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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

disease pathogen.

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

used interchangeably.)

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

“comfort level.”

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

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