African Crop Science Journal, Vol. 9, No. 1, March 2001 267-278
Berga Lemaga, D. Siriri1 and P. Ebanyat2
Code Number: CS01057
Potato bacterial wilt caused by Ralstonia solanacearum is a major threat to potato production in Sub-Saharan Africa. It is believed that yield losses due to bacterial wilt increase with decreasing soil fertility. A soil amendment experiment was therefore conducted for 3 consecutive seasons, 1998A, 1998B and 1999A at Kachwekano at an altitude of 2200 meters) in southwestern Uganda. Organic materials: Sesbania sesban (S) and Leucaena diversifolia (L.) were applied in amounts sufficient to supply 100 kg N ha-1 either singly or combined with P and PK. Also added were NP and NPK from inorganic sources. The organic materials were incorporated into soil one week before planting, while the inorganic fertilisers were side-dressed at planting all at rates that would supply 100 kg ha-1 of N, P and K. Nitrogen in the form of urea was split-applied at planting and one month after. Bacterial wilt incidence differed with treatments and seasons. Disease incidence was lowest with treatments NP and S+ PK and highest with the control. Application of organic manures alone did not necessarily result in reduced wilt incidence except in a few cases. Both marketable and total tuber yields were consistently highest with S + PK and differed significantly from the control in all seasons. A combined analysis over the three seasons showed that the treatment S + PK gave a significantly higher yield (20.8 ha-1) than all other treatments, while the control yielded significantly lower (9.7 ha-1) than the other treatments. Sesbania as an organic manure performed better than Leucaena and potassium was found to be a useful nutrient for crop performance. When K was applied with NP, LP and SP, it brought about marketable yield increases of 11, 23 and 37%, respectively. Generally, the rate of wilt development, expressed in wilt incidence per unit time, was highest at early stage of growth, thereafter, it declined and stabilised during much of the tuber bulking stage The interaction between soil fertility and bacterial wilt incidence merits further studies in different environments.
Key Words: Organic materials, inorganic fertilisers, polyphenols, ware yields, wilt incidence
La bactériose causée par Ralstonia solanacearum constitue une contrainte majeure de la production de la pomme de terre dans la région sub-Saharienne de l'Afrique. Il est connu que les pertes de rendements causées par la bactériose augmentent suite au déclin de la fertilité du sol. Un essai d'amandement du sol a été conduit pendant les saisons culturales 1998A, 1998B, 1999A et 1999B à Kachwekano (2000m asl) au Sud-Ouest de l'Uganda. Les matériaux organiques: Sesbania (S) et Leucaena diversifolia (L.) ont été utilisés en quantités suffisantes pour produire 100 kg N ha-1 soit seuls ou combinés avec P et PK. Les engrais inorganiques NP et NPK ont été aussi ajoutés. Les matériaux organiques ont été incorporés au sol une semaine avant la plantation alors que les engrais inorganiques ont été placés près des plantes lors de la plantation tous à des doses pouvant produire 100 kg ha-1 de N, P et K. L' azote en forme d'urée a été divisée en deux, la moitié appliquée à la plantation et l'autre moitié un mois après la plantation. L'incidence de la bactériose a été differente selon les traitements et la saison. L' incidence a été faible avec les traitements NP et S+PK et a elle été plus élevée avec le contrôle. L'application de la fumure organique seule n'a pas nécessairement conduit à une réduction du mildiou excepté dans peu de cas. Ensemble les tubercules commercialisables et les tubercules totaux étaient constament élevés avec le traitement S+PK et étaient significativement differents du contrôle pour toutes les saisons. Une analyse combinée pour les trois saisons a montré que le traitement S+PK a donné un rendement significativement élevé (20.8 t ha-1) plus que d'autres traitements, alors que le témoin a eu un rendement (9.7 t ha-1) le plus bas de tous les autres traitements. Sesbania en tant fumure organique a eu une meilleure performance plus que Leucaena et le potassium a été trouvé utile pour la performance de la culture. Quand le K a été appliqué avec NP, LP et SP, il a conduit à une augmentation du rendement commercialisable de 11, 23 et 37 % respectivement. Génerallement le taux de développement de la bactériose, exprimé en incidence par unité de temps, était plus élevé au premier stade de croissance, plus il eut un déclin et puis elle s'est stabilisée au stade de grossissement des tubecules. L'interaction entre la fertilité du sol et l'incidence du mildiou nécessite des études futures dans des environnements differents.
