced leaf disease levels. -30-