November 13, 2007
By Ralph Pearce
Disease can have high impact on yield.
Despite the shortfall in planted acreage for Ontario winter wheat in 2006,
the general trend in the next few years is to continue increasing the number
of acres of wheat. High prices in the short-term are a significant part of that
trend, as is Ontario's reputation as a supplier of specific classes of wheat
both at home and abroad. If these two developments hold up, growers will likely
have to learn more about dealing with take-all, a fungal disease that arguably
has the greatest potential to impact yield given its effect on wheat heads and
overall plant health.
Although it has been mentioned only recently at meetings, crop tours and in
agronomy reports, take-all is not new to wheat fields. In fact, when Ontario
had more forage acres, take-all was commonly seen after forages. The disease
complex that causes take-all in wheat lives on other species including most
grasses. A soilborne fungus, it is a specific pathogen that infects through
the roots and can limit the plant from tillering at an early stage, although
the tell-tale symptom in its final stage is a bleaching effect on the heads.
Lower levels can reduce yields without the bleached appearance. Growers often
mistake take-all for fusarium, yet the heads are largely empty with take-all
compared to the tombstone kernels that develop with fusarium. Take-all also
tends to affect smaller patches in the field, with high infection rates in those
patches; fusarium can devastate whole fields, as was seen in 1996.
Expecting higher acreages and increased wheat production will help create a
more suitable environment for take-all. "We're seeing more fields and more
areas of the province where wheat was planted every five or six years and are
now seeing wheat every third year," says Albert Tenuta, field crops pathologist
with the Ontario Ministry of Agriculture, Food and Rural Affairs. "In some
cases like 2007, there are areas where wheat followed wheat and that's where
you'll see it more."
Roots become shiny black
In addition to the bleached heads, take-all affects the roots as well, displaying
a dark brown or shiny black appearance on the base of the plant in the basal
root area. Depending on the severity of the infection, that black layer can
develop extensively on the bottom of the plant. Since it is a fungal pathogen,
the mycelium can overwinter on plant residue, which makes tillage an important
"Another contributing factor is the weather; conditions such as the past
few springs where we've had cycles of warm and cold favour the fungus,"
explains Tenuta, adding that soil temperatures ranging from 10 to 20 degrees
C are optimum. "Moisture is another key to disease development. In some
parts early on, we had good soil moisture conditions which would have helped
The best news Tenuta can provide is that despite the projection that take-all
will continue to increase in frequency, it is controllable through maintaining
the standard corn-soybean-wheat rotation.
Pat Lynch agrees with Tenuta's control methods, but emphasizes that take-all
has been around for a long time, in spite of recent mentions and references.
"To me, it's one of the biggest limiting factors in worldwide wheat production,"
says Lynch, the Stratford, Ontario based field agronomist with Cargill. "When
they talk about the theoretical things we could do to increase yield, if we
could come up with a cure for take-all, our yields would increase. Our current
controls consist of some varietal control and seed treatments, but the seed
treatments really only protect the seed, not the seedling of the plant."
Like Tenuta, Lynch says corn will break the cycle of take-all in wheat, and
points to areas of southwestern Ontario like Essex County where soils might
be considered too heavy for corn production. "Growers there will grow soybeans
and wheat and those fields are where this root rot complex will be worse,"
he says, adding that take-all is generally confined to wheat and not the other
Lynch also cautions that take-all could enhance the severity of barley yellow
dwarf virus, a disease that is also appearing with greater frequency. "I
believe we have a lot more barley yellow dwarf in these fields in 2007, showing
up as these yellow leaves, but I believe there are a lot of fields out there
with low levels, and they're not going all the way up to higher leaves,"
states Lynch. "But when you have background diseases like take-all, the
barley yellow dwarf is going to be showing the symptomology easier, better,
sooner and more."