On rolling landscapes, crop and disease development are seldom uniform.
November 29, 2007 By Bruce Barker
On rolling landscapes, crop and disease development are seldom uniform. Differences
in organic matter, moisture levels and nitrogen (N) mineralization influence
how the crop develops and the micro-climate where disease can incubate. To see
if farmers could use this knowledge to better target fungicide applications,
Agriculture and Agri-Food Canada (AAFC) research scientists, Randy Kutcher and
Sukhdev Malhi set up a four year trial at the Conservation Learning Centre near
Prince Albert, Saskatchewan. The results show that canola diseases develop differently
according to landscape, but that growing season weather has a lot to do with
"When we started looking into precision farming seven to eight years ago,
we thought we would be able to farm by the metre, and the technology to do so
existed," says Kutcher. "Now, we're finding that the weather and other
poorly understood agronomic factors have a lot to do with how the crop responds
at different slope positions. Farming by the metre doesn't look very easy today."
In theory, Kutcher explains that lower slope positions were expected to be
at greater risk of infection by sclerotinia stem rot than upper slopes because
of greater soil-available moisture and fertility, which would have promoted
a thick crop canopy conducive to disease development. This trend was observed,
but was not borne out more clearly because sclerotinia stem rot incidence was
usually low on either slope.
|Rolling topography can affect disease development, says research
scientist, Randy Kutcher.
Rather, sclerotinia stem rot was not consistently associated with either slope
position, or varying N rates. Kutcher says that sclerotinia stem rot incidence
in 1998 was so low that it was biologically meaningless, although the disease
was found only in the lower slope that year. Fungicide application did reduce
sclerotinia incidence by 2.5 percent in 1999 and 11 percent in 2000, years of
good rainfall, but the infestation levels were low, so that the application
was not economical. In 1999 and 2000, incidence was lowest without N fertilizer
and increased with increasing N rate. In 2001, a very dry year, results were
opposite: incidence was greatest when no fertilizer was applied and decreased
as N rate increased. Additional N in 2001 appeared to delay flowering, moving
it into the drier part of the summer.
The limited variation of sclerotinia stem rot between upper and lower slopes
may have been because the primary inoculum, transported by air movement, was
distributed across the landscape uniformly, and because canopy density on both
lower and upper slopes was similar in years of good rainfall, explains Kutcher.
Fungicide application significantly reduced sclerotinia incidence in three of
four years. However, because the impact of the disease on yield at low disease
levels is limited, the benefit of fungicide treatment for sclerotinia stem rot
control was also limited. As a result, fungicide treatment to control sclerotinia
stem rot based on topographical variation was not warranted in this study.
"The principles of precision farming may still apply to sclerotinia control.
In areas of heavier pressure, you could expect more in the low spots if those
areas develop a heavier crop canopy than the upper slopes," says Kutcher.
"You may be able to target those areas with a fungicide, if it appears
that the upper slopes are less affected. But under conditions similar to this
study, targetting fungicide to slope position would have little benefit."
Using a disease prediction model may help to determine if the upper slopes
are less susceptible to sclerotinia than lower slopes. But with the lack of
solid scientific evidence for applying fungicides based on slope position, the
practice can only be suggested on an experimental basis.
Blackleg developed differently on upper and lower
Where Kutcher did see a difference was in blackleg disease development. Incidence
of blackleg usually increased as N rate increased and the disease was associated
with the upper slope position in two of four years, although the differences
were small. However, with the development of blackleg resistant varieties that
are on the market today, fungicide application is not generally required.
Looking at N rates, Kutcher found, as other studies on canola have, that for
optimum yield, lower slopes would require somewhat less fertilizer than upper
slopes: 63 pounds of N per acre for lower slopes versus 78 pounds of N per acre
for upper slopes, on average over the four years.
The lower slopes likely had greater availability of soil-N from mineralization.
The differences in yield did not appear to be due to differences in precipitation
in years of 'normal' rainfall. Kutcher says the results are consistent with
the general trend of relatively better soil productivity at lower than upper
The difficulty in deciding what to do in any one year lies with weather variability.
Kutcher says upper slopes in this part of the Parkland can be more productive
than lower slopes in wet years. That makes deciding where to put nitrogen fertilizer
inputs difficult. "It comes down to predicting the weather," says
Kutcher. "There are many other variables that impact crop yield besides
Likely, the best place to start is with a careful assessment of soil moisture
conditions in the spring. After that, a good understanding of field topography
and landscape soil fertility will be the best guide to developing an agronomic