Building Better Stalk Rot Resistance
When it comes to sunflower diseases, Sclerotinia gets top billing among researchers and producers. Why? Because when ‘flowers get white mold, there is really nothing you can do to protect the infected crop.
Sclerotinia or white mold infects the sunflower plant in three possible ways: root infection, mid-stalk and head. The mid-stalk and head infections occur from the airborne ascospores, which can be abundant when soils are wet to initiate the little mushroom-like “apothecium.” A further requirement for mid-stem or head infection is a prolonged (24-48 hr) period of constant leaf wetness, whether caused by mist, fog or light rains.
Stalk rot (basal stem rot or wilt) occurs when the sunflower roots come into contact with sclerotia in the soil. The fungus grows up the feeder roots to the tap root, and then to the lower stem. A plant with stalk rot usually dies and produces no seed. Additionally, plants with stalk rot are very vulnerable to lodging. Sclerotia bodies, whether they are produced in the head or stalk, drop to the ground at harvest, ready to infect the next crop, which could be dry beans, soybeans, canola or sunflower.
Sunflower is the only commercial broadleaf crop that is susceptible to Sclerotinia root infection. Researchers assume that the sunflower roots give off a particular chemical, which may stimulate the sclerotia to germinate. Tom Gulya, USDA-ARS sunflower pathologist in Fargo, N.D., has developed a novel system to test hybrids and inbreds for resistance to root infection.
He grows the fungus in the lab on millet (several hundred pounds per year) and then knives it into the soil about 8 inches from the plant in the late vegetative stage. This is the second year Gulya has used this system, and he is confident that the plants are being exposed to a considerable amount of the disease.
Seed companies were invited to submit up to seven hybrids for stalk rot resistance evaluation, with at least two of them being their top selling hybrids. In 2005, Gulya had six testing locations throughout the N.D. and Minn. region with each hybrid replicated four times at each site. Three locations yielded excellent results. Those sites are Carrington and Mapleton in North Dakota and East Grand Forks in Minnesota.
Averaging data of all three locations, disease incidence on a per entry basis varied from a low of 10% to a high of 71%. Plot averages were lowest in Mapleton at 24.7% followed by East Grand Forks at 40.9% and Carrington at 49.5%. Even the lowest incidence (Mapleton) was sufficient to statistically distinguish between hybrids.
The results are published in the NDSU Extension sunflower yield results, and available on the NSA website, www.sunflowernsa.com under the grower link.
“We know that some hybrids are more resistant to this disease than others, and this is our first concentrated effort to quantify differences among commercial hybrids, a number of which are still experimental,” says Gulya. If a farmer is putting sunflower on last year’s soybean ground or in close rotation with other susceptible crops and is concerned about potential of Sclerotinia stalk rot, then this data should be very helpful in deciding which hybrids to consider.
Gulya points out that this is one aspect of selecting a hybrid, since yield and oil content still are the major factors in hybrid choice. In the 2005 National Sunflower Association crop survey, 90 fields were surveyed for stalk rot. Overall, only 2.4% of the plants surveyed were infected with stalk rot, and the worst field had an infection rate of 26%. That compares to the 2003 survey in which 3.6% of the plants were infected with this disease. Stalk rot did not show up in states other than North Dakota in the 2005 survey, although it is well documented that white mold is present in all sunflower-producing states, where it also affects other broad leaf crops as well.
Gulya notes that genes providing resistance to stalk rot may not provide resistance to head rot, and vice versa. Gulya entered the 20 best hybrids from the 2004 stalk and head rot trials for re testing this year. Ten of last year’s ‘elite’ hybrids are in the top 1/3 of the 2005 stalk rot trials, showing that the results are consistent from year to year. A USDA example is HA412 X RHA409 developed by USDA-ARS geneticist Jerry Miller. These two USDA parental lines were both selected specifically for stalk rot resistance, and with genes in both parents, the resultant hybrid is quite resistant, coming in sixth out of 88 entries.
Gulya advises that if a grower is looking for stalk rot resistance he could choose any of the hybrids within the top 30 entries. “Statistically they would all be regarded as highly resistant,” he says. Some of these hybrids have been in trials for two years and have performed consistently. Most companies have at least one hybrid that fits this category.
It is noteworthy that the former Northrup King Sunbred 277 (a traditional linoleic hybrid) is the resistant check in the stalk rot trial. Averaged over three locations, it was the best entry in the stalk rot. The point is that Sclerotinia resistant genes have been in the private sector and in some hybrids for a long time. Also notable is that a confection is among the top three resistant hybrids.
The challenge for breeders and pathologists is to get as many Sclerotinia resistance genes into a hybrid, and have all of the other desirable agronomic characteristics such as oil, yield, herbicide resistance, as well as resistance to other major diseases such as Phomopsis, downy mildew and rust.
Researchers recognize that Sclerotinia resistance to both head and stalk rot require multiple genes. Breeders are now crossing their best stalk rot parent lines with their best head rot parent. There could be as many as eight or more genes responsible for the resistance. This is compared to a single resistant gene for downy mildew and rust. Getting all of those genes into one hybrid, PLUS keeping yield and oil at a maximum is a daunting challenge for plant breeders.
Concentrated efforts are underway to identify the resistance genes through molecular genetics at USDA ARS in Fargo. However, in the foreseeable future, field plot testing is the only way to determine levels of resistance.
Gulya notes that all of this work, be it head or stalk rot, is being federally funded by the National Sclerotinia Initiative (www.sclerotinia.com). This fund provided by Congress directs dollars for work on this disease for sunflower, canola, dry beans, soybeans and pulses. Significant progress has been made in a few short years. “We will never have total immunity to Sclerotinia in sunflower, but we are getting much closer to some good resistance,” says Gulya. “We want to be able to dodge those catastrophic years when the weather conditions are ideal for Sclerotinia.” – Larry Kleingartner
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