African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 9, Num. 1, 2001, pp. 287-292

African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 287-292

Code Number: CS01059

ABSTRACT

Leaf petiole and stem blight disease is commonly encountered on sweetpotato in Kenya. A study was carried out to confirm the causal agent of the disease in different agro-ecological zones of Kenya and determine the importance of soil pH as well as field conditions on disease development. Diseased leaf and vine tissues were collected and pathogen isolated. The Kabete, Kakamega, Busia (Alupe), and Kabondo isolates were identified as Alternaria bataticola while the Kisii isolate was identified as Alternaria alternata. Pathogenicity and virulence of different isolates of A. bataticola and A. alternata were investigated under greenhouse conditions. Within 4 - 6 days following artificial inoculation, and depending on sweetpotato cultivar, the fungus produced characteristic symptoms typical of Alternaria leaf petiole and stem blight. All the isolates were pathogenic and differences in virulence depended on isolate source and conidial concentration. The Kabete isolate was more virulent than Kakamega, Kabondo and Alupe isolates, whose virulence did not differ significantly from each other (P<0.05). In a greenhouse study the different pH levels did not influence disease severity (P<0.05). However, there were significant differences in disease severity among the cultivars evaluated. Alternaria leaf and stem blight caused by Alternaria bataticola was monitored in field plots of sweetpotato accessions selected from the CIP germplasm collection field. Cultivars Viola (440046) and Yanshu 1 (440024) were more susceptible than other entries. The lowest disease levels were observed on cultivar Jayalo.

Key Words: Alternaria bataticola, A. alternata, virulence, Ipomoea batatus, Kenya

RÉSUMÉ

La maladie de la rouille de la tige et de la pétiole est communément rencontrée sur la patate douce au Kenya. Une étude a été conduite pour comfirmer l'agent causal de cette maladie dans les differentes régions agro-écologiques du Kenya et déterminer l'importance des niveaux du pH du sol ainsi que les conditions de champs sur le développement de la maladie. Les feuilles malades et les tissus des boutures ont été collectés et les pathogènes isolés. Les isolats de Kabete, Kakamega, Busia (Alupe) et Kabondo ont été identifiés comme Aternaria bataticola, alors que l'isolat de Kisii a été identifié comme Alternaria alternata. La pathogénécité et la virulence des differents isolats de A. bataticola et A. alternata ont été étudiés en serre. Entre 4 et 6 jours après l'inoculation artificielle et dépendamment du cultivar, le champignon a produit des symptômes typiques caractéristiques d'Alterinaria de la pétiole de feuille et de la rouille de tige. Tous les isolats étaient pathogéniques et les differences dans la virulence dépendaient de la source d'isolat et la concentration en conidies. L'isolat de Kabete était le plus virulent plus que ceux de Kakamega, Kabondo, et Alupe, dont la virulence n'a pas été significativement differente entre eux. Dans l'étude en serre, les differents niveaux de pH n'ont pas montré une sévérité significative de la maladie. Cependant, il y avaient des differences significatives dans la sévérité de la maladie entre les cultivars étudiés. L' Alternariade la feuille et la rouille de la tige causée par Alternaraia bataticola a été suivie dans les parecelles de champs des asseccions sélectionnées à partir du germoplasme du CIP. Les Cultivars Viola (440046) et Yanshu (440024) étaient sensibles plus que d'autres entrées. Les niveaux les plus faibles de la maladie ont été observés sur le cultivar Jayalo.

Mots Clés: Alternaria bataticola, A. alternata, germoplasme, virulence, Ipomoea batatus, Kenya

INTRODUCTION

Sweetpotato (Ipomoea batatus (L.) Lam) is an important food crop in several countries of Sub-Saharan Africa (Horton, 1988), Uganda being the largest producer in the continent (Hakiza et al., 2000). Among the production constraints that currently limit its production are damages by various diseases and pests. Some are quite common and others have been observed only in certain geographic areas (Clark, 1987). Alternaria leaf petiole and stem blight is one of the most important fungal disease in East Africa (Lenné, 1991) and South East Asia (Lopes et al., 1994). The amount of damage to the sweetpotato plant is dependent on the stage of growth and prevailing environmental conditions (Skoglund and Smit, 1994). In very severe attacks, vines may be killed within a very short period of symptom development leading to 100 % crop failure, while milder attack causes defoliation (Skoglund and Smit, 1994). Attack in latter stages of plant development results in no loss in root yield (Lenne, 1991).

