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Sclerotinia Head Rot of Sunflower
Biology of Sclerotinia
Sunflower Wilt
Sclerotinia overwinters as sclerotia (the hard black bodies formed in stalks and heads) in the soil and in plant debris. In summer when the growing sunflower roots come in contact with sclerotia, the sclerotia germinate and infect the roots. The fungus grows upward in the infected root, into the tap root, and forms a canker at the base of the plant. The plant wilts and dies. This is called Sclerotinia wilt or stalk rot. Adjacent plants in the row may be infected through root to root contact. Sclerotinia wilt can occur any time that sunflower is planted into an infested field. Its development is independent of weather conditions, with infection occurring at almost any soil moisture capable of supporting sunflower growth.
Sunflower Head Rot
When excessive rainfall occurs, resulting in saturated soil, the sclerotia germinate to produce tiny mushroom-like bodies the size and shape of golf tees. These bodies, called apothecia, liberate millions of airborne spores. The spores infect sunflower during wet weather at flowering time producing middle stalk rot and head rot. The spores do not infect healthy plant tissue, but need dead plant matter as a food source in order to begin growth. Dead flower parts are the preferred food source. Once growth is established, the fungus produces oxalic acid, which kills plant tissues. It also produces extracellular enzymes which dissolve and digest plant tissues. By this means the Sclerotinia fungus attacks the healthy green plant tissues. There has been much debate about whether hail or other wounds favor Sclerotinia infection. The consensus of opinion in the Plant Pathology Department is that hail wounds may favor infection. These wounds may not produce much dead tissue, but wounds on the back of the head are sites for the collection of water and flower parts, favoring infection.
Where do the spores come from?
Recent NDSU research indicates that the half life of these spores under normal atmospheric conditions is about 12 hours. Thus, spores may be blown from infested fields to non infested fields. In some cases, spores may be blown many miles down wind, causing infection in areas previously free of Sclerotinia. This also explains why growers who have had good crop rotations and have not had problems with Sclerotinia may suddenly have a serious problem with Sclerotinia head rot. In response to the question "What did I do wrong?", the answer is "Nothing".
Sclerotinia Management in Infested Fields.
Outbreaks of head rot may result in infestation of many fields in an area that previously was free of Sclerotinia or that had only low levels of Sclerotinia. These infestations pose a threat to other susceptible crops grown in subsequent years, if wet weather occurs at flowering. They also pose a threat to sunflower any time that sunflower is planted in an infested field, since Sclerotinia wilt may occur any year, regardless of weather conditions. This necessitates special precautions with sunflower, and this crop should not be planted in badly infested fields for at least 5 years. Broad leaved crops that can be grown in rotation with crops that are highly susceptible to Sclerotinia include flax and semi-leafless peas.
If sclerotia are buried more than 2 inches by tillage, they cannot produce apothecia in future years. Preferably, burial should be deep enough that shallower tillage in future years will not return the sclerotia to the soil surface.
Managing Sclerotinia Head Rot in Sunflower
1. Hybrid Selection
Research data on Sclerotinia stalk rot and head rot is gathered at various research sites in North Dakota. Not every year has significant head rot and data is not available for all years. Bulletin A-652, "North Dakota Hybrid Sunflower Performance Testing" generally will include available data on hybrid performance and tolerance ratings for stalk and head rot. The mechanism of tolerance for sclerotinia stalk rot is different than for tolerance to head rot. Even though a hybrid may be tolerant to stalk rot it may not be tolerant to head rot. Using the most tolerant available hybrids for head rot may help reduce the amount of sclerotinia produced in the crop.
2. Rotation
One should always plant sunflower with an interval of four years from a previous sclerotinia susceptible broadleaf crop. Rotation is not as big a factor for reducing head rot as for reducing stalk rot. Spores that blow in from some distance can cause head rot on sunflower, even though it may be planted in a field with no broadleaf crop history at all. Rotation will not prevent head rot in sunflower but will be beneficial for other reasons.
