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Managing Sclerotinia

Monday, March 28, 2011
filed under: Disease

Sclerotinia continues to be a serious challenge for most broadleaf crops — and sunflower is certainly no exception.

In many areas of the Upper Midwest, the rotation scheme of crops like soybean, sunflower, canola, edible beans, peas and lentils has narrowed considerably. All these crops and many weed species are susceptible to the disease. The disease produces sclerotia that drop to the soil and can persist for numbers of years. Under the right weather conditions, the hard-bodied sclerotia produce spores that can land on susceptible plants and begin a new disease cycle.

Great strides are being made in breeding Sclerotinia resistance in sunflower and other crops. New fungicides scheduled for 2012 introduction will add another important dimension in protecting crops against this difficult disease.

A strategy that has been around for some time is to reduce the sclerotia load in the soil. This, of course, is why crop rotation has been stressed over the years.

Enhancing the natural degradation of the sclerotia load can be accomplished by adding a natural fungus to the soil. Contans® is a biological fungus that is labeled for this use and has been available for several years. Dr. Luis del Rio, plant pathologist at North Dakota State University, has conducted research on the use of Contans and provides the following insights. — Larry Kleingartner

Under what field/disease conditions would you recommend that a farmer consider using Contans?

Let me start by indicating that for disease to occur, three factors have to be in place. The conjunction of these factors is called the “disease triangle” — a susceptible host, in this case sunflower; a pathogen, in this case Sclerotinia sclerotiorum, in the form of sclerotia in the ground and/or ascospores in the air; and favorable weather.

When talking about weather, I am referring to conditions that prevail during a period that starts approximately three weeks prior to the opening of the first flower and lasts until the end of the flowering period. Under appropriate conditions, sclerotia will take approximately three to four weeks to produce an apothecium and start releasing ascospores.

In the absence of appropriate weather conditions, sclerotia may not produce apothecia but will remain in the field in a dormant state. If conditions improve later on, the sclerotia could germinate and release spores after the flowering period is over; or it may simply remain dormant until the next growing season.

Also, it is important to keep in mind that sclerotia act as a seed that can germinate either directly producing hyphae when sunflower roots are in close proximity to them or by producing apothecia. Direct germination could result in wilt of sunflower plants, whereas production of apothecia could result in stem rot or head rot. To produce an apothecium, however, sclerotia have to be located in the upper 2” of soil. Sclerotia located deeper than that will either not be stimulated to produce apothecia, or if it is, it would not be able to break through — just like seeds that have been planted too deep.

Back to the question.

The active ingredient of Contans is Coniothyrium minitans, a fungus that feeds on the sclerotia of Sclerotinia sclerotiorum, the causal agent of head rot and Sclerotinia wilt of sunflower, white mold of dry bean and soybean, and Sclerotinia stem rot of canola, among many other crops. These organisms follow a predator-prey population dynamics, where the predator is C. minitans and the prey is the sclerotia of S. sclerotiorum.

Sclerotia are survival structures produced by Sclerotinia on infected tissues. On sunflower plants, sclerotia can be produced on the head if the plants have sunflower head rot and in the stems if affected by Sclerotinia wilt. Both diseases are caused by the same organism.

The highest impact of Contans will be obtained when applied in fields that have abundant sclerotia, i.e., a field where the current crop was hard hit by Sclerotinia, or fields with a history of Sclerotinia problems in previous crops. Contans will feed on sclerotia independently of the weather conditions above ground. That is why a Contans application will not go to waste even if dry and warm weather prevents the disease from showing up.

For the farmer who has had a significant disease event, should he fall-till or leave the sclerotia on the soil surface?

The benefits of incorporating sclerotia into the ground will depend on the characteristics of the soil and the length of the rotation practiced. In general, soils rich in organic matter will harbor a wide variety of microorganisms, some of which could antagonize or feed on sclerotia. Light incorporation of sclerotia into this type of soil could help speed up their degradation by other microbial organisms, though “speeding up the process” could mean most sclerotia will be dead within the next four years rather than within the next six years.

Deep incorporation of sclerotia into this type of soil could actually help sclerotia survive, since most microbial activity will be concentrated in the upper 6” of soil. In soils not so rich in organic matter, leaving sclerotia on the surface may expose them to rain and solar radiation. The repeated moistening and drying of sclerotia will create microscopic wounds on their surface. Bacteria and other organisms will penetrate through these openings and destroy sclerotia over time.

If Contans is to be applied, growers will obtain its maximum effect if they spray when sclerotia are still exposed on the soil surface. A light harrowing of the upper inch of soil immediately after application of Contans will cover the treated sclerotia protecting the spores of Contans from sunlight and helping them germinate. The incorporation of Contans in the top layer of soil will help reduce the amount of sclerotia that are able to produce spores.

Should he apply Contans on that soil surface in the fall or can he wait until spring?