Mots Clés: Matériels organiques, engrais inorganiques,
polyphénoles, les rendements commercialisable, incidence de la bactériose
Potato (Solanum tuberosum L.) is an important cash and food crop in southwestern Uganda (Low, 1997). The yields however are low due to a number of factors, of which bacterial wilt caused by Ralstonia solanacearum is an important one (Tusiime et al, 1996a, b; Low, 1997; Adipala et al, 2000). Bacterial (BW) is destructive and widespread in the tropical, subtropical and warm temperate regions of the world and attacks a wide range of crop species.
The incidence of the disease is exacerbated under conditions of moisture stress,
low soil fertility and low pH (Woltz and Jones, 1968; Shekhawat et al, 1978).
Ferralsols, the predominant soil type in Kabale (southwestern Uganda), has been
elsewhere reported to be conducive to bacterial wilt due to its lower pH and
organic matter contents that support multiplication of the pathogen (Ramesh
and Bandyopadhyay, 1993). Shekhawat et al (1978) also showed that lower soil
pH favoured BW. Productivity of soils in the highlands of Kabale, like most
other areas in East Africa, is limited by low nitrogen (Rao et al, 1998). Recent
trials on limiting nutrients in Kabale indicated high response of cereal crops
to nitrogen resulting in a yield advantage of over 70% upon nitrogen application,
particularly on the upper degraded two-thirds of the terrace (AFRENA, 1999,
unpubl.). The same study also found responses to potassium on 60% of the sites
studied, while phosphorus limitation was observed mainly on soils of volcanic
origin. Besides nutrient limitations, soil physical constraints in the form
of high bulk density, low water conductivity and reduced effective rooting depth
of crops on the upper parts of terraces have been shown to greatly affect crop
performance (Siriri, 1998). This is a result of a soil-scouring phenomenon caused
by downward cultivation and soil erosion, which results in movement of good
topsoil down the slope exposing infertile sub-soils on upper terraces.
Under limited resource conditions in southwestern Uganda and unavailability of inorganic fertilisers, soil fertility could be improved by addition of organic manures through leaf litter and plant residues and closely managing the nutrient flows (Sanchez et al, 1997). To provide adequate nitrogen in the soils through organic matter, high quality organic materials (Giller et al, 1997) that are free of Ralstonia solanacearum need to be used.
Since the contribution of most organic materials to nutrient supply is limited, an appropriate strategy in nutrient resource flow management will be to use them together with inorganic chemical fertilisers (Palm et al, 1997). This approach optimises nutrient availability to plants. Addition of inorganic fertilisers with legume organic materials has been reported to increase yields and fertiliser use efficiency, and helps to better synchronise nutrient availability to crop demand (Jones et al, 1997).
The type of leguminous organic materials to be used depends on their quality with respect to decomposition and nutrient release characteristics and on their availability. Two species, Sesbania sesban and Leucaena diversifolia produce a fairly high quantity of nutrient-rich biomass, 34, 1.5, 11-21 kg N, P and K, respectively, per ton of dry material (Palm and Rowland, 1997). Sesbania is indigenous to Kigezi highlands, where the experiment was conducted and is mostly found in swampy places, while Leucaena is exotic but grown on contour hedges and as fodder. Both release high N in a short time; for example, Lueceana releases 50% of N within 3-4 weeks from time of application (Mittal et al, 1992; Handanyonto et al, 1994).
Most of the research on organic and inorganic soil amendments mainly focused on yield responses and nutrient recovery (Goyal et al, 1992; Mittal et al, 1992; Xu et al, 1993; Jones et al, 1996) and exhibited that test crops performed better under combined applications than under sole applications. In contrast, information on management of soil nutrients to reduce bacterial wilt is scanty. This study was therefore conducted to quantify the effects of sole and combined application of organic and inorganic fertilisers on bacterial wilt incidence and potato yields.
MATERIALS AND METHODS
Study site.The study site was located at Kachwekano Agriculture Research and Development Centre in southwestern Uganda (01°15'S and 29°57'E) on soils classified as isomesic typic palehumult (Yost and Eswaran, 1990) at an elevation of 2200 metres. Soil characteristics of the experimental site (Table 1) indicate low levels of mainly nitrogen. The area has a rugged terrain with hillside farming done on developed terrace benches. The rainfall is bimodal with a mean of about 1000 mm per annum. Mean maximum and mean minimum temperatures are 23°C and 10°C, respectively. Weather conditions during the experimental period are given in Figure 1.