3. Desiccation of Mature Sunflower
Desiccants are available for confection flowers to kill and speed dry down of the sunflower plant and seed. Use of desiccants is not common but can be useful when the crop is physiologically mature and when earlier than normal harvest would be advantageous to the grower. When head rot of sunflower is present early dry down of the plants may slow or stop the development of head rot and reduce total sclerotia and destruction of seeds. This is somewhat dependent on weather following desiccation. Wet weather following desiccation may increase the problem in desiccated sunflowers as compared to flowers that are still green. White mold does attack dead tissue and while little data is available, grower experiences have indicated an enhancement of head rot when wet weather follows desiccation as compared to green fields left undesiccated. One would need to weigh the weather probabilities and the advantage of early harvest compared to the risks of wet weather following desiccation.
4. Delaying Harvest
Development of head rot in sunflower causes the head to lose structure and be subject to break up due to wind, rain and natural causes which causes the seed and head contents to fall to the ground. This, of course, removes damaged seed and sclerotia from the combine. Disadvantages of waiting for head rot to cause disintegration of the head is additional loss of good seed by natural weather conditions and bird depredation.
Delay in harvest is an option mostly considered by confection seed growers because confection seeds processors have very low tolerance for sclerotinia and sclerotinia damaged seeds. Sclerotinia damaged seeds when roasted turn dark in color and taste rancid and bitter to the consumer. Tolerances for dark roast are generally less than 1% of seeds. A grower has an indication of the number of dark roast seeds that might be in a sample by examining samples for off colored or bleached seeds in the sample. Typically dark roast seeds would be one-half to two-thirds the number of bleached seeds in the sample. A grower can get a quick estimate of dark roast seeds by selecting one hundred seeds and counting off colored or bleached kernels and multiplying that number by .5 to .67. If the number of dark roast seeds exceeds one, the sample may not be accepted for human consumption. The grower needs to consult with the processor concerning their specifications and would be advised to take a harvest sample and submit it prior to combining the whole field to make sure that harvested seed is acceptable to the processor. For oil seed growers, harvest delay offers no advantages as generally sclerotinia would be treated as foreign material. There is an expense involved in trucking and handling foreign material.
Bird seed specifications are generally less stringent than those for human consumption. Dark roast seed is not a factor in bird seed markets but the amount of sclerotia is a factor. Generally, levels above 3% are expensive and difficult for processors to work with and levels higher than that may be rejected even for bird food markets.
5. Combine Settings
Adjusting combines will offer only a minimal opportunity to reduce sclerotia in harvested seed. While little work has been done on combine settings, it would appear that for confection hybrids, low cylinder speeds and the most open concave that will thoroughly thrash the seeds would be preferred over high speeds and closer concave openings. If the head is moist and the seeds are above 15% moisture, the head and the receptacle area may not thoroughly disintegrate upon thrashing with low speeds as compared to high speeds and may reduce the amount of sclerotia in the hopper. For oil seed sunflower use appropriate concave openings and cylinder speeds that thoroughly separates seeds from the head at thrashing time. Sieves and wind should be set to reduce the amount of foreign material in the sample without losing seeds.
6. Fungicide Use
No fungicides are currently labeled for use in sunflower to reduce sclerotinia head rot. Data suggests sclerotinia may infect the sunflower head over a long period of time and thus timing and residual control with any product becomes a concern. White mold fungicides are expensive and would need to control the problem to be economically feasible to use. Timing, amounts to be used and effectiveness have not been researched to date and there are no options available for fungicides.
7. Storage
Confection seeds that are rejected at harvest may be eligible for FSA loan. Storage allows the market to sort out needs and quality needs. Storage is an option that may result in opportunities later in the year. It is risky in that market needs may not change over time. Quality factors involved in obtaining a loan need to be understood as they may be a factor when closing out the loan.
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