Sorghum ergot is a disease caused by a fungus (Claviceps africana) that infects the ovaries of sorghum flowers, converting each into a white, fungal mass that is visible between the glumes. The most obvious external symptom of infection is the exudation from infected flowers of drops of an amber-colored, sticky fluid, or "honeydew", which contains numerous fungal spores. Honeydew production can be so abundant that it drips onto the leaves and soil. Secondary spores may be produced on the surface of the honeydew droplets giving them a white-scum to powdery appearance. These spores are spread rapidly by wind, although the fungus can also be spread by sorghum seed infested with ergot sclerotia (durable, compact fungal tissue) or coated with dried honeydew containing spores.
Ergot only successfully infects unfertilized ovaries. Once fertilized, an ovary becomes resistant to infection. Sorghum crops most severely affected by ergot are those with male sterility and pollination difficulties (such as those caused by low temperatures below 55 F 3-4 weeks prior to flowering) which extend the time that the ovary remains unfertilized. Commercial grain sorghum hybrids will likely have a negligible incidence of ergot during normal growing seasons in Texas, because their high self-fertility results in rapid pollination, greatly reducing the risk of infection. Male-sterile (female parent) sorghums used in hybrid seed production can be severely infected by ergot. The flowers of male-sterile varieties produce no pollen, so they remain unfertilized long enough to become infected by C. africana. Pollen sterility caused by cool temperatures increases chances of ergot infection in normally-resistant commercial grain sorghum hybrids and in forage sorghums. High humidity and moist conditions also favor ergot infection. Johnsongrass is also a host of the sorghum ergot fungus.
HOW IS IT A THREAT TO TEXAS?
Commercial grain sorghum hybrids probably will sustain little damage from ergot, except in situations where cool temperatures (i.e. night temperatures below 55 F 3-4 weeks prior to flowering) might increase pollen sterility. Moist conditions also favor ergot infection. Grain sorghum grown in the fall may encounter such disease-favorable conditions and experience increased infection. Male-sterile sorghums in hybrid seed production face a significant risk from ergot. Triazole fungicides applied to the heads of flowering sorghum plants in Brazil and other areas have provided adequate control. Commercial seed companies are anticipating the need for fungicidal protection and altered pollination mangaement strategies to address the ergot threat. The use of fungicides on sorghum production for grain is considered unnecessary under normal growing conditions.
Forage sorghums, especially sterile forage hybrids, will likely support considerable ergot infection, because they have no pollen production and tillering provides a succession of sorghum flowers that can provide for rapid and prolonged buildup of inoculum. The high incidence of ergot in these forages can produce abundant amounts of honeydew that can interfere with harvest and support abundant saprophytic fungal growth in moist weather.
Fortunately, evidence to date suggests that the fungus which causes ergot in grain and forage sorghums has little or no negative effect or toxicity on livestock.
WHAT TO DO?
To help prevent the spread of ergot and limit its impact where it occurs, consider the following:
1. Timely planting to avoid low evening temperatures (below 55 F) during the 3-4 weeks prior to flowering and 5 days after flowering. The former induces pollen sterility, and the latter slows pollen tube growth, both allowing the seed to become prone to infection.
2. Sow seed produced in ergot-free areas, and be sure that the seed is treated with captan or thiram fungicide.
3. Maintain good johnsongrass management within and around the border of the field.
4. After harvest, disk the fields to prevent sorghum ratoon and sorghum volunteer development.
5. After leaving a known ergot-infected field where fresh or dried honeydew is observed, change and/or launder clothes and shoes before entering another field. Dried honeydew clinging to clothes could provide for long-term survival and spread of conidia contained within it.
6. With ergot-infected forage sorghums, harvest prior to heading.
7. If ergot is suspected in a field, contact your county extension agent, or bring him a sample in a SEALED CONTAINER to avoid spreading any spores. Also contact Dr. Gary Odvody (512-265-9201), Dr. Tom Isakeit (210-968-5581), Dr. Harold Kaufman (806-746-6101), Dr. Mark Black (210-278-9151), Dr. Richard Frederiksen (409-845-1227), Dr. Joe Krausz (409-845-8001), Dr. Chip Lee (817-968-4147), Dr. Cloyce Coffman (409-245-0877), Dr. Steve Livingston (512-265-9203) or Charles Stichler (210-278-9151).
More information on sorghum ergot can be found on the internet at:
SICNA International Sorghum and Millets Newsletter (good introduction with pictures)
http://primera.tamu.edu/pubs/ERGOT.HTM (has some good pictures)
http://www.ars-grin.gov/ars/SoAtlantic/Mayaguez/USAergot.html