To obtain the best effect, Contans applications should be made when sclerotia are exposed on the soil surface. Once sclerotia are incorporated into the soil, whether because of tillage practices or wind and rain activity, their chances of escaping infection by Contans spores increases.

After getting in contact with sclerotia, spores of C. minitans (Contans) will penetrate and start feeding on it. Under appropriate soil temperature and moisture conditions, infected sclerotia will be destroyed within three to four weeks. Thus, applications prior to planting in the spring are beneficial in reducing sclerotia, although a fall application would allow growers a wider window of application.

The bottom line is that growers can decide which method to use based on their tillage practices. In either case, they must keep in mind that a shallow incorporation should be conducted after application. This incorporation will help control the sclerotia that are most likely to produce spores.

Should he do a light till to incorporate the Contans, or will a rain event accomplish the incorporation?

Working the soil surface after a Contans application will protect the Contans spores from UV radiation, contribute to moisten sclerotia, and will in general help Contans infect sclerotia. While rain could incorporate the spores into the ground, too, there is no guarantee that a good rain will occur soon after the application.

What rate do you recommend, and are there any compatibility issues with tank mixing other pesticides or water quality issues to be considered?

The recommended dose ranges from 1 to 2 lbs/ac, depending on time of application (heavier in spring, lighter in fall applications). Soils with high pH should receive the heavier doses as well.

Few studies have been made on compatibility studies, but it is known that Contans can be mixed with a number of herbicides. It is recommended that once a herbicide is mixed with Contans, the mixture should be applied as soon as possible. As most labels suggest, a compatibility test is usually advised before mixing two products. When a herbicide is in question, check with your supplier on the compatibility.

After applying Contans, can a producer expect 100% control of the sclerotia?

How long will it take?

Expectations of 100% control are unrealistic. Even the best foliar chemical fungicides can only reduce disease intensity by a bit more than one-half in the presence of high disease pressure. However, multiple applications over time will help reduce the sclerotial population in the ground and to keep it low.

U.S. studies indicated the application of Contans could result in an average reduction of more than 80% of sclerotia, whereas a five-year study conducted in The Netherlands in the early 1990s indicated repeated applications of C. minitans could reduce the number of apothecia in fields by 90%.

Can Sclerotinia spores from an adjacent field infect my crop?

Yes, spores of Sclerotinia can be transported by wind quite some distances. However, the distance at which they could still cause economic damage — i.e., “effective spread” — varies with crops and has not been established for sunflower.

In North Dakota, Sclerotinia stem rot on canola and soybean drops in intensity by 50% within 90’ from the border of a field. At that distance, you will still see a few plants get infected, but a fungicide application will cost you more than what you will get in return.

Research is needed to establish effective spread distances for sunflower. But two traits of sunflower plants make us suspect it would be much longer than that of canola or soybean. These traits are (1) the production of a single head per plant and (2) its position above the canopy.

Having a single head per plant drastically limits sunflower’s ability to compensate for yield when a head becomes infected. Other crops, like canola and soybean, have a much better ability to compensate since they can produce higher number of pods or increase the number of branches.

The position of the sunflower heads on the plants and their orientation make them act like a “catcher’s mitt” that could collect airborne spores. Contrary to a canola or a soybean plant where, after infection, the fungus has to travel down a petiole and into the stem, once Sclerotinia lands on a sunflower head, the fungus does not need to travel much anymore. All the food that it needs is right there. So the possibility of producing economic damage is higher on sunflower.

In these times of high crop prices, does it make economic sense to use a tool like Contans?

It makes absolute sense. The best strategy to manage monocyclic diseases, like those caused by Sclerotinia in canola, dry bean, soybean and sunflower, is to attack the source of the inoculum.

Sclerotia produce apothecia, which in turn produce the spores that will infect plants. A single apothecium can produce a few million spores over a period that could last up to 10 days. Destroying a single sclerotium will take those millions of spores out of the equation — and by thinning down the spore concentrations that could become airborne, also reduce the effective spread distance.

In conclusion, can you recommend five (or more or less) points to follow after a farmer has had a serious outbreak of Sclerotinia?

1. After a serious outbreak, apply Contans before tilling the field. Lightly incorporate the residues after the application.

2. Avoid planting sunflower or any other Sclerotinia alternate host the following two years. Planting wheat or another cereal is the best option.

3. Prior to coming back to that field with sunflower, apply Contans either in the fall or the spring prior to the new sunflower planting.

4. When planning your rotations, try to organize the schedule in a way that fields that are being planted this year with sunflower will not be to the side of a field that was planted to it last year. If necessary, talk with your neighbors to try to coordinate plantings.

5. Contans, like many other management strategies, should not be considered a single-bullet solution to the Sclerotinia problem, but rather part of a larger plan that combines efforts in different areas.
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