DEVELOPMENT OF AN INTEGRATED BEAN ROOT ROT CONTROL STRATEGY FOR WESTERN KENYA
R.M. OTSYULA, S.I. AJANGA, R.A. BURUCHARA^1 and C.S. WORTMANN^1
Kenya Agricultural Research Institute, P. O. Box 1629, Kakamega, Kenya
(Received 30 January, 1997; accepted 27 October, 1997)
Root rot severely constrains bean (Phaseolus vulgaris L.) production in parts of Western Kenya. Genetic and soil management options were evaluated for an integrated root rot management approach. Four hundred genotypes were progressively screened in the field for resistance to root not over six seasons. A sequence of problem-solving trials on cultural methods was conducted over four seasons. All trials were conducted in root rot infested fields of farmers. No resistant varieties were found in the Kenyan germplasm collection of 374 accessions. Ten resistant varieties were identified from 26 introductions which were known to be resistant to root rots in Rwanda. Farmers preferred MLB-49-89a because of its early maturity, SCAM 80-CM/5 and RWR 532 because of their high yield and seed type, and the late maturing RWR 719 as it was seen as a replacement for a previously popular cultivar which succumbed to root rots. Crop tolerance to root rots was greatly improved by improving soil fertility through application of diammonium phosphate or certain organic manures, and less so by sowing on ridges. Application of several rapidly decomposing green manures, including Tithonia diversifolia which was abundantly available in field borders, resulted in improved crop tolerance to root rot.
Key Words: Phaseolus vulgaris L., resistance, root rot, soil fertility, integrated disease management
Les pourritures racinaires constituent une contrainte majeure pour la production du haricot (Phaseolus vulgaris L.) dans certaines zones de l'Ouest du Kenya. L'utilisation du materiel genetique et la gestion du sol ont ete evaluees comme une approche de lutte integree contre ces maladies racinaires. Quatre cent genotypes etaient progressivement cribles dans le champ pour resistance sur une periode de six saisons culturales. Une serie d'essais sur les methodes culturales pour le controle de ces maladies etait conduite pendant quatre saisons. Tous les essais etaient etablis dans les champs des fermiers infestes avec les pourritures racinaires. Aucune variete n'etait trouvee resistante dans la collection du germoplasme kenyan de 374 accessions. Dix varietes etaient cependant identifiees a partir de 26 varietes introduites qui etaient anterieurement connues au Rwanda comme resistantes aux pourritures racinaires. La lignee 'MLB-49-89 A' etait preferee par les paysans a cause de sa maturite precoce; 'SCAM-80-CM/5' et 'RWR 532' preferees a cause de leur haut rendement et le type de graine; tandis que 'RWR 719' vu comme un cultivar tardif etait considere comme le remplacant d'un cultivar populaire qui a succombe anterieurement suite aux pourritures racinaires. La tolerance du haricot aux pourritures racinaires etait grandement amelioree en ameliorant la fertilite du sol par l'application de di-ammonium phosphate (DAP) ou certains fumiers organiques, et elle etait aussi moins en semant sur des billons. L'application d'engrais verts se decomposant rapidement y compris Tithonia diversifolia lequel etait abondamment disponible dans les abords des champs, a resulte dans l'amelioration de la tolerance du haricot face aux pourritures racinaires.
Mots Cles: Phaseolus vulgaris L., resistance, pourriture racinaire, fertilite du sol, lutte integree contre la pourriture racinaire
Bean (Phaseolus Vulgaris L.)production in Western Kenya is severely constrained by root rot, especially where soil fertility is low and bean production is intensive. Root rot has been observed to be serious under similar conditions in parts of Burundi, Central Kenya, Uganda, Rwanda, and Zaire (CIAT, 1992). In Western Kenya, root rot is primarily caused by Fusarium solani sp. phaseoli, Rhizoctonia solani, and Pythium species (R. Buruchara, pers. comm.). Root rot pathogens attack beans at all growth stages and cause damping-off at the seedling stage, yellowing of the leaves, stunted growth, and death if severe. Bean is a major food crop in Western Kenya (Gitu, 1992) and small-scale farmers are reluctant to decrease the frequency of sowing bean on a piece of land. Potentially effective crop rotations to maintain the pathogen at low levels (Hall and Phillips, 1992) are not currently acceptable.
The ability of a bean crop to tolerate root rots is related to soil nutrient supply. With high soil fertility, bean grows vigorously and tolerates root rot infections (Otsyula and Ajanga, 1994). Application of fertilizers or readily decomposed organic manures have been shown to improve crop tolerance to root rots (CIAT, 1992; Mutitu et al., 1985 and 1989). This effect appears to be primarily due to the plants' improved ability to obtain adequate nutrients. Addition of fertilizer or organic manures may affect the pathogens, either directly or indirectly, e.g. through attack of root rot pathogens by soil micro-organisms or competition for some essential substrate (Papavisas and Davey, 1960; Cook, 1995 ; Liu et al., 1995).
