African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 9, Num. 1, 2001, pp. 251-256

African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 251-256

Code Number: CS01055

ABSTRACT

A survey of bacterial wilt (BW) incidence was carried out in three potato producing districts of Kenya, namely, Nyeri, Nyandarua, and Meru in 1997. The survey was carried out in 30 randomly selected farms in each district. Incidence of BW was highest (18.8%) in Nyeri district, intermediate (16.7%) in Meru and lowest (10.4%) in Nyandarua. A significant and negative correlation (P=0.05; r = -0.34) between bacterial wilt incidence and altitude was observed. Fifteen potato cultivars, Nyayo, Desiree CIP-800048, Roslin Tana, Kerrs pink LB-5, Golof (Dutch Robjin), B53, Tigoni CIP-381381.13, Rutuku CIP-720097, Asante CIP-381381.20, Kenya Dhamana CIP-800228, Mauritius Clone (89016), KP93739.26, KP92633.26, and Cruza148 CIP-72011 were subsequently evaluated for their reaction to Ralstonia solanacearum in artificial and natural infections in the greenhouse and in the field. Although none of the cultivars appeared resistant, there were significant (P=0.05) differences in bacterial wilt incidence and severity among the cultivars. Cultivars Kenya Dhamana, Mauritius and Cruza (CIP-720118) had low bacterial wilt severity and incidence and were rated tolerant, whereas the other cultivars were rated moderately susceptible or very susceptible.

Key Words: Bacterial wilt, incidence, Kenya, Solanum tuberosum, tolerance

RÉSUMÉ

Une enquête sur l' incidence de la bactériose (BW) a été conduite en 1997 dans trois districts producteurs de la pomme de terre au Kenya; Nyeri Nyandarua et Meru. L'enquête a été conduite dans 30 fermes sélectionnées aléatoirement dans chaque district. L'incidence de BW était élevée (18.8%) dans le district de Nyeri, intermédiaire (16.7%) dans Meru et faible (10.4%) dans Nyandarua. Une corrélation négative significative (r=-0.34, P<0.05) entre l' incidence de la bactériose et l'altitude a été observée. Quinze cultivars de pomme de terre, Nyayo, Desiree CIP-800048, Roslin Tana, Kerrs pink LB-5, Golof (Dutch Robjin), B53, Tigoni CIP-381381.13, Tutuku CIP-720097, Asante CIP-381381.20, Kenya Dhamana CIP-800228, Mauritius Clone (89016), KP92633.26, and Cruza 148 CIP-72011 ont été conséquemment évaluées pour leurs réactions au Ralstonia solanacearum par une infestation artificielle et naturelle en serre et en champs. Bien qu' aucun cultivar sembait être résistant, il y avaient des différences significatives (P<0.05) pour l'incidence et la sévérité de la bactériose entre les cultivars. Les cultivars Kenya Dhamana, Mauritius and Cruza (CIP-740118) ont une faible sévérité et incidence de la bactériose et ont été considérés tolérants, alors que les autres cultivars ont été classés modèrement sensibles ou très sensibles.

Mots Clés: Bactériose, incidence, Kenya, Solanum tuberosum, tolérance

INTRODUCTION

Bacterial wilt is a major disease of potatoes (Solanum tuberosum L.) in the tropics and sub-tropics (Hayward, 1990; Boucher et al., 1992; Jaunet and Wang, 1999). In Kenya the potato industry is threatened by bacterial wilt (BW) because most production areas are infested with the wilt-causing bacterium and over 50% yield losses have been reported (Ajanga, 1993). Spread of the disease is difficult to contain due to the use of infected potato tubers as seed by the growers.

Use of resistant varieties is undoubtedly the cheapest and environmentally friendliest means of controlling plant diseases (French et al., 1996), particularly for countries with underdeveloped or developing agriculture (Prior et al., 1994). Breeding programmes have been initiated in many parts of the world, but acceptable varieties with good resistance to bacterial wilt are yet to be identified. This situation is worsened because some potato cultivars exhibit variable reactions in different environments and in different seasons within the same environment (Tung et al., 1990; Grimault and Prior, 1993; French, 1994). The situation is exacerbated by the fact that the pathogen has a high level of genetic polymorphism (Dristig and Dianese, 1990; Yabuuchi et al., 1992; Frey et al., 1996; Thierry et al., 1999). Lack of detailed information on occurrence of bacterial wilt and the reaction of potato cultivars prompted us to undertake a survey and evaluate potato germplasm for resistance to bacterial wilt. The objectives of the study were to establish (a) the incidence of bacterial wilt in Kenya and (b) the reaction of selected potato cultivars to R. solanacearum race 3 under field and greenhouse conditions.

MATERIALS AND METHODS

A survey of bacterial wilt. A survey of BW was carried out in three potato growing districts of Kenya, namely, Nyeri, Nyandarua and Meru. The survey was carried out at the flowering stage of the crop during the short rain season (November - December 1997). Thirty potato fields in each district were randomly selected at intervals of 3-5 kilometers. In each field, ten rows of about 100 plants were selected at random and wilt disease identified using the procedure described by French and Martin (1985). Bacterial wilt incidence was based on the number of plants showing symptoms expressed as a percentage of the total number of plants observed (James, 1974). Disease prevalence was based on the number of potato fields with bacterial wilt expressed as percentage of the total number of fields assessed. Spot altitudes of the fields visited were taken using an altimeter.