Trial layout and treatments. The study was conducted for three seasons, i.e., during 1998A (mid May to mid August), 1998B (mid October to mid January) and 1999A (late March to mid July). To avoid effects of the previous season's treatments on soil fertility and Ralstonia solanacearum inoculum pressure, each season had a new field, but in the same location. The trial was laid in a randomised complete block design with 3 replications. Plot size was 16.2 m2 to which disease-free potato seed tubers of var. Victoria (CIP-381 381.2)) was planted and amended with either organic materials (Sesbania sesban or Leucaena diversifolia) or inorganic fertilisers (N, P, K) or different combinations of these. The treatments were:
The sesbania and leucaena were applied as fresh leaves and twigs of in quantities needed to supply 100 kg N ha-1 according to the chemical characteristics summarised in Table 2 and incorporated into soil a week before planting to leave sufficient time for decomposition. These are mainly leguminous green manures high in N but low in lignin and polyphenols that release N quickly (Constantinides and Fownes, 1994; Handayanto et al, 1994; Palm, 1995, Giller et al, 1997). The major limitation of the organic materials is usually low amounts of phosphorus (Palm et al, 1997; Buresh, 1999).
Inorganic fertilisers N, P and K were side-dressed each at adequacy rates of 100 kg ha-1 as urea, triple super phosphate (TSP) and muriate of potash (KCl), respectively. One-half of the nitrogen was applied at planting and the other half at one month after planting, while TSP and KCl were both applied at planting.
Disease monitoring and crop yield assessment. Assessment of the bacterial wilt disease started with the onset of first wilt symptoms, after which counting of wilted plants was done on a weekly basis. Plants that showed either complete or partial wilting were all considered wilted and staked to avoid double counting in subsequent assessments and also to avoid the possibility of missing out those that completely die early in the growth period. Previous studies (Bergaand Priou, unpubl.) confirmed that BW in the experimental area is caused by race 3 of Ralstonia solanacearum only. Thus the results given here hold true for this specific race. Wilt incidence was then calculated as percentage of total number of plants emerged. For statistical comparisons, percent wilted plants were transformed into square roots before analysis. Plants were harvested after about 75% of the plants showed senescence. At harvest, total and marketable ware potato yields were recorded. Data were statistically analysed using Genstat 4.23 Release.
Effect of soil amendments on bacterial wilt incidence. Bacterial wilt incidence varied with the seasons and was highest in 1998A and lowest in 1999A (Table 3). The control treatment enhanced wilting and resulted in the highest incidence in 1998A (66.5%) and 1998B (24.5%) that differed significantly from all other treatments except for the S and L treatments during the first season and for L and L + P during the latter. Treatments NP, S+PK and NPK resulted in lowest bacterial wilt incidences, except for a few cases, that were mostly significant from other treatments. Generally, application of green materials alone did not necessarily have a big effect on BW incidence, however, when they were applied together with inorganic fertilisers the effects become evident.
The progress of BW incidence was dramatic at early stage of crop growth and the trend declined rather rapidly between 40 and 61 days after planting (DAP). Thereafter, it appeared to stabilise up to 70 DAP and showed slight increases after that (Fig. 2a). This suggests that highest disease incidence and progress occurred during early stages of the crop growth. Accelerated disease build-up with time was found within the first 61 DAP of growth for all treatments, except for the treatment NP, where rapid disease accumulation stopped about two weeks earlier (Fig. 2b)
Yield response to treatments. Application of soil amendments brought about increases in both total and marketable ware potato yields (Table 3). However, only the S+PK treatment resulted in significantly higher marketable yields in all the three seasons compared to the control and accounted for 230, 218 and 33% yield increases for 1998A, 1998B and 1999A seasons, respectively. In 1998B all the treatments gave significantly higher marketable yields than the control. The treatment NP was significantly superior to the control in both 1998A and 1998B seasons with marketable yield increases of 200 and 97%, respectively. Marketable yield increases for the different treatments over the control ranged from 42 to 230% in 1998A and from 97 to 218% in 1998B. Tuber yields were lowest in the first season when BW incidence was highest.