The effectiveness of other cultural practices in contributing to the control of root rots has been demonstrated in Rwanda (Buruchara, 1991). Planting on ridges can be useful where soils are not well aerated (Pieczark and Abawi, 1978; Miller and Burke, 1985; Buruchara and Rusuku, 1992). Proper use of fungicides results in effective control. Hilling-up soil around the stems of seedlings encourages growth of adventitious roots. Varietal tolerance, expressed as ability to produce adventitious roots and recover from attack, and resistance, expressed as low levels of infection, has been found in bean germplasm (CIAT, 1992 and 1993). The effectiveness of individual practices varies with environmental conditions and additive effects, and sometimes positive interactions, often result from combining two or more cultural practices (Abawi and Pastor-Corrales, 1990; CIAT, 1992); hence an integrated root rot management approach is preferred.
Utilising available information on root rot management, an adaptive research programme was initiated in 1993 to develop alternative components for integrated management of bean root rots. The objectives were to identify resistant or tolerant genotypes which were acceptable to farmers and consumers, and to evaluate the effectiveness of different cultural practices in the control of the root rot complex.
MATERIALS AND METHODS
Farmers' fields with heavy infestation of root rot were identified in Vihiga District in Kenya in 1992, and subsequently used for trials. Weather conditions varied across sites but the characteristics for the Western Agricultural Research Station (WARS) of the Kenya Agricultural Research Organisation are considered to be typical for the research sites: altitude, 1530 m asl; latitude 0 degrees 18'N; longitude 34 degrees 45'E; a bi-modal rainfall with 60% reliability of at least 800-900 mm during the first season and 600-700 mm during the second season; and little variation from the mean maximum (28.6 C) and minimum (12.8 C) temperatures. The red soils of the trial sites were not characterised in detail but were well-drained, deep, and of sandy clay to clay texture (humic/ferralo-chromic/orthic Acrisols (Jaetzold and Schmidt, 1982). The soils in this area are typically low in available soil N and P.
Genotype evaluation for resistance to the root rot complex and a series of experiments to test the effectiveness of alternative cultural practices were initiated in 1993. Observations made on these trials included: plant survival at two weeks after planting (WAP) and stand at harvest; severity of lesions on the taproot scored (1, 3, 5, 7, 9 as no, slight, moderate, severe and complete discolouration, respectively); growth of adventitious roots scored (1, 3, 5, 7 as no, few, many, very many adventitious roots, respectively), and grain yield. The sowing pattern throughout was with a spacing of 50 x 10 cm.
Genotype evaluation for resistance to root rots. In the 1993a season, 374 entries from the Kenyan germplasm collection, and 26 introductions previously identified as resistant in Rwanda, were tested on two farms. Single row plots, 3 m in length, were sown with one replicate per farm. GLP 2, a well-adapted but susceptible Calima-type variety, was sown every five rows to account for variation in disease severity throughout the fields. Forty two genotypes were selected from this trial based on plant survival.
In the 1994a season, the 42 genotypes (16 accessions and the 26 entries from Rwanda) were evaluated in plots of two rows of three meters length on two farms with two replicates per farm with GLP 2 as the check variety. Sixteen genotypes were selected based on plant survival and further evaluated under root rot stress in the 1994b season.
Ten resistant entries were selected based on grain yield under root rot stress and further subsequently evaluated in two row plots of three meter length in the 1995a season, and in six row plots of four meter length in the 1995b season, on farmers' fields under root rot stress. There were three replications on each of two farms each season. In 1995b, the two outer rows were used for destructive sampling and the four inner rows were harvested to estimate yield. In addition, 10 farmers were given one or more of these 10 lines in the 1995a and 1995b seasons to evaluate in large plots under their own management, and for farmer observation and evaluation.
Evaluation of cultural practices for root rot control. Several cultural practices were evaluated during the 1993a&b and 1994a seasons on farmers' fields with high levels of root rot infestation. The treatments included: seed dressing with Benlate @ 28 g kg^-1 of seed; muriate of potash (KCl) applied at 100 kg ha^-1; urea applied at 87 kg ha^-1; diammonium phosphate (DAP) applied at 150 kg ha^-1; certified seed assumed to be free of root rot inoculum; green manure (either Sesbania sesban or Leuceana spp.) applied and incorporated into the soil 14 days before sowing at 10 t ha^-1 fresh material; and bean sown on top of small ridges spaced 50 cm apart. The design was a randomised complete block design with three replications and a plot size of 3 x 3 m. The outer two rows were used for destructive sampling and the inner four rows were harvested to determine yield. The variety sown was the susceptible GLP 2. The trial was conducted on four farms for two seasons.
After determining that application of DAP and green manures were effective in root rot management, these practices were further evaluated with two bean varieties in the 1994b season in a split plot design with main plots as variety and sub-plots as soil amendment treatments. The varieties were susceptible GLP 2 and tolerant, but not resistant, GLP x92. The trial was conducted on four farms, each farm representing one replication. The plots were 10 rows wide and five meters long. Rates and methods of applying DAP and green manure were as described above. The outer two rows were used for destructive sampling and the inner eight rows were used to estimate yield.
