Ergot disease of sorghum is not new to Africa - the causal

pathogen, Claviceps africana Frederickson, Mantle, and de Milliano was first recorded in Kenya as far back as 1923. However, ergot disease only began to gain recognition as a potential problem in sorghum production in the 1960s, when all the A-lines in Nigeria's national breeding program became infected, to the near complete exclusion of seed production (Futrell and Webster 1965)

Frederickson, D E

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A smallholder farmer in Tanzania, one of the earlyadopters of MaciaISMN 41, 2000 7 Macia has white, medium-sized grains, a thin pericarp,and a white pearly endosperm. It produces white flour,which is universally preferred throughout the region.Farmer-participatory assessments have confirmed thegrain quality characteristics of the variety and itspotential for widespread adoption. Macia also has thepotential for improving fodder supplies—it has broadleaves that stay green even after maturity, a key advantagein mixed crop-livestock systems. Analytical laboratorytests by SMIP have shown that Macia has a high SDU value(Sorghum Diastatic Unit), indicating its suitability formalting. Current efforts in the five countries where it hasbeen released are geared towards sustainable productionand distribution of seed to farmers.Sorghum Ergot—a Sticky DiseaseProblem in Southern AfricaD E Frederickson (INTSORMIL Pathologist,SADC/ICRISAT Sorghum and Millet ImprovementProgram, PO Box 776, Bulawayo, Zimbabwe)Ergot disease of sorghum is not new to Africa - the causalpathogen, Claviceps africana Frederickson, Mantle, and deMilliano was first recorded in Kenya as far back as 1923.However, ergot disease only began to gain recognition asa potential problem in sorghum production in the 1960s,when all the A-lines in Nigeria's national breedingprogram became infected, to the near complete exclusionof seed production (Futrell and Webster 1965).The pathogen has now been recorded in everysorghum-growing country on the continent. By 1997,when it had reached the Americas, Australia, and theUSA, ergot disease had gained notoriety as the majorbiotic constraint to sorghum production globally(Bandyopadhyay et al. 1998). Ergot causes annual lossesof 12-25% and 60% in hybrid seed production inZimbabwe and South Africa respectively.The pathogen, Claviceps africana, is a fungus thatspecifically targets the sorghum ovary. The spores, orconidia, germinate on the stigma and the germ tubes trackdown the ovary wall to the distal nutrient supply. Once thefungus has access to this sugar-rich nutrient source thereis enormous proliferation of hyphae and almost completedestruction of the ovary to form the soft, white, globosebody or sphacelium. The sphacelium produces millionsmore conidia in a sticky exudate called honeydew.Conidia represent the asexual form of the fungus,enabling rapid clonal propagation during the sorghumgrowing season. Honeydew droplets may collect on thetips of infected florets or may drip to smear the wholepanicle or drip onto the leaves or soil.Vol. 2, No. 1 (Jul 2000)Sorghum Variety Macia Released inTanzaniaThe improved sorghum variety Macia (SDS 3220) wasreleased on 14 Dec 1999 by the Tanzania National VarietyRelease Committee. Macia is a high-yielding, early-maturing, white-grained variety developed jointly byICRISAT and national scientists in southern Africa.It has so far been released in five SADC countries—Mozambique, Botswana (under the name Phofu),Zimbabwe, Namibia, and now Tanzania. It is suitable forareas with a growing season of 3-4 months.Macia was developed by mass selection in the F4generation from material originally developed atICRISAT-Patancheru (India). The new line, indexed asSDS 3220, was selected by breeders at the SADC/ICRISAT Sorghum and Millet Improvement Program(SMIP) in 1984/85. A series of preliminary and advancedtrials at SMIP demonstrated its potential, after which SDS3220 was evaluated in collaborative trials at multiplelocations across the region between 1988/89 and 1990/91.It then underwent multilocational testing in Tanzania forthree seasons—1991/92 (7 sites), 92/93 (5 sites), and93/94 (2 sites). Grain yields of Macia in these trials were15% higher than those of two released, improvedvarieties, Pato and Tegemeo. In on-farm trials conductedin northern Tanzania, Macia gave yields comparable to orbetter than Pato and Tegemeo.The new variety has several other advantages. It haslarge heads and a high degree of uniformity. It maturesearlier than other improved varieties, and is thus lesssusceptible to terminal drought. Plants are short, makingbird scaring easier. It is also a multipurpose variety,suitable for food, fodder, and other uses.Under conditions of high relative humidity, thehoneydew turns from clear or brown throughout tosuperficially white. Here the first-formed primary ormacroconidia germinate to form secondary conidiawhich, unlike macroconidia, are windborne. Secondaryconidia