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Squelching Sclerotinia

Wednesday, December 15, 2004
filed under: Research and Development

After the many bumps and bruises doled out by the 2004 growing season, wet weather favorable for Sclerotinia was nothing less but salt in the wound for some growers in the Northern Plains.

It was a sour reminder that the fungal disease remains a significant threat for a number of broadleaf crops. It frustrates producers and crop scientists alike, since it is a complex disease that has proven difficult to bring under control in the field and in the research lab.

For sunflower, Sclerotinia (referred to as white mold in other crops vulnerable to the disease, which includes canola, soybeans, dry edible beans, peas, and lentils) is multi-faceted. Sunflower plants can contract the disease through root infection from sclerotia (the black hard material produced from a previous infected crop) in the soil and get what is commonly called “sunflower stem rot” or “sunflower wilt.” The sunflower plants that contract the disease in this way often wilt and die around bloom. In most cases entire fields are not devastated by the disease, but infected plants will have smaller heads and often lodge.

The other form of Sclerotinia is head rot, which is a result of the sclerotia producing airborne spores which infect the head anytime from flowering until harvest. Under the right environmental conditions of fog and rain, this form of the disease can be devastating. Sclerotinia head rot has gained the most attention from researchers internationally, and will be the subject of an update in the February issue of The Sunflower.

Stalk rot is also getting attention from researchers, thanks in part to the Sclerotinia Initiative (a federal funding source) and work by USDA ARS sunflower pathologist Tom Gulya and others.

Gulya has been working on Sclerotinia and other sunflower diseases for over 25 years. For stalk rot disease to occur in sunflower, the roots of the plant have to come into contact with a sclerotia body in the soil. Once contact occurs, the sclerotia germinates, infects, and decays the roots. The fungus then grows into the stem of the plant, choking off water and nutrients to the rest of the plant, causing death. New sclerotia are formed at the base of the plant and are returned to the soil.

For Gulya, working on this disease has been challenging and often times frustrating over his years of research. The same variability in soil and weather conditions that makes Sclerotinia development unpredictable in growers’ fields has often made the disease more difficult to study in research plots as well.

“I was always on the look out for fields heavily infested with sclerotia, or we would establish disease nurseries by adding sclerotia to the soil,” he explains. “But these ‘disease nurseries’ usually last about two to three years, and then the soil sclerotia appear to die from naturally occurring mycoparasites, fungi that attack sclerotia. So the disease inoculum builds up in the soil and nature takes its course to bring it back under control. For a researcher, that situation is very frustrating because a year’s worth of research is lost.”

He gives an example of a production field with a high level of naturally-occurring sclerotia that was disappointing in generating the conditions needed to research the disease the following year. Several years ago, a producer planted sunflower on a field that had previously been planted to crambe, a crop that seems to add lots of sclerotia to the soil. By late summer, the sunflower was laying flat with high levels of Sclerotinia stalk rot. The following year, researchers planted sunflower variety trials in that field to test for differences among hybrids and breeding material. However, despite the history of that field – a year of crambe followed by a year of sunflower infected by Sclerotinia stalk rot – the sunflower plants in that third year were hardly affected. “That’s part of the frustration when working with this disease,” says Gulya.

To deal with this inconsistency, Gulya has developed a novel, but simple process to ensure a uniform level of disease in field plots by placing Sclerotinia-infected millet near the roots of the developing sunflower plants. He infects the millet by growing the fungus in the lab for 7 to 12 days. The infected millet is then placed in-furrow next to rows about six weeks after planting.

Further, Gulya and others have designed a tractor-drawn field applicator allowing researchers to conduct much larger and multiple testing sites. A key question for Gulya in using the infected millet, however, was timing of the application, as well as how much millet to put down with the sunflower plants.

“As a researcher, I want to have good disease infection, but I don’t want to overwhelm the plants with disease and kill all of them,” he explains. Over the last two years, Gulya has answered most of those questions, however, and feels comfortable with one application about six weeks after planting. Rainfall or irrigation after the infected millet placement in the soil is important for the infection process.

Measurable progress

Gulya has been testing both commercial hybrids and inbred lines for future breeding. In 2004, Gulya had five locations, from Crookston Minn. to Brookings S.D. Each location had the same 75 hybrids, which included NuSun hybrids, confections and a few conventional oil hybrids. Three of the five locations were lost to either hail, rabbits or downy mildew – not an unusual situation in field testing, and further testaments to the difficulty of achieving useable research data. But the remaining two locations (Carrington N.D. and Breckenridge Minn.) withstood the rigors of nature and have provided good data.

When combining the results of the two locations in 2004, Gulya finds that the percentage of diseased plants in particular hybrids ranged from a low of 16% to a high of 86%. There were eight hybrids that outperformed the resistant checks, and most of these were NuSun hybrids.

For Gulya, the initial look at the results of the two locations points to this method of placing infected millet in the row as having a great deal of potential in rating hybrids for tolerance to stalk rot.

Measurable progress is rewarding to see for researchers and producers alike. “This is the direction we need,” says Don Schommer, a Munich N.D., farmer who serves on the NSA board of directors. “We think it is important for farmers to be able to look at Sclerotinia stalk rot ratings for hybrids as we select which hybrid to plant. Here in northeastern N.D., we have a lot of broadleaf crops in close rotation including canola, edible beans, soybeans and sunflower. The best way to keep sunflower in a rotation is to understand the potential disease resistance of a hybrid before we plant it in a field that may have sclerotia from previous crops.”

USDA geneticist Jerry Miller agrees that now, there are noticeable differences in existing hybrids in terms of resistance to both stalk and head rot. He continues that it is important to able to quantify those differences in reliable field tests. Miller emphasizes that developing total immunity to Sclerotinia might be a tall order, but high levels of resistance is very possible, and indeed, may already be in place in some hybrids. “We just have not had the mechanisms for reliable and consistent testing,” says Miller.

Gulya believes his field tests are a “worst case disease scenario” – evaluations under a heavy line of fire. “We are exposing these hybrids to a high level of disease, something you are not likely to have in a real field situation,” he says.

While the same hybrid under evaluation may have slightly different levels of disease at different plots or different years, Gulya says the rankings of the hybrids should remain relatively constant. Thus, a grower can look at this data and feel confident that hybrid A is more resistant than hybrid B. He cautions, however, that there will always be a variability of hybrid response to disease pressure, given the variability of environmental conditions.

Next year, Gulya’s plans are to conduct at least five replicated stalk rot trials, and hopefully get good results from at least four locations to begin the process of rating hybrids with confidence. “We will also compare the stalk rot and head rot trial results to determine if there is consistency between the two tests,” he says. The plan is to expand the irrigated misting head rot trials to at least five locations in 2005 as well.

Once there is enough data to make valid “statistical comparisons,” the information will be published in the annual university hybrid trial publications, put out by North Dakota and State Dakota Extension Services.

Schommer says that getting a handle on Sclerotinia is the highest research priority within the NSA. “It’s been very worthwhile for the NSA to work with Congress on funding the Sclerotinia Initiative, which has put additional dollars into public research programs focusing on Sclerotinia,” he says. “We know it’s been a difficult disease to study for researchers, but it’s great to see that progress is being made both in the public and private sectors.” – Larry Kleingartner

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