African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 15, Num. 2, 2007, pp. 93-99

African Crop Science Journal, Vol. 15, No. 2, June, 2007, pp. 93-99

Nematode Distribution and Damage to Yam in Central and Eastern Uganda

J. Mudiope^1,3, P.R. Speijer¹, D. Coyne^1*, R.N. Maslen² and E. Adipala³

¹International Institute of Tropical Agriculture, Eastern and Southern Africa Regional Centre, P. O. Box 7878, Kampala, Uganda *Correspondence address: c/o Lambourn & C., Carolyn House, 26 Dingwall Road, Croyodon CR9 3EE, UK.
²Natural Resources Institute, University of Greenwich central Avenue, Chatham Maritime, Kent ME4 4TB, United Kingdom
³Department of Crop Science, Makerere University, P. O. Box 7062, Kampala, Uganda

Received 2 January, 2007; accepted 19 June, 2007

Code Number: cs07011

ABSTRACT

Yams (Dioscorea spp.) are food crops of growing significance in sub Saharan Africa. Unfortunately, nematodes are major pests to their production. A study was undertaken in major yam growing areas of Uganda to investigate the association of plant parasitic nematodes with damage symptoms. Nematodes were assessed from tubers, roots and surrounding soil for seven cultivars belonging to Dioscorea alata, D. bulbisiana, D. burkilliana and D. cayenensis at harvest. Pratylenchus sudanensis was found in the greatest density and was followed by Meloidogyne spp. Higher densities of both nematodes were observed in the tubers rather than roots. Although symptoms of cracking and galling were relatively low, P. sudanensis incidence was strongly associated with cracking and Meloidogyne spp. with galling. Pratylenchus sudanensis and Meloidogyne spp. incidence their associated damage were, however, negatively correlated, suggesting inter-species competition. Principal components analysis of data suggested that differences in susceptibility to P. sudanensis occur between yam cultivars and species.

Key Words: Dioscorea, Meloidogyne, Pratylenchus, tuber cracking

RÉSUMÉ

Les ignames (Dioscorea spp.) sont une culture alimentaire d’importance croissante en Afrique sub–saharienne. Malheureusement, les nématodes sont des parasites majeurs qui gênent sa production. Une étude était menée dans les principales régions productrices de l’igname en Ouganda en vue de pouvoir mener investigation sur l’association des nématodes parasitant les plantes avec les symptômes de dommages. Les nématodes étaient évalués à partir des racines, tubercules, et sol environnant pour 7 cultivars appartenant à Dioscorea alata, D. bulbisiana, D. burkilliana et D. cayenensis à la récolte. Pratylenchus sudanensis était considérée l’espèce à plus haute densité et était suivie par Meloidogyne spp. Des densités plus élevées des nématodes étaient observées plus dans les tubercules que dans les racines. Bien que les symptômes de craquelures et d’irritations étaient relativement bas, P. Sudanensis et Meloidogyne spp. étaient respectivement associées à la craquelure et à l’irritation. Pratylenctus Sudanensis et Meloidogyne spp. présentaient des damages associés, qui étaient cependant négativement corrélés suggérant une compétition inter-espèces. L’analyse principale des données a suggéré des différences à la susceptibilité du P. Sudanensis surviennent entre cultivars et espèces d’ignames.

Mots Clés: Dioscorea, Meloidogyne, Pratylenchus, craquelure detubercules

INTRODUCTION

Yams (Dioscorea spp.) are tropical tuber crops grown as a source of carbohydrates, but also for use in ceremonial activities (Degras, 1993). They are cultivated world wide, but principally in West Africa, where approximately 95% of world annual production (37 million tonnes) in grown (FAO, 2004). In East Africa, yams constitute an important subsistence food crop and component of the farming system (Wanyera et al., 1996).

In Uganda, yam production is widespread, but is primarily concentrated in the Eastern and Central areas, although its cultivation has steadily declined (Wanyera et al, 1996). The water yam (Dioscorea alata) and its yellow counterpart (Dioscorea cayanensis), are primarily grown in Uganda, although other species are also cultivated. The white Guinea yam (Dioscorea rotundata), which is globally the most popular species due to its palatability, is not traditionally grown in Uganda, but is becoming increasingly popular with yam growers through recent introductions (Khizzah, 1995; Wanyera et al., 1996). Compared to other root and tuber staple crops, yams are rich in proteins, fats and vitamins (Degras, 1993).

Amongst the various constraints to production of yam, nematode pests are of significant importance (Bridge et al., 2005). In West Africa, the yam nematode, Scutellonema bradys, is a major causal agent of dry rot (Adesiyan and Odihirin, 1977; Hahn et al., 1989; Coyne et al., 2006); while in Central America, Pratylenchus coffeae causes similar damage to that of S. bradys (Acosta and Ayala, 1976). Meloidogyne spp. can also cause in-field and post-harvest losses to yam, although information for Meloidogyne spp. is less conclusive (Bridge et al., 2000). A survey of parasitic nematodes on root and tuber crops in Uganda (Coyne et al., 2003) established that both Meloidogyne spp. and Pratylenchus sudanensis were common on yam. It has also been shown (Mudiope et al., 1998) that yams could be severely damaged by Meloidogyne spp. in Uganda, substantially affecting newly introduced, high yielding cultivars of D. rotundata. However, the pathogenicity and the extent of damage caused by P. sudanensis to yam in the field is currently unknown. This study was undertaken to establish the nematode species causing tuber damage to Dioscorea spp. in the major yam producing regions of Uganda.