are crucial to the pathogen for rapid diseaseincrease to epidemic proportions, and for spread overmoderate distances of tens to hundreds of kilometers(Frederickson et al. 1989, 1993).Towards the end of the sorghum growing season, theergot pathogen ceases spore production and may form a more resilient type of tissue adjacent to the sphacelium.For Claviceps species in general, this sclerotial tissueenables pathogen survival and perennation from oneseason to the next.In Africa, where an alternate host with a role inperennation has yet to be found, C. africana sclerotia maypossibly assume this role by germination to produce thesexual stage. However, conidia in crop residuescontaminated with sphacelia/sclerotia may also assumethat function. Thus inoculum may possibly be seedborneby one or both routes.There are no ergot-resistant sorghum genotypesanywhere in the world. Pollen and pathogen take theidentical route into the ovary and any biochemical ormechanical stigmatic or gynoecial exclusion mechanismprobably has mutual consequences: no ergot but no seed!Fertilization of the ovary precludes infection, so varietieswith good fertility tend to "escape" ergot most of the time;and in evolutionary terms, no other "resistance"mechanism has probably ever been necessary.It is obvious, therefore, that male-sterile A-lines inhybrid seed production are at greatest risk. Al l ovaries ofall sorghums are potentially susceptible, however,depending upon environment. Cold nights (temperaturesbelow 12oC) at pollen microsporogenesis, inducingpollen sterility, lead ubiquitously to reduced fertility andincreased ergot severity (McLaren and Wehner 1992).Selecting varieties, hybrids, and their parents with cold-tolerance has had some success in minimizing ergotseverity. However, rainfall or high RH at anthesis willhave an overriding influence, because these factorsdisrupt pollen-shed, deposition and germination, andsimultaneously induce secondary conidiation, splashdispersal, and favor germination of pathogen spores.Screening for cold-tolerance in sorghum is clearly onegood strategy to reduce ergot. Few other methods arecurrently available. Early planting, which in Zimbabwemeans before the end of December, reduces the risk ofergot, in part by avoiding flowering around the time oflowered night temperatures.The protection of valuable germplasm by theapplication of triazole fungicides at heading mayoccasionally be warranted in seed production but severelyreduces the already slim profit margins.The reason for the lack of alternative control strategiesis that there are wide gaps in our knowledge of the life-cycle and biology of the pathogen. The INTSORMIL(International Sorghum/Millet Collaborative ResearchSupport Program) project, networking with otherscientists in the region and globally, focuses primarily onthe very important question of inoculum mentioned earlier.The key question is:What is the source of initial inoculum each season (e.g.honeydew, conidia, sphacelia, sclerotia)? To answer thisquestion requires research in the following areas:• The effect of environment on the survival ofmacroconidia in honeydew, in sphacelia/sclerotia, andon the survival of secondary conidia• Formation, survival, and germination of sclerotia toproduce ascospores (sexual reproduction)• How frequently does sexual reproduction occur innature; how much variability does the pathogenexhibit in Zimbabwe and regionally1 ? Answers to these questions will help us target thecritical stages of the life-cycle of C. africana, enabling usto formulate new strategies and integrate multiplestrategies for better control of ergot on all sorghumgenotypes.ReferencesBandyopadhyay, R., Frederickson, D. E., McLaren, N.W.,Odvody, G. N., and Ryley, M.J. 1998. Ergot: a new diseasethreat to sorghum in the Americas and Australia. Plant Disease82: 356-366.Frederickson, D.E., Mantle, P. G., and de Milliano, W.A..I.1989. Secondary conidiation of Sphacelia sorghi on sorghum, a novel factor in the epidemiology of ergot disease. MycologicalResearch 93: 497-502.Frederickson, D. E., Mantle, P. G., and de Milliano, W. A. J.1993. Windborne spread of ergot disease (Claviceps africana)in sorghum A-lines in Zimbabwe. Plant Pathology 42: 368-377.Futrell, M. C. and Webster, O. J. 1965. Ergot infection andsterility in grain sorghum. Plant Disease Reporter 8: 680-683.McLaren, N. W. and Wehner, F. C. 1992. Pre-flowering lowtemperature predisposition of sorghum to sugary disease(Claviceps africana). Journal of Phytopathology 135: 328-334.8 ISMN 41, 2000Dr Frederickson is very keen to network with anyone in the regionwith an interest in sorghum ergot, or who would be able to providesamples from their country.Please e-mail d.frederickson@cgiar.org.

Sorghum ergot - a sticky disease problem in southern Africa

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Sorghum ergot - a sticky disease problem in southern Africa

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