Plant Pathology
Factsheet

PPFS-AG-F-2

RISK FACTORS FOR SCLEROTINIA CROWN AND STEM ROT IN
FALL-SEEDED ALFALFA

Alfalfa seeded during late summer is susceptible to a destructive disease-Sclerotinia crown and stem rot (SCSR). Fall-seeded stands often have not had sufficient time to develop adequate resistance by the time infectious spores are produced in late October. In contrast, spring-seeded stands have developed sufficient resistance to withstand attack should spores of Sclerotinia trifoliorum be present in the field. (This is not the same species as that which attacks tobacco in float beds, canola, or vegetable crops; that one is Sclerotinia sclerotiorum.)

Recognize that almost all commercial alfalfa varieties are highly susceptible to SCSR for the first few months of growth. Even varieties reported to exhibit partial resistance in other states have usually not shown any resistance response under Kentucky conditions. (In one UK test, Cimarron VR exhibited a small but statistically significant improvement in ground cover over other cultivars after the first year of growth. However, UK research has shown clearly that no variety will provide acceptable control of SCSR under high disease pressure, including Cimarron VR.)

This disease can be so destructive that growers who have had wipeouts in fall-seeded fields often forsake fall seedings in the future. On the other hand, many growers successfully seed alfalfa this time of year, and don't understand all the fuss about Sclerotinia. What accounts for the divergence of experience?

It should be understood that plant diseases only develop when all three sides of the disease triangle are in place: a susceptible plant, a virulent pathogen, and a favorable environment. Producers who have successfully fall-seeded alfalfa have not had all sides of the disease triangle in place in their fields, often because S. trifoliorum is absent or present at levels that are too low to cause damage.

If one has a history of successfully fall-seeding in a particular field, there is probably minimal risk in doing so again, assuming the same cultural practices are being followed and the disease has not developed to high levels in neighboring fields. However, even a history of previous success is no guarantee, because the farmer may be planting alfalfa into a highly infested pasture for the first time. Furthermore, season-to-season variation in disease security is quite high, which means that the disease can be severe in a field one year and mild the next. In any case, if the farmer has little experience with fall seeding, he/she should consider the following risk factors relative to this disease.

1. Cropping history of site. This fungus only attacks forage legumes, so sites with a long history of row cropping are likely to have low levels of the fungus. However, I emphasize the word "long". This fungus can survive in the soil at high levels for 5-6 years without any forage legumes. Also recognize that the fungus can maintain itself indefinitely on volunteer clovers in a pasture.

2. Time of seeding. Seeding by mid-August is preferable to seedings in late August or September, because this gives plants more time to develop some natural resistance. Of course, spring seedings have very little risk of the disease.

3. Tillage. I don't recommend plowing as a disease control practice, since soil conservation is important to us and future generations. However, moldboard-plowed fields have the least risk, followed by reduced tillage, with no-till seedings having the most risk (assuming the fungus is there). Plowing buries the survival structures (sclerotia), reducing spore levels in the fall.

4. History of adjacent fields. Even if SCSR has not been observed in the field being sown, the field may still be at risk if the disease has been detected in adjacent fields. Adjacent fields can be a source of airborne spores for the newly seeded fields. Also, the detection of SCSR in adjacent fields suggest that local conditions have allowed SCSR outbreaks in the past and may allow them in the future.

5. Field size/shape. Let's assume, for example, you are going to seed a 2-acre field surrounded by pasture. The field has been planted to tobacco for the past 10 years, and was plowed and disked to create a good seedbed. What's the risk of Sclerotinia? Keep in mind that this is an airborne fungus. The pasture surrounding the field could easily provide enough inoculum to cause a serious outbreak. The larger the field, and the less border it shares with possible sources of inoculum, the less the risk.

Written: Aug 1993

Revised: Oct 2000

Reviewed by: WCN

Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!

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