Fungicides to Control Sclerotinia Head Rot
Sclerotinia has long been a thorn — sometimes a very painful one — in the side of many sunflower producers. Though definite progress has been made in the identification and incorporation of varying degrees of hybrid tolerance to Sclerotinia, there currently are no commercial hybrids that can be classified as “resistant” to this complex disease.
While hybrid resistance is the best long-term answer for Sclerotinia, what about the use of fungicides for short-term protection? Are any available? Do they work?
The answer to the first question is easy. No fungicides are presently labeled in the United States or Canada for control of Sclerotinia in sunflower. However, a handful of new products are in the commercial pipeline, with registration within sight.
The second question has been the basis for research conducted in Manitoba and northeastern North Dakota during the past several years. In both cases, these field trials have focused on the head rot form of Sclerotinia.
Khalid Rashid, research scientist with Agriculture and Agri-Food Canada at Morden, Man., has conducted Sclerotinia fungicide trials since 2003. He has tested various experimental fungicides as well as products already labeled for use on other crops and diseases. The take-away message from his evaluations, Rashid says, is that some fungicides do provide a significant measure of Sclerotinia head rot control in sunflower.
As to the number and timing of applications, “two applications — at flowering and 15 days later — are better than either application alone,” Rashid reports, adding that the early application is more effective than the later treatment.
“Some fungicides reduce the head rot incidence by up to 60-70%,” he says, “and reduce the yield losses from head rot by up to 50%.”
Rashid’s 2010 Sclerotinia fungicide trial at Morden was heavily infested with sunflower midge, so yield data were very skewed. However, the trial still revealed useful information on head rot control levels. Several fungicides were applied at manufacturer-recommended rates. Some plots received two applications (once at early flowering and again two weeks later); other plots received a single application (at either early or late flowering). To ensure adequate disease levels, all of the plots were artificially inoculated with ascospores of the Sclerotinia fungus, supplemented with ground Sclerotinia-infected millet seed. A misting system was turned on for three weeks to enhance disease infection and development.
On those plots where two applications were made, “all fungicides used were effective at different levels in reducing the Sclerotinia head rot,” Rashid reports. Some products reduced head rot by 60-90%, compared to the inoculated control plot. Only two of the products improved sunflower yield in the midge-damaged tests, “while in previous years, most of these fungicides improved the yield by 10-50%,” according to the southern Manitoba researcher.
Rashid also observes that the effective fungicides reduce the amount (%) of sclerotia in his trials’ harvested samples. That’s an especially important quality issue in the grading of confection sunflower intended for dehulling or roasting.
About 35 miles to the south of Morden, Scott Halley has been conducting Sclerotinia fungicide trials at North Dakota State University’s Langdon Research Extension Center since 2005. Halley, who has tested a variety of both labeled and experimental products (and different adjuvants), says his research has indicated “there are no ‘silver bullets’ out there.” The best-performing tier of products seems to vary from year to year, with nothing that’s “head and shoulders” above the rest. High-end rates and high water volumes tend to produce the best results, but that brings into play economics and logistics — i.e., how much is a grower willing to invest in fungicide applications, and how much water can the applicator realistically be expected to apply?
The answer to the economic portion of that question, Halley suggests, depends upon crop prices and a given field’s yield prospects. “Where a grower is going to make money [by spraying] is where he has the potential for 2,500 lbs and the disease could take him down to 2,000 or so. That’s where the product will have a fit,” he says. Such a scenario implies that the fungicide application(s) would be made early on, prior to disease infection and progression. Waiting until head rot is established and thriving is too late.
“Environment is the big key,” Halley observes. “If you get the big crop potential and the economics are there where you can afford to spray . . . and you get rain 10 days ahead of flowering, the ground is wet and the sclerotia are germinating . . . that’s where the fungicide will pay.
“To get infection, you need two events: the soil water at the soil surface (for the sclerotia to germinate and expel the ascospores), and that wet or humid period during flowering for the ascospores to cause disease.”
Harvested sunflower yields in the Langdon trials (which, like Morden, are artificially inoculated and kept moist by a misting system during flowering) have typically been quite low when head rot is present — around 500 to 600 lbs/ac. The NDSU plots also have dealt with other detrimental yield influences like insects, blackbirds and deer.
The Sclerotinia fungicide trials will be expanded beyond Langdon in 2011, with additional collaboration at Carrington, N.D., Crookston and Sabin, Minn., and one South Dakota location. Halley says there are several areas of research to be pursued. Fungicide screening similar to what’s been done at Langdon and Morden is certainly needed “to find two or three fungicides that have potential to be labeled and potential to provide consistent efficacy for growers.” Also, “we need to determine how to get the most value for growers. Should we combine two fungicides? Should we make two applications? Do we spray with a higher volume of water?”
For a disease like Sclerotinia — resistance for which encompasses multiple genes — a combination of genetic tolerance along with fungicides hopefully will be the formula leading to significantly less incidence and damage. The fungicide studies to date have provided much valuable information, so researchers should be able to “hit the ground running” in their expanded 2011 testing. — Don Lilleboe
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