Reaction of potato cultivars to R. solanacearum. A glasshouse test was conducted between January and May 1998 to determine the reaction of potato cultivars to R. solanacearum. The cultivars tested were Nyayo, Desiree CIP-800048, Roslin Tana, Kerrs pink LB-5, Golof (Dutch Robjin), B53, Tigoni CIP-381381.13, Rutuku CIP-720097, Asante CIP-381381.20, Kenya Dhamana CIP- 800228, Mauritius Clone, KP93739.26, KP92633.26, and Cruza148 CIP-720118. Certified tubers of each cultivar were sown in heat-sterilised soil:sand mixture (2:1) contained in 20 cm-diameter pots. Twenty-one days after planting, potato plants were inoculated with a bacterial suspension containing 10⁹ cfu/ml of R. solanacearum race 3. Inoculation was done using the method of Winstead and Kelman (1952). The inoculum consisted of a mixture of R. solanacearum isolates obtained from Nyeri, Meru and Nyandarua districts. Virulent colonies growing on tetrazolium chloride medium were selected and grown on nutrient agar for 48 hr at 30-33°C. Treatments were arranged in a completely randomised design with three replications. Plants were rated at six day intervals for bacterial wilt severity, from the time symptoms were first observed to 60 days after inoculation (DAI). A scale of 0-5 was used where; 0 = no symptoms, 1 = 1 leaf wilted, 2 = 2 or 3 leaves wilted, 3 = All the leaves wilted except the top 2 or 3 leaves, 4 = All leaves wilted, 5 = plant dead.

A field experiment to determine the reaction of the potato cultivars to R. solanacearum was conducted in a naturally infested field at the University of Nairobi's Kabete Campus. The field is situated at an altitude of about 1880 m and receives an average rainfall of 1500 mm p.a. with a mean temperature of 23°C. Tubers were sown in 2 m x 3 m plots at a spacing of 30 cm x 60 cm. Treatments were arranged in a randomised complete block design with four replications. From each plot, six plants were randomly selected and labeled. Disease severity was monitored on the labeled plants from which a mean for the whole plot was computed. Reaction of the potato genotype was rated using the genotypes scale of 0-5 described earlier. The total number of plants infected per plot was also recorded weekly and expressed as a percentage of the total number of plants per plot (incidence). All data were subjected to analysis of variance and means were separated at P = 0.05.

RESULTS

Incidence of Bacterial wilt. Bacterial wilt was observed in 71.1% of the farms visited. Prevalence of bacterial wilt was highest in Meru district (86.6%), intermediate (73.3%) in Nyeri and lowest (56.7%) in Nyandarua. On the other hand, incidence was highest in Nyeri district (18.8%), followed by Meru (16.7%), and lowest (10.4%,) in Nyandarua (Table 1). Significant (P=0.05) variations in bacterial wilt incidence among divisions in the districts were observed. Nkuene division had the highest incidence (23.1%) while Ndaragwa division of Nyandarua district had the lowest (4.9%) (Table 2).

The incidence of bacterial wilt at the different altitude ranges is depicted in Figure 1. The highest incidence (19.8%) was recorded in sites within the altitude range of 1800-2000 m, while the lowest was observed in sites located 2600 m and above. The correlation (r = -0.34) between bacterial wilt incidence and altitude was significant (P=0.05) and negative indicating that bacterial wilt incidence decreased with increase in altitude (Fig. 1).

Some form of crop rotation was practiced in 25.6% of the farms visited. The mean bacterial wilt incidence in farms where crop rotation was practiced was 7.7% as opposed to 17.6% in farms where potatoes were grown continuously. By the student's t-test procedure, a significant (P=0.05) difference in bacterial wilt incidence was observed between farms that practiced crop rotation and those that did not.

Reaction of potato cultivars to Ralstonia solanacearum. Symptoms of bacterial wilt were first observed on potato cultivars Asante and Tigoni 13 days post inoculations in the greenhouse. There were significant (P=0.05) differences in bacterial wilt severity among the cultivars (Table 3). Severity of bacterial wilt was highest (3.2) in cv. Rutuku (CIP 720097) and lowest (1.2) in cvs. Cruza and the Mauritius clone. However, the severity of wilt during the field screening was highest (3.2) in cv. Rutuku (CIP 720097) and lowest (0.9) in cv. Kenya Dhamana.

DISCUSSION

There was high prevalence of bacterial wilt indicating that the disease has steadily spread to most potato growing areas in Kenya. The use of infected potato seed and accumulation of R. solanacearum in the soil over a time may have also contributed to the spread of the disease. Bacterial wilt incidence varied from one district to the other and also among divisions within each district. Factors such as temperature, rainfall, potato cultivar and cropping systems could account for the disparities (Nyangeri et al., 1984; Ajanga, 1993).

