System Will Aid Testing for Head rot Resistance
Wednesday, March 1, 2000
filed under: Disease
You can call her what you wish; just don’t call her “predictable.”
Despite big advances in meteorological science, Mother Nature still
rebels at the insinuation that she can be typecast. And the regularity
with which TV weathermen revise their forecasts is a testament to her
While Mother Nature may scoff at the notion of “environmental
consistency,” many agricultural researchers conducting multi-year
studies do wish, at least for their own purposes, that she would
relent. Certain types of research are often hamstrung by the simple
fact that the growing season environment in regions like the Northern
Plains changes significantly from year to year.
Case in point is the search for hybrid tolerance or resistance to
Sclerotinia head rot in sunflower. For years, breeders and plant
pathologists have been screening sunflower hybrids and parental lines
for tolerance to this troublesome form of this vexing disease.
However, the degree to which head rot “rears its ugly head” in a given
field within a given year is largely dependent upon the type of weather
being experienced. Wet and warm conditions during flowering are much
more likely to result in significant head rot, for instance, than would
a cool and dry climate during the bloom period.
So testing for tolerance to Sclerotinia head rot can become a real
“hit or miss” proposition from year to year. Even in a disease-prone
year like 1999, it’s difficult to determine whether a low incidence of
head rot in a particular field is due to hybrid tolerance as opposed to
planting or bloom date or some other weather-related factor that cannot
be explained by a single year’s experience. Researchers need
multiple-year results before they can attest, with confidence, to a
hybrid’s inherent level of disease tolerance or resistance.
If Mother Nature balks at providing a consistent environment, the
logical response is to attempt to create one artifi-cially. In terms of
effectively screening for Sclerotinia head rot, researchers are now
hoping to use an irrigation misting system to create optimum disease
conditions year after year.
The National Sunflower Association (NSA) is actively supporting
this strategy so that USDA and private sunflower breeders can test their
genetic material. With financial assistance from NSA, efforts are
moving forward to purchase the necessary misting system equipment. The
system would be installed at the North Dakota State University Research
& Extension Center near Carrington.
The key word in this project is “mist.” For Sclerotinia head rot
to develop, sunflower plants need to remain wet for at least seven to 10
days, 24 hours a day. To ensure that the disease is present,
researchers would inoculate the plants with a laboratory-produced
Several such misting systems are already in use for other crops in
the United States and foreign nations. A University of Illinois misting
system, installed to test for soybean tolerance to Sclerotinia, will be
in its fifth year of operation in 2000. At North Dakota State
University, researchers working on scab resistance in small grains found
their misting system to be extremely useful.
Speaking to an audience of sunflower scientists during the January
NSA Sunflower Research Forum, UI plant pathologist and North Dakota
native Wayne Pedersen emphasized that a misting system is essential for
this type of research. “Some researchers have tried sprinkler
irrigation systems and have failed,” he reported. “They either washed
the disease inoculum off the plants or ‘drowned’ the plants with water.”
The Illinois group added sunflower plots into its misting nursery
last year and successfully developed head rot. The UI scientists
inoculated the sunflower plants with mycelia (disease fungi) produced in
the laboratory and then sprinkled by hand onto the plant heads.
Their success is based on the misting process. “The system is
timed so that the plants are covered with a fine mist every 15 minutes,
24 hours a day,” for several consecutive days, according to Pedersen.
While the misting system keeps plants wet, very little water actually
reaches the soil surface. “We can walk in the plots without our shoes
getting muddy,” he notes.
Jose Bruniard, a researcher with an Argentine seed firm and current
Ph.D candidate at North Dakota State University, has extensive
experience in screening for resistance to Sclerotinia head rot in
sunflower. Head rot is a more-consistent problem in Argentina than in
the United States, and seed companies there have been breeding for
resistance for quite some time.
Bruniard concurs that in testing for head rot resistance, it’s
essential to employ a system that keeps the plants sufficiently moist
during the infection period. Artificial inoculation with the disease
fungus also is crucial, Bruniard adds. His company infects the plants
with the same type of ascospores that would cause infection in a
farmer’s field. The ascospores have been produced in the laboratory
from sclerotia bodies. (The sclerotia produce the mushroom-like
apothecia, which in turn are the source of the infecting ascospores.)
Tested hybrids are grouped according to maturity range in order to
remove the bloom date factor when inoculating and determining resistance
Speaking this winter to a group of U.S. sunflower breeders and
other researchers, Bruniard advocated testing finished hybrids first.
“Start from the top and work down,” he suggested. “Some hybrids might
show tolerance even though their parents do not display a similar
Bruniard utilizes the term “resistance,” not “tolerance,” in his
head rot testing program. (Though the word “resistant” implies total
immunity to a disease or pest, “resistance” and “tolerance” are often
He also uses a three-tiered rating system to compare tested hybrids
against a known susceptible hybrid. If, for example, the susceptible
“check” shows a 10-percent head rot infection level, those being tested
would need zero head rot to be classified as having “resistance.”
Should the check hybrid incur 50-percent infection, those being tested
could be no more than five to 10 percent to meet the criterion.
Finally, should the susceptible check suffer 100-percent infection, the
tested hybrids can have no more than 50-60 percent.
Assuming it becomes a reality — and the signs are very promising —
the Carrington misting facility will provide space for any private seed
company to submit hybrids for testing on a fee basis. “We will be
learning as we go forward, testing a number of variables,” says Bob
Henson, NDSU Carrington-based plant pathologist. “But we hope to
minimize errors by learning from the experiences of others.”
For private seed companies and public breeders alike, the
Carrington system will offer the first U.S. opportunity to test their
sunflower hybrids and parental lines on a consistent basis under a
consistent environment. No company is eager to promote a hybrid’s
apparent tolerance based on just one year’s data. At least three years
of reliable test results usually are desired before there’s an adequate
Given the complicated and elusive nature of Sclerotinia, even a
three-year testing period may leave questions in the minds of some
scientists. But it’s a huge step in the right direction — a step whose
ultimate beneficiaries will be those producers who farm in areas where
Sclerotinia head rot can be a serious threat to the yield and quality of
their sunflower crops. — Larry Kleingartner