African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 3, Num. 2, 1995, pp. 203-207

Sorghum diseases in Tanzania

S.B.A. MANSUETUS

Ilonga Agricultural Research and Training Institute, P.O. Ilonga, Kilosa. Tanzania

Code Number: CS95027
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ABSTRACT

In Tanzania sorghum (Sorghum bicolor L.) is grown in all agro-ecological zones and is a subsistence crop in the semi-arid areas. Twenty two diseases and Striga weed have been identified to be major constraints of sorghum production in the country. Grain mould, gray leafspot, anthracnose, leaf rust, leaf blight and covered kernel smut are the most important and widespread diseases of sorghum, while the incidence of downy mildew, ergot and sooty stripe is increasing. Different sources of resistance to grain mould, leaf blight, anthracnose, downy mildew and Striga have been identified. Effective and durable resistance to grain mould will probably require simultaneous resistance to a series of pathogens because different sibling species or mating populations within Fusarium section Liseola exist on sorghum in Tanzania. The deployment of biotechnologically transformed sorghum could potentially control sorghum diseases such

as grain mold.

Key Words: Biosafety, diseases, grain mould, Sorghum bicolor

RESUME

En Tanzanie, le sorgho (Sorghum bicolor L.) qui est cultive dans toutes les zones agro-ecologiques, est une culture de subsistance dans les zones semi-arides. Vingt deux maladies et la mauvaise herbe Striga constituent la contrainte majeure a la production du sorgho dans le pays. La moisissure de graianes, 'gray leaf spot", l'anthracnose, la rouille, "leaf blight" et "covered kernels smut" sont les maladies plus importantes et les plus repandues tandis que l'incidence du mildiou, de l'ergot et "sooty stripe" progresse. Differentes sources de resistance aux maladies majeures et au Striga ont ete identifiees. Une resistance durable et effective a la moisissure necessitera probablement une resistance simultannee a toute une serie de pathogenes a cause de la difference d'especes et de mode d'accouplement de populations de la fusariose de la section Liseola existant sur le sorgho en Tanzanie. La diffusion de sorgho biotechnologiquement transforme pourrait potentiellement faciliter le controle des maladies du sorgho telle que la moissisure de graines. La Tanzanie a besoin d'etablir un comite de biosecurite et de mettre en place des politiques et procedures reglementant la biotechnologie.

Mots Cles: Biosecurite, maladies, moisissure de graines, Sorghum bicolor

INTRODUCTION

Sorghum (Sorghum bicolor L. Moench) is a very important cereal crop in Tanzania after maize and rice. It is grown in all agro-ecological zones and is a subsistence crop in the semi-arid and frequently drought stricken areas. Sorghum is mainly utilized as food and feed. Cultivars with a red pericarp are also used for brewing alcoholic beverages both locally and industrially. Recently, DarBrew, a brewing company in Tanzania succeeded in extracting pure starch from sorghum (Msangula, personal communication). This starch can be used for different purposes.

The most important biotic factors limiting sorghum production are diseases, insect pests and the parasitic weed Striga. Twenty-two diseases and three species of Striga have been identified on sorghum in Tanzania (Table 1 ). Grain mould, gray leaf spot, anthracnose, leaf blight. leaf rust and covered kernel smut are the most important and widespread diseases. Downy mildew, ergot, and sooty stripe are diseases of growing incidence (Mbwaga et al., 1993). No detailed studies have been done to determine yield losses due to these diseases in Tanzania. Visually. up to 100% yield loss has been observed due to Striga. Grain mould causes losses through poor seed viability, reduced test weight and grain quality. The threat of mycotoxin contamination due to grain mould pathogens has not been determined in Tanzania.

SORGHUM DISEASE CONTROL IN TANZANIA

The integrated pest management strategy will continue to be the best approach in the management of diseases of sorghum in Tanzania (SADCI ICRISAT. 1992). Many farmers in the country control diseases of sorghum through cultural means such as intercropping or varying times of planting and harvest.

TABLE 1. Sorghum diseases, their causal organisms and parasitic weed species identified at different locations in Tanzania, 1986-1990.