The mean effects of inorganic and organic soil amendments across the seasons were significantly higher than the control for both total and marketable ware yields (Table 4). Amending the soil with sesbania alone yielded as high as NPK application, but the highest total and marketable yields that were significantly different from others were obtained when sesbania and PK were combined resulting in marketable yield increases of more than 110% over the control. The corresponding increases with S + PK over sole application of sesbania and NPK accounted for 25 and 23%, respectively. No significant responses were obtained to application of P either alone or with organic amendments.
When P was in adequate amounts, potassium application with inorganic and organic sources (both mainly supplying N) resulted in yield increases of 12, 23 and 37% respectively for NP, LP and SP treatments (Fig. 3). This demonstrates that application of K was important to increase yield on the soils under consideration disagreeing with the general belief that East African soils are rich in K and do not respond to K application. The effects were more prominent when it was added together with organic materials than with N from inorganic sources. Sesbania in all the seasons performed better than leucaena.
Relationships between bacterial wilt incidence and potato yield. Application of inorganic and organic treatments resulted in significant effects on mean bacterial wilt incidence and marketable ware potato yields across the three seasons (Fig. 4). The highest bacterial wilt incidence occurred with the control treatment, which also brought about the lowest tuber yield. On the other hand the treatments S+PK and NP, which suppressed bacterial wilt incidence the most yielded higher, with the former treatment giving the highest yield of 22.4 t ha-1. Potato marketable yields had a significant (P<0.001) negative relationship (R2 = 0.56) with BW incidence (Fig. 5). A unit increase in bacterial wilt would apparently result in a 2.5 unit yield reduction.
Bacterial wilt incidence significantly varied with seasons with the highest incidence being recorded during the 1998A season and the lowest in 1999A. This variability could be attributed to differences in weather and R. solancearum populations in the soil. The season 1999A received the least amount of precipitation with almost no rainfall in May and June. Several investigators, for example, Akiew (1986), reported a positive relationship between soil moisture and BW. Akiew (1986) observed that the decline of the R. solanacearum population with decreasing soil moisture was faster with race 3 than with race 1 of the bacterium. Lemaga and Priou (unpubl.) showed that the location where this experiment was conducted has only race 3. In Guadeloupe, hydration of oxisols reduced the suppressiveness of the soil and increased bacterial wilt incidence (Prior et al, 1993).
Highest rate of bacterial wilt incidence was recorded at 40 DAP and the progress declined with time (Fig. 2). A similar observation was made in another experiment conducted at the same site (Lemaga et al, 2001, this volume). This could be attributed to the fact that early stage coincides with active root formation providing openings for the organism to enter into the plant. Sequeira (1993) reported that the bacterium penetrated into the roots of its hosts along the points of emergence of secondary roots. He also asserted that injuries caused to roots mechanically or by nematodes facilitate entrance of the organism and effective infection occurs with fewer bacterial cells, but the organism also enters the roots in the absence of wounds. In the later case, more bacteria may be needed to infect the plant. It is thus possible that the higher rate of infection at early stage reported in this study could also partially be due to the delicate tissues the potatoes had that could not adequately resist penetration by the pathogen.
In many cases, soil amendments significantly reduced BW incidence as compared to the control signifying the importance of soil fertility in reducing the disease. Well-nourished plants are stronger and could better withstand disease organisms than poorly nourished ones (Muchovej et al, 1980) probably due to absorption of adequate nutrients through a well-established root system, which may not all be infected. There is evidence that the population of R. solanacearum race 3 declines in soils that have high organic matter content (Moffet et al, 1983) and high nitrogen content (Prior et al, 1993; Dhital et al, 1997). However, in the current study it was not always the case because application of S and L treatments alone in two of the three seasons resulted in comparable cumulative percentage of bacterial wilt incidence to the control. The discrepancy can be explained partly by differences in supressiveness of the soils used in various experiments and soil moisture contents.