Stem blight is manifested in the wet season as stem necrosis and dieback and is especially serious in drier periods. Disease and lesion size increase as altitude increases (Ndamage, 1988). High relative humidity or free water is necessary for infection and sporulation. The fungus survives in debris and spores are spread through infected planting material, wind, splashing rain and water (Skoglund and Smit, 1994). Alternaria stem and petiole blight caused by Alternaria alternata has been reported in the southern and western highlands of Papua New Guinea (Waller, 1984; Lenne, 1991) and New Caledonia (Bugnicourt and Marty, 1961). Leaf spot and stem blight caused by Alternaria bataticola was first recorded in Brazil in 1994 (Lopes et al., 1994). In Africa, Alternaria leaf and stem blight caused by Alternaria solani has been reported in Burundi and Rwanda (Ndamage, 1988). Alternaria tax sp IV occurs in Ethiopia (Van Bruggen, 1984), and an Alternaria sp. has been recorded in Zambia (Angus, 1963), Zimbabwe (Whiteside, 1966), Ethiopia (Terefe and Amanuel, 1992), Uganda (Bashaasha et al., 1995), Nigeria (Arene and Nwankiti, 1978) and in Kenya. The disease is serious particularly in infertile, acid soils, and at higher altitudes (Ndamage, 1988).

Although some workers have investigated the relative susceptibility of different germplasm of sweetpotato to Alternaria leaf and stem blight (Van Bruggen, 1984; Simbashizweko and Perreaux 1988; Lopes et al., 1994) no information is available on the relative susceptibility of different sweetpotato germplasm found in Kenya. Yet host resistance is a key disease management strategy, especially in susbsistence agriculture.

The objectives of this study were (a) to establish the causal agent of Alternaria leaf petiole and stem blight in Kenya, (b) to study the effect of soil pH on the disease development, and (c) to assess the reactions of selected Kenyan sweetpotato accessions to the disease.

MATERIALS AND METHODS

Vines and leaves showing characteristic symptoms of Alternaria leaf petiole and stem blight were collected from various parts of Kenya, including Kabete, Kakamega, Kabondo and Kisii. From each area, 20 samples were randomly picked from sweetpotato growing fields. In the laboratory, small pieces of infected leaf petiole and stem/vine tissues were surface sterilised and placed in moist chambers for 24 hrs to enhance sporulation (Van Bruggen, 1984).

The fungus was identified using a modification of the slide culture technique described by Riddle (1950). When suitable growth and sporulation had occurred the cover slip was then mounted in a drop of clear lactophenol and viewed under the microscope. The nature of mycelial growth and conidiophore formation, arrangement of conidia on conidiophores and shape of conidia was noted. Mounting was similarly repeated in sterile distilled water for colour observation of various structures. The size of conidia (length and width) was determined using Dynazoom compound microscope with a micrometer.

Effect of soil pH on disease severity. Soils of known pH levels were obtained from the Department of Soil Science, University of Nairobi, for this experiment. Sweetpotato accession lines Viola (440046), Jayalo, Kemb 36 and TIS 83/0138 (440102) were planted in pots filled with sterilised soil of pH levels 4.4, 4.6, 6.7, and 7.6. A completely randomised design (CRD) with three replicates was used in this experiment, with soil pH levels and cultivars being studied in a factorial combination. The plants were inoculated with a uniform conidial concentration of 5.4 x 106 condia/ml. Control plants were inoculated using sterilised distilled water. The inoculated plants were examined for disease development on weekly basis. The disease severity was scored using a modified scale of Van Bruggen (1984) as 0 = no disease, 1 = few, small lesions, 2 = from 1-10%, 3 = from 11-25%, 4 = from 26-50% infection, 5 = > 50% lesion coverage.

