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return to Farmers
Independent Weekly
July
11, 2002
By Lakhdar
Lamari, Department of Plant
Science |
Leaf
spot diseases of wheat in Manitoba: They were not always here!
For
those who farmed the land in Manitoba in the sixties and seventies,
talking about leaf spot disease must bring up memories of
disease-free, green fields extending over the horizon. Back
then, scientists had finally put the cereals rusts in their
place and the future looked very bright.
That is,
as long as energy prices were low. The "oil shock"
of the mid seventies made soil-conservation practices attractive,
in that they required lower inputs. The implementation of
minimum- and zero-tillage, in addition to a reduction in stubble
burning, led to better soil protection and greater stubble-retention.
However,
success does not come cheap. A new disease emerged at about
the same period in the major wheat-growing areas of the world
and rapidly became destructive. Tan spot of wheat, a disease
of until now negligible importance, had come of age.
Several
studies conducted in the USA, Australia and Canada suggested
that the rise in tan spot was intimately associated with stubble
retention. Unintentionally, farmers were depriving the pathogen,
for centuries, of its overwintering habitat through crop rotation,
deep ploughing, stubble-burning and summer fallow.
While
this is true, one should not overlook other factors such as
the introduction of tan spot susceptible cultivars. For instance,
Australian researchers found that cultivars released in their
country before the 1960's were, generally, more resistant
than the more "modern" cultivars. If stubble alone
could explain the rise of tan spot in Manitoba in the seventies,
why did it take another two decades for our second leaf spot
disease to show up?
Septoria
leaf blotch, caused by a stubble-borne fungal pathogen, appeared
in 1990-1991 in Manitoba and has been ever since one of the
most significant foliar pathogens in our region. Why didn't
it come at the same time as tan spot? While the stubble buildup
can be implicated, it does not explain it all.
We are
currently painstakingly sorting out the facts from the myths
as we attempt to provide a sustainable solution to these two
leaf spot diseases. We have made considerable progress in
our understanding of these diseases and can now develop genetic
resistance. It is just a matter of time and resources.
So, next
time you visit your wheat field, do not see it only for what
it is at the moment. Diseases are dynamic and have histories.
We can influence, and have generally influenced, the behavior
of plant pathogens over the years, centuries and millennia.
Today, the challenge is to be a "positive" influence.
 How
about some practical stuff! Tan spot can be seen very early
in the season, in fields with infested stubble, provided that
there is sufficient moisture such as rains or dews. It is
not uncommon to see tan spot at- or before- the tillering
stage (Figure 1). The overwintering structures of the fungus
appear as tiny black balls, firmly attached to the wheat straw
(Figure 2). The mostly oval-shaped lesions are tan in color
and have, generally, one small dark spot in the center (Figures
1 and 3). The disease favors high moisture as the pathogen
requires free water on the leaf surface to infect. High temperatures
(> 27 0C) can stop disease development. It is not uncommon
to see a field "blasted" in June and totally recovered
in July, if the weather turned hot and dry. For this reason,
it is not advisable to rush into fungicide sprays.
Septoria
leaf blotch, the newest major leaf disease of wheat in Manitoba,
is generally seen later than tan spot (July) and can be recognized
as rectangular and speckled lesions (Figure 4). The "speckles"
(Figure 5) are little black sacs containing fungal spores.
Septoria leaf blotch has a unique "look" among the
many foliar diseases found in Manitoban wheats. If the lesions
are rectangular and have a speckled appearance, the disease
is definitely not tan spot (never speckled) and you can almost
be sure that it is septoria leaf blotch (Figures 4 and 5).
Tan spot
and septoria leaf blotch, both caused by stubble-borne pathogens,
are controlled in a similar fashion. For the moment, the easiest
method of control is the elimination of stubble through crop
rotations, away from cereals for at least three years.
Fungicide
control is effective but costly. It should be weighed against
the returns and the real risks of yield loss. If it is necessary
to spray (i.e. good yields forecasted, evidence of heavy infestation
in your field, high moisture levels, and "good"
wheat market prices), protecting the flag leaf should be a
reasonable goal for a one-time application. A good question
for a Manitoba farmer should be "do I really need to
spray?" and not "when do I spray?"
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