Top Crop Manager

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Tillage has less effect on crop diseases than other factors

Disease is generally not a concern in reduced tillage systems.


November 19, 2007
By Top Crop Manager

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The rural myth that crop disease can be greater under direct seeding and no-till
is just that, a myth. While research has shown that a few diseases can be slightly
worse under no-till, for the vast majority of diseases that infect prairie crops,
disease is no worse when tillage is reduced or eliminated.

"I don't know of any farmers who have gone back to tillage because of
disease," says Roger Andreiuk, an agronomist with Alberta Reduced Tillage
Linkages (RTL) at Leduc. "It isn't on the radar screen, and that's because
there has been a lot of research and experience to show that there isn't a concern
with disease and no-till."

Perhaps, the root of the myth is that with no-till, crop residue and the resulting
disease pathogens are left on or close to the soil surface, supposedly to infect
subsequent crops. Plus, no-till produces changes to soil moisture and temperature,
which can impact the biological activity of soil microflora and fauna. However,
these changes may favour some pathogens while limiting the growth of others.

"I always like to use the example of blackleg in canola to illustrate
the point that tillage has little effect on the disease, and that other factors
are more important," explains research scientist Kelly Turkington, with
Agriculture and Agri-Food Canada at Lacombe, Alberta. "Blackleg devastated
canola in Saskatchewan in the 1980s, but remember what cropping practices were
like? It usually consisted of a fall discing or cultivating and several more
cultivations in the spring before seeding. Yet we had severe outbreaks. The
reason was that we had very susceptible varieties like Westar, tight rotations
and weather that favoured the disease."

Turkington also explains that the persistence of canola stubble on the soil
surface is relatively short, even with no-till. In a study at Beaverlodge, Alberta,
AAFC researchers found that one year after canola, the amount of canola residue
found on the surface was slightly higher with no-till than conventional, but
after two years the amount of residue was similar between the two systems. He
explains that to best manage blackleg, a canola rotation of four years that
uses a resistant variety is the recommended approach, and the type of seeding
system will not matter.

"Rotations, seed quality, variety resistance and especially the weather
are more important in controlling any disease than the tillage system used,"
says Turkington.

That is not to say there are not any impacts on disease caused by tillage.
Jeannie Gilbert, a research scientist with AAFC at Winnipeg, Manitoba, says
that a few diseases are actually less of a problem in zero-till, while a few
others are increased.

Gilbert notes that although the commonly isolated leaf spot pathogens that
cause tan spot, septoria leaf blotch, stagonospora nodorum blotch and spot blotch
all overwinter on infested crop residue, a study in southern Manitoba found
that tan spot was more prevalent in no-till fields, septoria leaf blotch and
stagonospora leaf blotch were more prevalent in conventional till fields, and
spot blotch was present at equal levels in both tillage systems. Studies in
Saskatchewan also reported higher levels of tan spot in reduced tillage systems
and higher levels of septoria leaf blotch in conventional till.

"We always hear the statement that stubble diseases will be worse under
no-till, but that's not always the case," says Gilbert. "If you grow
wheat every other year, then yes, the older stubble can infect the crop with
tan spot. To manage tan spot, the rotation just needs to be extended."

Turkington also looked at leaf diseases in Alberta barley fields in the mid
1990s. Scald and net blotch levels were assessed throughout Alberta. Fields
were classified by tillage regime, variety resistance and crop rotation. In
all three years of the study, tillage did not influence the probability of higher
levels of scald or net blotch. However, the level of scald or net blotch resistance
and the previous year's crop did. Fields previously planted to barley were three
to four times more likely to have greater scald severity than fields planted
to non-host crops like oilseeds or pulses. The previous crop affected net blotch
severity in one of three years. During the three years, susceptible varieties
were four to eight and five to 12 times more likely to have higher levels of
scald and net blotch, respectively, compared with resistant varieties.

"We consistently see that crop rotation and cultivar susceptibility are
the biggest factors for these diseases," says Turkington.

Common root rot is another disease that is influenced by tillage, or the lack
of it. All crops are susceptible to root rots caused by Rhizoctonia solani
AG8 and to pythium species. Gilbert says research has found that rhizoctonia
root rot is generally more severe when wheat and barley are sown directly into
cereal stubble than when sown into a seedbed using conventional tillage. However,
the use of modified narrow openers that disturb the soil beneath the seed depth
consistently reduced the level of rhizoctonia root damage. She explains this
is not due to a reduction or elimination of the pathogen, but enhanced root
growth in the loose soil which compensates for loss of root mass due to disease.

Research in Saskatchewan by Karen Bailey of AAFC Saskatoon found that common
root rot levels were lower under reduced tillage compared with conventional
tillage. Bailey also found that tillage did affect the recovery of fungal species
associated with common root rot. For example, prevalence of the pathogen Cochliobolus
sativus
was reduced under zero-till while Fusarium spp. increased.

Turkington says that some of his studies in the mid 1990s showed trends where
there was less root rot in peas and common root rot of wheat and barley under
direct seeding than conventional tillage. Root rot in peas is caused by a complex
of organisms including Fusarium spp., Pythium spp. and Rhizoctonia
solani
.

For ascochyta blight, Gilbert explains that research shows tillage management
was not found to be important except in rotations with short re-cropping intervals.
Following a disease outbreak, at least two non-host crops were needed between
successive lentil or chickpea crops to substantially reduce inoculum of the
pathogens that cause ascochyta blight of lentils and chickpeas.

Fusarium head blight, caused by Fusarium graminearum readily survives
in crop residues and is another disease that may be affected by tillage. The
population of F. graminearum decreases once colonized host tissues decompose.
While F. graminearum might persist for unknown periods of time, mainly
on the debris in no-till plots, researchers have concluded that weather conditions
are more influential than tillage practices in the development of FHB, Gilbert
explains.

"For fusarium head blight, hot, moist weather is the key factor. It doesn't
like cool weather," says Gilbert. She also cautions that if tan spot attacks
durum wheat later in the season, it can cause red smudge (pink kernels) that
looks similar to FHB affected kernels, and growers should seek expert opinion
to help assess their grade.

Turkington summarizes that while any one individual disease may or may not
increase under no-till, weather, rotation and variety resistance are much more
important factors to consider. Establishing a healthy, competitive crop by using
crop rotation, disease resistant varieties, high quality seed, registered seed
treatments and fungicides when warranted, and seeding shallow into a firm moist
seedbed, will help the crop fight off disease stress.

"Disease is not a reason to avoid reducing tillage. Reduced tillage farmers
usually end up with a more diverse rotation, so disease can be easier to manage,"
explains Andreiuk. "Also, established direct seeded soils ones that have
been direct seeded for a number of years, have a more active and diverse biological
component which decomposes crop residues quite quickly." -30-