Significant total and marketable yield responses to application of soil amendments were obtained in each of the seasons (Table 3). Lowest yield responses to treatments occurred in the 1998A season and highest responses in 1998B in which all the yields were significantly higher than the control. This could be attributed to differences in disease incidence, which was highest in 1998A (Table 3) as well as a higher and better distribution of rainfall. Probably because of the rainfall factor, highest yields were obtained in 1998B despite the fact that wilt incidence was higher than in 1999A. In this study, plants that showed both partial and complete wilting were considered wilted; however, partially wilted plants yielded apparently healthy looking tubers that contributed to total and marketable ware potatoes. It is therefore important that care must be exercised when there is need to use such tubers for seed, as the tubers may be latently infected and can spread BW. It is known that latently infected tubers are a major cause of wilt dissemination worldwide (Martin and French, 1985). Tusiime et al (1996b) also reported that seed in the highland of Uganda was latently infected posing a danger of wilt spread.
The S+PK, treatment consistently gave significantly higher total and marketable yields than the control in each of the seasons. This could be attributed to a better nutrient balance that is achieved when organic and inorganic nutrient sources were used. It further appears that organic materials in addition to releasing high amounts of N also help to improve other soil conditions, which together with balanced nutrient supply results in good crop performance.
Organic residues are reported to improve moisture retention (Palm et al, 1997) and biological activity (Woomer et al, 1994) in soils hence improving nutrient availability and nutrient use efficiency (Palm et al, 1997). It is not certain why application of Sesbania increased yields in the presence of ample amounts of P, but it may be because of increased K supply through fast decomposition and also retaintion of it against leaching losses that presumably improved its use efficiency. Thus, combined application of organic and inorganic soil amendments could be a better option than only organics, however, the quality of the organic material and specific soil constraints to be addressed must be carefully considered (Giller et al, 1997). Sesbania performed better than leucaena when supplied alone or in combination with inorganic fertilisers due probably to its lower polyphenol content.
Improving soil fertility (though not fully quantitatively and qualitatively investigated in this study) can suppress bacterial wilt incidence and hence increase potato yields. As can be seen in Fig. 5, increased incidence in bacterial wilt lowers marketable yields. The S+PK treatment provided the best results (Fig. 4) but the economic feasibility of this approach is yet to be evaluated.
CONCLUSIONS AND RECOMMENDATIONS
Wilt incidence greatly varied with seasons vividly showing the strong interaction between Ralstonia solanacearum and the environment. Within a season the rate of wilt development was highest at early stages of growth, which declined and stabilised as time elapsed. Soil amendments helped to reduce wilt incidence and increase potato yields, but best results were obtained when organic and inorganic fertilisers were combined. The presence of K in any combination of amendments was important for both wilt reduction and yield increases. The effect became more pronounced when K was added with organic than with inorganic N sources. Sesbania as an organic material performed better than leucaena. The data indicated that a combined application of organic and in organic soil amendments could be one of the components for the integrated control of BW to increase ware yields. It is however important to investigate whether same thing applies for seed potatoes as well.
We recommend that (1) the availability of organic materials and the economic viability of soil amendments to increase potato production in areas that have a wilt problem and (2) the interaction between soil fertility and bacterial wilt incidence merit further studies in different environments.
This experiment was part of the activities of integrated pest management of the African Highlands Initiative (AHI). The authors also thank Mr. G. Manzi, AHI assistant in Kabale, for careful data collection and consistent and reliable follow-up throughout the research. The technical support obtained from Drs. P. Ewell, S. Priou and E.R. French of CIP is highly acknowledged. The strong collaboration of the AFRENA Project in Kabale is duly appreciated.
TABLE 1. Site soils characterisation
TABLE 2. Composition of the organic amendments
Source: Palm et al (1997); Mafongoya et al (1997)
TABLE 3. Effect of soil amendments on bacterial wilt incidence and potato tuber yields
* Figures in parenthesis are square roots; NS = Not significant at P<0.05
TABLE 4. Effect of soil amendments on mean potato yields over three seasons (1998A- 1999A)
* Means followed by same letters in columns are not statistically different at P<0.05
Figure 1. Trends in mean monthly rainfall, maximum and minimum temperatures during the experimental period. Source: Meteorological Department, Kabale.
Figure 2. Effect of organic and inorganic amendments on progress of bacterial wilt during the course of growth (a) and on cumulative bacterial wilt incidence (b) during 1998A season.
Figure 3. Yield increase due to potassium as affected by N source, P in abundant amounts.
Figure 4. Effect of organic and inorganic soil amendments on bacterial wilt incidence and marketable yield over three seasons.
Figure 5. Relationship between marketable yield and bacterial wilt incidence.
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