Although crop rotation reduces bacterial wilt incidence (Melton and Powell, 1991), it is not practical due to land pressure in most potato growing areas (Berrios and Rubigiri, 1992; French, 1994). In most cases where rotation was practiced, solanaceous crops were included thus nullifying the objective of rotation. However, in divisions such as Ndaragwa and Ojororok of Nyandarua district, where average land sizes per household were large (2 ha or more per household), incidence of the disease was lower because most potato crops were grown on previously fallow land.

The incidence of bacterial wilt correlated negatively with altitude thus confirming reports by Nyangeri et al. (1984) that the disease is more prevalent at lower altitudes. However, bacterial wilt was observed at high altitudes (above 2500 m) with high frequency. This is contrary to Harris (1976), that bacterial wilt is inhibited at altitude of > 2400 m.

Abdullah (1983) and Ho (1988) reported that high soil moisture resulting from high rainfall especially towards the end of the growing season favours bacterial wilt development. The high incidence of bacterial wilt in Nyeri (18.8%) and Meru (16.7%) as opposed to Nyandura (10.4%) may have been related to high rainfall experienced in these two districts (Table 1).

Most farmers used tubers saved from a previous crop or potato purchased from the local markets. This might have contributed to the spread of the disease. Popular cultivars in the area such as "Nyayo" and "Kerr's Pink" were susceptible to R. solanacearum. This, together with the use of uncertified seed might have led to the build-up of the pathogen in the soil.
The high incidence and severity of bacterial wilt in the inoculated cultivars confirmed previous reports that it is difficult to obtain stable resistance to R. solanacearum (Saumtally et al., 1993; Bhagsari et al., 1994). According to Smith et al. (1995) and Frey et al. (1996), the fact that the pathogen exhibits a high level of genetic dimorphism makes the problem worse. Kenya Dhamana-CIP-800224, Mauritius clone and Cruza CIP 720118 however, performed relatively well and were rated as tolerant to bacterial wilt. Asante CIP-381381.20, Tigoni , and CIP 381381-13 were highly susceptible, contradicting the reports of Ng'ang'a et al. (1995).

Contrasting results in bacterial wilt severity were obtained when potato cultivars were inoculated with R. solanacearum underfield and greenhouse conditions. This observation is in agreement with that of Tomlinson and Gunther (1986) and French (1994) who conducted similar experiments under field and greenhouse conditions at different times of the year. Greenhouse and field environments differ considerably, suggesting that resistance against R. solanacearum may be influenced by environmental factors.

REFERENCES

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Ajanga, S. 1993. Status of bacterial wilt in Kenya. In: Bacterial wilt. Hartman, G. L. and Hayward, A. C. (Eds.), pp. 338-340. ACIAR Proceedings No. 45, Canberra, Australia.

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TABLE 1. Bacterial wilt incidence, rainfall and temperature in Meru, Nyandarua and Nyeri districts of Kenya

District	Bacterial wilt Incidence (%)1	Rainfall (mm)2	Temperature (°C)2
Meru	16.7	1299	18.5
Nyandarua	10.4	1125	13.7
Nyeri	18.8	1220	16.1
Lsd (P=0.05)	3.7
CV (%)	83.2

TABLE 2. Bacterial wilt incidence in the different divisions of Meru, Nyeri and Nyandarua districts of Kenya
Division Wilt incidence (%)¹

Division	Wilt incidence (%)1
Meru
Timau	13.0
Nkuene	23.1
A. West	14.0
Nyeri
Kieni West	13.1
Othaya	22.8
Kieni East	18.3
Nyandarua
Ojororok	3.3
Olkalou	13.0
Ndaragwa	4.9
LSD (P=0.05)	6.1
CV (%)	79.4

TABLE 3. Incidence and severity of bacterial wilt in fifteen potato cultivars 60 days after noculation with R. solanacearum in the greenhouse and under field conditions

Cultivar	Greenhouse		Field
	Incidence (%)	Severity 1	Incidence (%)	Severity 1
Rutuku CIP-720097	56.1	3.2	52.9	3.2
Dutch Robjin	51.3	2.9	51.9	2.9
B53	33.4	2.9	32.4	1.9
KP92633.26	30.5	2.9	29.7	1.2
KP93739.26	23.9	2.8	23.5	1.1
R. Tana	29.0	2.7	27.7	1.5
Asante CIP 381381.20	56.2	2.6	52.5	3.6
Tigoni	55.3	2.6	52.2	3.4
Nyayo	43.6	1.9	42.9	1.8
Kenya Dhamana	21.1	1.8	22.6	0.9
K. Pink	36.8	1.8	30.6	2.0
Desiree CIP-800048	40.3	1.8	37.8	1.9
Furaha	45.6	1.6	42.2	2.5
Cruza 720118	24.0	1.2	24.8	1.1
Mauritius Clone	21.4	1.2	21.3	1.0
LSD (P=0.05)	3.9	0.5	5.0	0.4

Figure 1. Effect of altitude on bacterial wilt incidence.

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