Field experiments. Field studies were conducted at the University of Nairobi, Kabete campus field station farm for two seasons to evaluate the relative susceptibility of 20 sweetpotato cultivars/accessions lines to Alternaria stem and petiole blight. The first trial was from May to August and the second from September to November 1999. Alternaria bataticola isolate from Kabete was used in this experiment because it was more virulent than the other isolates. The tested varieties were Mar Ooko, Habare 127 (440380), New Kawogo (440165), Kalam Nyerere, Yanshu 1 (440024), Kemb 36, 420006, PEPA (440050), PM-04-4 (440333), Camote Negro (420021), Mugande (440163), Mafuta, KSP20, Naveto (440131), Viola (440046), Jayalo, Iguro iwe, LM88.014 (188001.2), Mwezi tatu, and TIS 83/1380 (440102) obtained from the International Potato Centre field multiplication plots at the University of Nairobi Kabete Farm.

A randomised complete block design with 5 replicates was used each season. The plants were inoculated with a uniform conidial concentration of 5.4 x106 conidia/ml. The inoculated plants were examined for disease development on weekly basis and scored as described earlier. Areas Under Disease Progress Curves (AUDPCs) were computed from disease severity ratings using the formula of Shaner and Finney (1977).

Averages of AUDPCs for each plot were examined using Analysis of Variance, and significant differences were identified using LSD at P=0.05.

RESULTS AND DISCUSSION

Pathogen identification. Isolation of the pathogen from infected sweetpotato leaf petioles and stem tissues yielded colonies typical of Alternaria sp. The Kabete, Busia, Kabondo and Kakamega isolates were similar in colony appearance and conidia size. Their conidia were solitary, elongate obclavate, muriform, transversely 5 to 12 septate and longitudinally 0 to 8 septate with a measurement of (75-) 120-160 (-210) x (13-) 15-18 (-23) µm. Their conidial beaks were long, filiform and colourless sometimes branching with an average dimension of 8 (4 - 12) x 71 (32 - 129) µm. These results compare well with the descriptions of Alternaria bataticola (David, 1991; Lopes and Boiteux, 1994). The Kisii isolate differed from other isolates in conidia shape and size and was identified as Alternaria alternata. Its conidia were brown, ellipsoidal, 20 - 60 x 75-15 µm (av.39 x 10.3 µm) in size, short beaked, with 2 - 6 transverse septa, 0 - 4 longitudinal septa and were catenulate at the apex of the conidiophores. But the slight differences in the conidia, conidial beaks, presence of conidia with branching beaks and conidiophores dimensions observed in this study could be due to effects of artificial conditions under which the fungus was cultured. Grogan et al. (1978) observed variations in sizes of beak, conidia and conidiophores of Alternaria alternata when cultured on artificial media. Van Bruggen (1984) also observed variation in conidia dimensions when he cultured Alternaria sp. (tax sp. IV) in different culture media and incubation conditions.

The isolates were sent to the International Mycological Institute (IMI) in Kew, England for confirmation of their identity. The Kabete, Kakamega, Busia (Alupe) and Kabondo isolates were confirmed as Alternaria bataticola and the Kisii isolate as Alternaria alternata (David, J.C., personal communication). The isolates are deposited at IMI culture collection under numbers IMI 379880, IMI 379881, IMI 379882, IMI 379883 and IMI 379884, respectively. This study confirms that Alternaria bataticola and Alternaria alternata cause Alternaria leaf petiole and stem blight in Kenya. Other species of Alternaria have been reported as causal agents of Alternaria leaf petiole and stem blight in other countries (Angus, 1963; Whiteside, 1966; Van Bruggen, 1984; Ndamage, 1988; Lenne, 1991; Terefe and Amanuel, 1992; Lopes and Boiteux, 1994).

Effect of soil pH on the disease development. Sweetpotato accession Viola (440046) recorded the highest mean disease severity rating under all the soil pH tested (4.9), followed by TIS 83/0138 (440102) (3.6), Kemb 36 (3), while Jayalo recorded the least (2) (Table 1). However, mean disease levels at the different pH levels were not significantly different. Simbashizweko and Perreaux (1988) reported that Alternaria blight (Alternaria solani) attack was more severe in infertile acidic soils in Rwanda under field conditions. This does not compare well with the results obtained under our greenhouse conditions for Alternaria bataticola, where differences in disease development varied only among the varieties evaluated regardless of the soil pH.

