sorghum ergot: new disease threat to the sorghum industry · where the disease is already present,...

What is it?Sorghum ergot is a disease causedby a fungus (Claviceps africana) thatinfects the ovaries of sorghumflowers and often converts theminto a white, fungal mass(sphacelia). The most obviousexternal symptom of infection isthe abundant exudation frominfected flowers of an amber-col-ored, sticky fluid, or “honeydew,”which often drips onto the leavesand soil. Spores of the fungus arecontained within the honeydew,and when these germinate theyproduce secondary spores on thesurface of the honeydew, giving it awhite-scum to powdery appear-ance. Wind rapidly spreads thesesecondary spores over long dis-tances. The fungus also can bespread by seed contaminated withsphacelia or honeydew. Under cer-tain conditions, Claviceps africanaproduces very durable, compactfungal structures called sclerotia.

Where is it? Sorghum ergot has existed in Africafor many decades. It appeared inBrazil in 1995, and since then it hasquickly spread throughout sorghumproduction areas in the Americas.It was first detected in the UnitedStates in Texas in 1997. During the1997 season, sorghum ergot spreadfrom the Lower Rio Grande Valleyto the High Plains of Texas, andinto Kansas, Nebraska, NewMexico and Georgia.

How Are DifferentTypes of SorghumAffected?Sorghum ergot infects only unfertil-ized ovaries. Once fertilized, anovary becomes resistant to infec-tion. The mechanism of this resis-tance is uncertain, but the resis-tance is highly effective. Any condi-tion that prevents or delays fertil-ization increases the risk of ergot.Sorghum plants with inherent malesterility or with pollination difficul-ties caused by cool temperaturesare most severely affected by ergot.

Because of their high self fertility,grain sorghum hybrids should sus-

tain very little damage from ergot,except where cool temperaturescan predispose the flower to infec-tion. Cool temperatures (averagedaily minimum below 55 degreesF) occurring 3 to 4 weeks beforeflowering inhibit pollen develop-ment, while cool temperaturesoccurring at flowering to 5 daysthereafter can delay fertilization byslowing pollen tube growth. There-fore, grain sorghum maturing dur-ing the cooler temperatures ofautumn is at greater risk of ergotthan sorghum maturing duringsummer months.

Male-sterile sorghum (female “A”line) used in hybrid seed produc-tion is at significant risk because ofa delay in pollination, since thefemale flower receives pollen froma different plant. Poor pollinationcaused by cool temperaturesincreases chances of ergot infectionin normally resistant commercialgrain sorghum hybrids. Highhumidity and moist conditions alsofavor ergot infection. Foragesorghums vary in their levels of fer-tility. Some forage sorghums, espe-cially sterile hybrids, are very sus-ceptible to ergot because they oftenproduce little or no pollen andtillering provides a succession ofsorghum flowers that can be con-ducive to the rapid and prolongedbuildup of inoculum. Johnsongrassis a host of the sorghum ergotpathogen and likely serves as anoverwintering source of ergotinoculum in south Texas.

E-4616-98

Sorghum ErgotNew Disease Threat to the Sorghum Industry

*Extension Plant Pathologists, The TexasA&M University System.

Newly formed honeydew drippingfrom an infected panicle.

Joseph Krausz and Thomas Isakeit*

Ho w is it a Threatto Texas?Each �o wer infected with ergotrepresents a direct loss of one seed.Additional losses occur because thestickiness of infected panicles inter-feres with the harvest of healthyseed and favors the growth ofsaproph ytic fungi, which can a�ectseed quality . A high incidence ofergot in sorghum forages also canresult in abundant amounts of hon -eydew that can interfere with har -vest by “gumming” the equipment.

Seed production �elds are very sus -ceptible to this disease . Texas pro-duces appro ximately 90 percent ofthe h ybrid sorghum seed planted inthe United States and 45 percent ofthe w orld supply . To protect thisseed supply , seed production �eldsmay need to be treated with afungicide and additional pollinatorrows may need to be planted .These measures will add to the costof h ybrid seed production.

E vidence to date suggests that C.africana , the sorghum ergot fungusin the United States, has little or notoxicity to livestock, especially atthe lo w quantities that might beconsumed if good disease manage-ment practices are employed.Ho wever, the presence of ergot ongrain sorghum may result in regula-tory restrictions on grain shipmentto other nations .

What to Do?The following practices can mini -mize the development of ergot andlimit its impact:

1. Time planting to avoid lowevening temperatures (below 55degrees F) during the period 3to 4 weeks prior to �oweringand from �o wering to 5 da ysthereafter . The former inducespollen sterility and the latterslows pollen tube growth. Bothsubject the �o wer o vary to therisk of infection.

2. When planting inergot-free areas , useseed treatment fungi -cides such as captan,thiram or Maxim®.Where the disease isalready present, or ifthe seed has beenstored for 3 or moremonths prior toplanting, seed treat -ments are of muchless importance thanother practices forergot management.

3. After harvest, disk the �elds toprevent sorghum ratoon andsorghum v olunteer de velop-ment. The ergot fungus canmaintain itself on sorghumratoon and volunteer plants notkilled by winter freezes.

4. Manage Johnsongrass withinand around the borders of the�eld .

5. Harv est forage sorghums priorto heading.

6. In h ybrid sorghum seed produc-tion, use an appro ved fungicideaccording to the label. Also usethe most e�ective pollen man-agement practices . It is not nec-essary to use fungicides forgrain sorghum production.

More information on sorghum ergot can be foundon the internet at:

(http://www .cgiar .org/icrisat)(http://www .ars .grin.go v/ars/SoAtlantic/Ma yaguez/sorghumne ws.html)

(http://www .agr.state .tx.us/comm/pr112.htm)(http://cygnus .tamu.edu)

(http://primera.tamu.edu.pubs/ERGOT.HTM)

White secondary sporulation of the ergot fungus on the surface ofhone ydew.

White secondary sporulation on honeydewthat dripped onto leaves.