In the 1995b season, Tithonia diversifolia, weathered FYM, Sesbania sesban and DAP were compared. Tithonia is a naturally but abundantly occurring plant on farms in this area, and its plant material is high in N and P (C. Palm, pers. comm.) The bean varieties were GLP 2 and GLP x92. The trial was a split plot design with main plots as variety and sub-plot as soil amendments. It was conducted on four farms with one replication per farm. Plots were 5 rows wide and three meters long. The outer two rows were used for destructive sampling while the three inner rows were for harvested for determination of yield.
RESULTS AND DISCUSSION
Genotype evaluations for root rot resistance. Ten resistant ( < 10% of taproot covered with lesions) and six tolerant (10-30% of taproot covered with lesions) lines were identified from the original set of 400 entries after two seasons of evaluation (Table 1). Of the 374 local accessions, only GLP x92 was found to be tolerant. Many of those which were resistant in Rwanda were found to be resistant or tolerant in western Kenya. Resistant genotypes represented a range of seed colours, growth habits and times of maturity.
In the on-farm trials conducted in 1995a, MLB-40-89a and SCAM 80-CM/5 gave the highest yields and the least plant loss (Table 2). RWR 432 and RWR 719 also yielded more than the tolerant control, GLP x92. MLB-49-89a and GLP x92 were early maturing, but had the most plant mortality. MLB-49-89a, RWR 1059 AND RWR 86B yielded less than GLP x92.
Farmers considered early maturity as an important trait as early harvest of bean provides food for subsistence during hunger periods which commonly occur before maize harvest. Seed colour and size were considered less important. Farmers liked MLB-40-89a because of its early maturity, although it has black seed and yields relatively less in the long rain season because of susceptibility to common bacterial blight and ascochyta. SCAM 80-CM/5 and RWR 432 which have smaller seed size than GLP2, but are early to intermediate in maturity and high yielding, were most preferred by farmers. MLB-40-89a is high yielding with small, chocolate-yellow seed, but was not liked by farmers because it matures late. Farmers liked RWR 719, a high yielding and late maturing variety with small red seed because of its similarity to GLP 185, a popular local cultivar before it succumbed to the root rot complex.
Evaluation of Cultural Practices for Rot Root Control. Root rot infection levels were high and similar for all treatments in the first set of trials as indicated by the lesion scores (Table 3). Application of DAP was the most effective treatment in improving crop tolerance as indicated by plant survival, growth of adventitious roots and grain yield despite root rot infection. Crop tolerance was improved by application of green manure, and to a lesser extent, by ridging. The other treatments did not have a significant effect over the farmers' practice although urea had a positive effect on some farms, probably in cases where low soil N was more limiting than low P. Generally, the results demonstrated the importance of adequate soil nutrient supply to crop tolerance.
Application of green manure and DAP were of similar effectiveness in improving crop yield where root rots were a major constraint (Table 4). The roots of GLP 2 were more infected than those of GLP x92 as indicated by the lesion scores. GLP 2 performed worse without soil amendment. However, survival rates for the two varieties were similar and GLP 2 generally produced more adventitious roots and had higher yields with soil amendments than did GLP x92.
The effectiveness of improving crop tolerance to root rots by applying DAP or organic manure was further verified in the 1995b season (Table 5). The best crop tolerance and yields with both varieties were with the application of tithonia or DAP, followed by application of sesbania or FYM. Soil amendment with FYM was less effective in improving crop tolerance than some other organic manures, probably due to a high C:N ratio due to poor management before application and a slow rate of decompostion.
There was a significant variety by soil treatment interaction. The roots of GLP 2 were more infected than those of GLP x92 as indicated by the lesion scores and GLP 2 performed worse without soil amendment. However, survival rates for the two varieties were similar and GLP 2 generally produced more adventitious roots and had higher yields with soil amendments than did GLP x92. This interaction was not expected and differs with the results observed elsewhere (CIAT, 1992) where combining tolerant or resistant varieties with improved soil management gave the best results. The interaction observed in these trials may be due to the lower yield potential of GLP x92 in this environment as compared to GLP 2 under conditions of adequate soil fertility.
Several genotypes with resistance to the root rot complex prevailing in Western Kenya have been identified. Farmer evaluations indicate that some of these will be well accepted by farmers and consumers and should be released. Improving soil fertility through application of DAP or certain organic manures improved crop tolerance to root rots. Tithonia is readily available but its potential for soil improvement was not recognised by farmers. Sesbania is well-adapted and easy to manage, with good potential as a major green manure crop. Investigation of possible synergistic effects of combining reduced rates of DAP with reduced rates of green manure is needed to improve the efficiency of use of these resources.
The East and Central Africa Bean Research Network (ECABREN) facilitated the exchange of germplasm between countries enabling us to get the root rot resistant genotypes. The role of bean researchers who first identified the resistant materials is acknowledged. Research assistant, Mr. Ambitsi, was instrumental to the implementation of on-farm trials. We are especially grateful to the farmers who allowed us to conduct trials on their fields and who participated in the evaluation of the varieties and cultural practices.
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Copyright 1998, African Crop Science Society
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