Disease/
parasitic weed   Causal organism^#           Location
--------------------------------------------------------------
Acremonium wilt   Acremonium spp.            Hombolo
Anthracnose       Colletotrichum graminicola Ifakara,
Ilonga,
                                            Hombolo, Ukiriguru
Charcoal rot      Macrophomina phaseolina    Ilonga
Downy mildew      Peronosclerospora sorghi   Ilonga,
Singida,                                               
Ukiriguru
Grain mould       Fusarium moniliforme       Ilonga,
Ifikara
Ergot             Claviceps africana         Ilonga,
Hombolo,                                               
Ukiriguru
Gray leaf spot    Cercospora sorghi          Ilonga,
Ifakara,                                              
Hombolo, Ukirigiri
Ladder leaf spot  Cercospora fusimaculans    Ifakara,
Hombolo,                                              
Ukirigiri 
Leaf blight       Exserohilum turcicum       Iduo seed
farm,                                       Ifakara, Hombolo,
Ukirigiri

Leaf spot         Phoma sorghina             Ukirigiri
Leaf spot         Mycosphaerella spp.        Ukirigiri
Rough leaf spot   Aschochyta sorghina        Sinigada
Leaf rust         Puccinia purpurea          Ilonga,
Hombolo,                                              
Manhala, Ukirigiri
Sooty stripe      Ramulispora sorghi         Ilonga,
Hombolo,                                               
Ukirigiri
Stalk rot         Fusarium moniliforme       Hombolo,   
                                                    Ukirigiri,
Ilonga
Headsmut          Sporisorium reilianum      Singida,
Dodoma
Covered kemel smutSporisorium sorghi         Singida,
Dodoma
Longsmut          Tolyposporium ehrenbergii  Bihawana,
Hombolo
Loose kernel smut Sphacelotheca cruenta      Ilonga,
Hombolo
Zonate leaf spot  Gloeocercospora sorghi     Ifakara,
Hombolo
Viral diseases    N.C.                       Hombolo, Ilonga
Bacterial diseasesN.C.                       Ilonga
Parasitic weed    Striga asiatica            Ilonga,
Hombolo,                                               
Manhala, Mwele,
                                         Naliendele, Ukirigiri

Parasitic weed    Striga hermontheca          Ukirigiri
Parasitic weed    Striga forbesii             Ilonga

# Causal agents were identified at IMI; 
N.C = Causal agents not confirmed or identified.

They do not depend on chemical control. Host plant resistance (HPR) is being emphasised in complement with other means of disease control because HPR optimizes disease management in promoting sustainable agricultural systems. However, before any disease control strategy is implemented, information is needed on yield loss levels, host-pathogen-environment interaction, physiological races or pathotypes of the pathogen, interaction among multiple pathogens, biology and epidemiology. In Tanzania, these criteria have not been met since only disease surveys have been made (Mbwaga et al., 1993). Also, due to limited research funds, only on-farm trials are given priority.

Sorghum disease resistance management in Tanzania. Efforts have been made within the sorghum improvement programme to broaden the genetic background of the working materials in order to develop favorable genotypes for farmers (Mitawa and Saadan. 1985). The main objective of diversifying the genetic base is to improve disease resistance while maintaining desirable agronomic characteristics and stability. Unfortunately, all the collections (local and exotic) were destroyed due to a power failure at Ilonga Research Institute. There is need for new germplasm for the programme. Through collaboration with ICRlSAT, SADC/ICRISAT. Texas A & M University, Kansas State University. and Mississippi State University, it is intended to have a world collection of exotic and indigenous elite accessions, and to evaluate their reaction to diseases under different environments, with sufficient disease pressure to produce reliable data. Landrace materials that generally contain agro-ecologically adaptable multiple disease resistance will also be evaluated.

Currently Framida, IS 24995, 1S 23599, and R 4244 are the red pericarp source of resistance to grain mould. For cultivars with a white pericarp, SDS 2293-6 and SDS 2293-1 could be good sources of resistance to grain mould. SAR29 and its hybrid SPL 38A x SAR 29 and Weijita have been identified to be resistant to Striga asciatica and S. hermontheca. SDS 2293-6 is moderately resistant to leaf blight but resistant to anthracnose and downy mildew. Therefore it is possible to incorporate resistance into agronomically desirable released varieties such as Tegemeo.

At Ilonga, there are no laboratory facilities for the preparation of pure cultures for artificial inoculation, and only natural inoculum is used for screening purposes. It is intended that a field test be established through the use of spreader rows and also resistant and susceptible checks to monitor pathogen spread, and to differentiate among races especially for leaf blight and sooty stripe diseases. Multi-location trials will be established to determine the stability of the resistance. Currently, the Sorghum Grain Mould Resistance Screening Nurseries are only at two locations. Depending on the availability of funds, it is intended to have an All-disease and Insect Nursery and Striga Resistance Observation Nursery.

Breeding strategies. Reports have shown that the mode of inheritance for grain mould is dominant or intermediate, dominant for covered kernel smut and leaf blight and dominant/ recessive/intermediate for sooty stripe (Rosenow, 1992). Therefore, population breeding will be used because it takes advantage of random mating and genetic stability and also because it is useful for monogenic and polygenic resistance. There is need to have durable resistance that is conferred by many genes and can exhibit a synergistic effect in host response to pathogens (Odvody and Hepperly, 1992). For instance, with foliar diseases, developing moderate resistance and avoiding high susceptibility may be more durable than developing high levels of resistance especially with monogenic traits. Considerable horizontal resistance that is non-specific, general, quantitative and durable exists (Fry. 1982; Agrios, 1988), and can be incorporated by cross breeding existing varieties. The wider the genetic base of a crop, the greater the likelihood that horizontal resistance can be identified and the less the need for the usually transitory vertical resistance.