In most cases where soil pH have been found to influence disease severity and development, they are associated with those pathogens that attack the root systems of the host plant. Alternaria bataticola, being an aerial pathogen, was thus little affected by the soil pH conditions. The exact mechanism of how soil pH influences disease development is not clearly understood. However, pH alterations may affect the biological and nutritional balance of the soil thus making either the pathogen more or less active or the host more susceptible or resistant to disease development.

Susceptability of Kenyan accessions. In this study, none of the genotypes evaluated under field conditions appeared immune to the pathogen. On the contrary, some accessions showed high degree of susceptibility to Alternaria bataticola. The AUDPCs ranged from 28 to 100.8 and 22.4 to 100.8 in season one and two, respectively (Table 2). Disease was less severe on cultivar Jayalo in both seasons, showing its tolerance to the pathogen. Alternaria blight was more severe on accessions Viola (440046), Yanshu 1 (440024) and TIS 83/0138 (440102). Sweetpotato cultivars considered indigenous to Kenya had lower AUDPCs as compared to CIP accessions introduced into the country. This compares well with observations made in Ethiopia by Van Bruggen (1984), where he reported moderate resistance in traditional varieties as opposed to introduced varieties. Simbashizweko and Perreaux (1988) also recorded different levels of disease severity among sweetpotato cultivars to Alternaria solani, while Lopes et al. (1994) recorded differences in sweetpotato reaction to Alternaria bataticola. In this study, those sweetpotato genotypes originating from Eastern Africa were tolerant to the disease as opposed to those originating from South East Asia. The high level of disease tolerance by the local cultivars may be due to their broad genetic base. However, even within the East African accessions there was considerable variation in the disease levels, a possible indicator that selection for disease resistance is still possible with the local accessions.

REFERENCES

Angus, A. 1963. Plant Pests and Diseases in Northern Rhodesia. Part 5. Mt. Makulu, Northern Rhodesia. 33 pp.

Arene, O. B. and Nwankiti, A. O. 1978. Sweetpotato diseases in Nigeria. Pest Articles and News Summaries 24:298-305.

Bashaasha, B., Mwanga, R. O. M., Ocitti P'Obwoya and Ewell, P. T. 1995. Sweetpotato in the farming and food systems of Uganda. A farm survey report. CIP, Sub-Saharan Africa Region, Nairobi, Kenya/National Agricultural Research Organisation, Kampala, Uganda.

Clark, C. A. 1987. Principal bacterial and fungal diseases of sweetpotato and their control. In: Exploration, Maintenance, and Utilisation of Sweetpotato Genetic Resources. Report of the 1st Sweetpotato Planning Conference, February, 1987, Lima, Peru. CIP. Pages 275-289.

David, J. C. 1991. Alternariabataticola. International Mycological Institute-IMI

Grogan, R. G., Duniway, J. M. and Kible, I. J. 1978. Influence of environment and culture media on spore morphology of Alternariaalternata. Phytopathology 68:29-34.

Hakiza, J.J., Turyamureeba, G., Kakuhenzire, R.M., Odongo, B., Mwanga, R.M., Kanzi-kwera, R. and Adipala, E. 2000. Potato and sweetpotato improvement in Uganda: A historical perspective. African Potato Association Conference Proceedings 5: (in press).

Horton, D. J. 1988. Underground Crops: Long-term trends in production of roots and tubers. Winrock International. 130 pp.

Lenne, J. M. 1991. Diseases and Pests of Sweetpotatoes: South East Asia, the Pacific and East Africa. Natural Research Institute Bulletin No.46.

Lopes, C. A. and Boiteux, L. S. 1994. Leaf spot and stem blight of sweetpotato caused by Alternariabataticola; a new record to South America. Plant Disease 78:1107-1109.

