By Bruce Barker
A shift in races call for cultural management.
Rust infestations in 2004 were at relatively low levels for sunflower growers
in Manitoba. However, if the weather in 2005 returns to more normal conditions,
rust infestations could return to levels seen in 2002 and 2003. The problem
is that current hybrids have little resistance to the new races that have evolved.
"2003 was one of the most severe rust infestations in recent years,"
says Khalid Rashid, a research scientist with Agriculture and Agri-Food Canada
at Morden, Manitoba. "2004 wasn't as bad because the rust got started later
and we had below normal temperatures, neither of which favoured the development
Sunflower rust is caused by the fungus Puccinia helianthi Schw. and can cause
serious yield loss. Rust increases rapidly during warm, moist weather. High
nitrogen rates and abnormally high plant populations can produce excessive crop
foliage, and may contribute to increased humidity in the canopy, which favours
Rashid says the pathogen is specific to sunflowers and unlike wheat rust, can
complete its life cycle on the sunflower plant. As a result, it can overwinter
on the prairies and produce new races of the pathogen here as well. Rashid has
noticed a shift in races over time.
Prior to 1990, Race 2 was most predominant in the Red River valley. "We
weren't sure if the recent increase in rust was due to new races or just favourable
conditions in 2002 and 2003. Through field surveys, we have confirmed that we
have shifted to races 3 and 4, with variations of those races present as well,"
In 2003, the infection was mostly found in the Red River valley, where surveys
showed that most fields had moderate levels of the leaf area infected, but some
fields had up to 80 percent infection. Information from Manitoba Agriculture,
Food and Rural Initiatives showed that yield losses under heavier infestations
can easily be in the 25 percent range.
Preliminary 2004 survey results showed that 64 percent of the sunflower rust
was a Race 3 or variation and 24 percent was a Race 4 or variation. That compares
with 2003 when 60 percent was Race 3 and 40 percent was Race 4.
Most oilseed hybrid sunflowers are resistant to Race 2 and a few to Race 3,
while most confection sunflowers are susceptible to all races. Provincial variety
recommendations show rust disease rating as the reaction to races 2, 3 and 4
under controlled indoor conditions.
Use cultural methods to minimize impact
Rashid says the best defense against rust is to plant the most resistant hybrid
possible. While many of the hybrids may still be attacked by some of the newer
rust races and their variants, at least the hybrid will be partially protected
against some of the older races.
Crop rotation is also important to reduce rust inoculum. Three years is the
recommended rotation, which is the same rotation used to minimize other sunflower
diseases such as sclerotinia. Infected sunflower stubble should also be incorporated
into the soil.
If possible, sunflowers should not be planted adjacent to heavily infected
sunflower stubble in the previous year. In adjacent or subsequent crops, control
of volunteer sunflower plants will also help reduce the inoculum. While this
helps to a certain extent, the rust inoculum can travel large distances on the
wind and it is not totally effective.
"Even on adjacent fields, the rust inoculum can blow across to sunflower
fields, so volunteers should be controlled," says Rashid.
Fungicide research shows potential
Rashid has conducted fungicide trials on sunflower rust. Currently, none are
registered for use on sunflowers. In 2004, he found Folicur, Stratego and a
new experimental fungicide from Bayer to be the most effective in reducing rust
infestations. For these fungicides, he found that one application significantly
reduced rust infestations and resulted in yield increases. A second application
always gave better control, but "economically, the cost would have to be
evaluated against the increase in yield." Dithane and Headline also showed
potential, but required two applications to reduce rust infestations. Rashid
plans on repeating the trials in 2005 to build the data necessary to draw adequate
conclusions on fungicide effectiveness. -30-