Pathogen variability. Variability in pathogen populations makes disease management and breeding for resistance more difficult. Variation within Fusarium section Liseola (Gibberella fujikuroi) provides examples of the difficulties that may be encountered in biological species. Mating populations designated by the letters A through F are known world-wide (Leslie, 1991) and all are present in Tanzania (Mansuetus, 1993).

Mating populations F and A, which share the F. moniliforme anamorph, were the most frequent on sorghum in Tanzania. Members of the A mating population, but not the F mating population, are known to synthisize the fumonisin mycotoxins (Leslie and Plattnet, 1991; Leslie et al., 1992). This toxin can cause iucoencephalomalacia in mouses (Kellerman et al. 1990) and pulmonary edema in swine (Rossetal., 1990),and is correlated with esophogeal cancer in humans (Rheeder et al., 1993). The danger posed to Tanzanian sorghum by fumonisin toxins needs to be determined.

To complicate the analysis further multiple strain types, as noted by differences in vegetative compatibility groups, VCGs, have been found in Tanzania (Leslie, 1993; Mansuetus, 1993). The presence of different VCGs within F. moniliforme on sorghum in Tanzania suggests that, if resistance to grain mould is to be durable, breeding efforts should be directed against numerous strains of F. moniliforme. In Tanzania, strains more virulent for utilization in the screening process for grain mould resistance have not been determined.

Because of the complex nature of resistance to grain mould, and the absence of immunity to this disease, it is anticipated that biotechnology will be the solution for the control of grain mould. The benefits of biotechnology outweigh the risks of utilizing this technology. Genetically engineered sorghum cultivars will, however, have to be catalogued, uniform for various traits, distinct and stable. In Tanzania, there is no variety protection act on sorghum utilization, hence, there will be no obstacles on the deployment of a genetically engineered sorghum. However, before introducing transformed plants, there is need to establish a national biosafety committee supported by an institutional biosafety committees in research institutes dealing with modern biotechnology. This committee will deal with the assessment and regulation of in-country research and the assessment of safety of imported biotechnology products.

REFERENCES

Agrios, G.N. 1988. Plant Pathology, 3rd Ed. Academic Press, New York, NY, USA. 803 pp.

Fry, W.E. 1982. Principles of Plant Disease Management. Academic Press, Orlando, FL,USA.

Kellerman, T.S., Marasas, W.F.O, Thiel, P.G. Gelderblom, W.C.A., Cawood, M. and Coetzer,J.A.W. 1990. Leukoencephalomalacia in two horses induced by oral dosing of Fumonisin B1. Onderstepoort Journal Veterinary Research. 57:269-275

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Leslie, J.F. 1993. Fungal vegetative compatibility. Annual Review of Phytopathology 31:127-150

Leslie, J.F. and Plattner, R.D. 1991. Fertility and fumonisin B1 production by strains of Fusarium moniliforme (Gibberella fujikuroi). Proceedings of the Sorghum Improvement Conference of North America 17:80-84.

Leslie, J.F., Plattner, R.D., Desjardins, A.E. and Klittich, C.J.R. 1992. Fumonisin B1 production by strains from different mating populations of Gibberella fujikuroi (Fusarium Section Liseola). Phytopathology 82:341-345.

Mansuetus, S.B.A. 1993. Mating Populations and Vegetative Compatibility Groups Within Gibberella fujikuroi (Fusarium Section Liseola) on Sorghum in Tanzania. Ph.D. Dissertation, Texas A&M University, College Station, TX, USA. 96 pp.

Mbwaga, A.M., Pande, S., de Milllano, W.A.J. and Karunakar, R.I. 1993. Diseases and parasitic weeds of sorghum in Tanzania: occurrence and incidence, 1986-1990. Crop Protection 12:183-188.

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Rosenow, D.T. 1992. Using germplasm from the world collection in breeding for disease resistance. In: Sorghum and Millets Diseases: A Second WorldReview. de Milliano, W.A.J., Frederiksen, R.A. and Bengston, G.D. (Eds.), pp. 319-324. ICRISAT, Patancheru 502324. A.P., India. 370 pp.

Ross, P.F., Nelson, P.E., Richard, J.L., Osweiler, G.D., Rice, L.G., Plattnet, R.D. and Wilson, T.M. 1990. Production of fumonisins by Fusarium moniliforme and Fusarium proliferatum isolates associated with equine leucoencephalomalacia and pulmonary edema syndrome in swine. Applied Environmental Microbiology 56:3225-3226.

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