Luke, H. H. and Berger, R. D. 1982. Slow rusting in oats compared with logistic and Gompertz models. Phytopathology 72:400-402.

Lyda, S. D. 1978. Ecology of Phymatotrichum omnivorum. Annual Review of Phytopathology 16:193-209.

Ndamage, G. 1988. Development et amelioration de la production de la patate douce (Ipomoea batatus L.) au Burundi. pp. 167-184. In: Improvement of Sweetpotato in East Africa. Report of the Workshop on Sweetpotato Improvement in Africa, held in ILRAD, Nairobi, September 28 - October, 2 1997: CIP, Lima, Peru.

Riddel, R.W. 1950. Permanent stained mycological preparations obtained by slide culture. Mycologia 42:265-270.

Shaner, G. and Finney, R. E. 1977. The effect of nitrogen fertilization on expression of slow-mildewing resistance in Knox wheat. Phytopathology 67: 1051-1056.

Simbashizweko, A. and Perreaux, D. 1988. L'anthracnose de la patate douce desciption de l'agent causal et mise au point d'une methode de criblagede la patate douce au Rwanda. In: Improvement of Sweetpotato in East Africa. Report of the Workshop on Sweetpotato Improvement in Africa, held in ILRAD, Nairobi, September 28-October, 2 1987. CIP, Lima, Peru.

Skoglund, L.G. and Smit, N. E. J. M. 1994. Major diseases and pests of sweetpotato in Eastern Africa. CIP, Lima, Peru. pp. 67-69.

Terefe, B. and Amanuel, G. 1992. Sweetpotato production constraints and research in Ethiopia. In: Root and Tuber Pest Management in East and Southern Africa. Proceedings of a regional workshop held in Mombasa, Kenya, 10-14 August 1992. Allard, G.B., Skoglund, L.G., Neuenschwander, P. and Murphy, R. J. (Eds.), pp. 49-50.

Van Bruggen, A. H. C. 1984. Sweetpotato stem blight caused by Alternariasp.: a new disease in Ethiopia. Netherlands Journal of Plant Pathology 90:155-164.

Whiteside, J. O. 1966. A revised list of plant diseases in Rhodesia. Kirkia 5:87-196.

William, E. F. 1982. Principles of Plant Disease Management. Academic Press. pp. 96-99

TABLE 1. Influence of soil pH on severity of Alternaria leaf petiole and stem blight on some cultivars of sweet potato in Kenya

Cultivar	Soil pH
	4.4	4.6	6.7	7.6	Mean
	Disease rating
Viola (440046)	5a2	5a	5a	4.7a	4.9
Jayalo	2d	2d	2d	2c	2.0
Kemb 36	3c	3c	3c	3b	3.0
TIS 83/0138	3.7b	3.7b	3.7b	3.3b	3.6
Mean1	3.4	3.4	3.4	3.3

TABLE 2. Mean areas under disease progress curves (AUDPC) of Alternaria blight disease severity on 20 sweetpotato accessions at Kabete, 1999

Accession/varieties severity	AUDPC-Disease1
	First Second	season season
Mar Ooko	33.6f	30.1hi
Habare 127 (440280)	65.7bc	64.4bcd
New Kawogo (440165)	65.1bc	65.8bc
Kalam Nyerere	38.5ef	33.6hi
Yanshu 1 (440024)	91a	90.3a
Kemb 36	65.8bc	65.1bcd
PEPA (440050)	67.9bc	67.9b
PM-04-4 (440333)	63.7bc	66.5b
Camote Negro (420021)	49.7de	48.3efg
420006	49.7de	48.3efg
Mugande (440163)	58.8cd	51.8cdef
Mafuta	56cd	58.8bcde
KSP 20	49.7de	51.1def
Naveto (440131)	38.9ef	38.5fgh
Jayalo	28f	22.4i
Viola (440046)	100.8a	100.8a
Iguro Iwe	35.7f	28.7hi
LM88.014 (188001.2)	39.9ef	35.7ghi
Mwezi Tatu	59.5cd	58.1bcde
TIS 83/0138 (440102)	76.3b	72.1b
Mean	56.7	54.9
C.V (%)	33.4	37.0