MATERIALS AND METHODS

This study was conducted in 1998. Seven geopolitical districts in the major yam growing areas of Central and Eastern Uganda, representative of the common agro-ecologies (Jagtap, 1993) were selected for this study namely, Rakai, Masaka, Luweero and Mukono in Central; and Iganga, Mbale and Kapchorwa in Eastern Uganda. In each district, between 4 - 7 farms neighbouring each other were randomly selected and visited for yam collection. Each of the 4-7 farms was considered as a single site.

At each site, at least seven tubers of the most common cultivar (cv) were collected from mature plants in separate fields, along with roots and soil from each plant. Samples of the less commonly grown cultivars were additionally collected in cases where they existed. Whole tubers and roots, and soil from the root zone were collected and stored separately in plastic bags. For each tuber, the levels of galling and cracking were scored using: 0 = no damage, 1 = slight damage (< 20% of the tuber surface affected with galls or cracks), 2 = moderate damage (20-50% of the tuber surface affected with galls or cracks), and 3 = severe damage (> 50% of the tuber surface affected with galls or cracks).

Motile nematodes were extracted from tubers, roots and soil separately. Nematodes were isolated from tubers from a single 2 g tuber core (1 cm wide and 2 cm deep), and removed using cork borer. The cores were rinsed briefly in tap water to remove soil, before chopping finely with a knife and placing on a modified Baermann sieve (Hooper et al., 2005) for seven days. Nematode extracts were removed daily and stored in a beaker at 4 ^oC and replaced with fresh water on each occasion. Nematode suspensions for each sample were combined, reduced to 25 ml and nematode population densities assessed from 3 x 2 ml- aliquots using a light microscope. Roots were first rinsed in tap-water to remove soil debris, dabbed dry with a paper towel and cut into approximately 2 cm pieces with a knife.

Nematodes were extracted from 2 g root subsamples removed from the chopped roots of each sample. Each sub-sample was macerated using a kitchen blender for 7 seconds and the suspension rinsed onto a modified Baermann sieve for extraction for 12 hours. Nematodes from the soil were assessed from a 50 g sub-sample using the modified Baermann sieve method, after first passing the soil through a 2 mm aperture sieve to remove soil aggregates, stones and debris.

Nematodes species identifications were confirmed at the Plant Protection Institute, South Africa from samples fixed in 4% formaldehyde solution. Nematode densities were estimated and recorded per 100 g or tuber fresh weight or per 50 g soil. Mean nematode densities were calculated for each of the districts involved. Protection Institute, South Africa from samples fixed in 4% formaldehyde solution. Nematode densities were estimated and recorded per 100 g root or tuber fresh weight or per 50 g soil. Mean nematode densities were calculated for each of the districts involved.

Analysis of variance was done on nematode density data for the two dominant species and damaged indices. Nematode population densities data were normalised using log_n (x + 1) transformation prior to analysis (Gomez and Gomez, 1984). Pairwise regression analysis was undertaken using the two nematodes species densities and the two damage indices, and between nematode densities from the different sources on plants. Components of variance were estimated using the VARCOMP procedure in SAS (SA, 1999) and principal components analysis undertaken on nematode densities and damage indices. Nematode means for yam cultivars were compared using the probability functions of the least square means procedure with SAS.

RESULTS

A total of seven cultivars within for species of Dioscorea (D. alata, D. bulbisiana, D. burkilliana and D. cayenemsis) were identified during the study (Table 1). The cv Kyetutumula (D. cayenensis) was encountered at all sites except in Kapchorwa, while cultivar diversity was relatively greater in Mbale.

Numerous nematode species were recovered from the samples (data not shown). The majority however, occurred infrequently and in low densities, particularly in the soil. Only P. sudanensis and Meloidogyne spp. were encountered in tuber samples, both of which consistently occurred and in high densities in soil and root samples. Analysis, therefore, focused on these two species. Relatively higher nematode densities were observed in tubers for both P. sudanensis and Meloidogyne spp., followed by roots, while soils had lower relative densities. Pratylenchus sudanensis densities varied among districts. Mean densities of approximately 1500 nematodes g^-1 tube fresh weight (TFW) were recovered from the Masaka and Rakai districts; and 245 per g^-1 TFW in Kapchorwa. Tubers from Mbale, Mukono and Iganga districts were infected with nematode densities less <100 P. sudanensis g-1 TFW, while no P. sudanensis was recovered in Luweero. No Meloidogyne spp. were isolated from tubers from Rakai and Mukono districts; while tubers from Kapchorwa and Luweero had the highest densities of Meloidogyne spp. (ca. 20 nematodes g^-1 TFW). Pratylenchus sudanensis root densities were positively correlated (P < 0.01, r = 0.4) with soil densities, however, there was no correlation between tubers and soil densities (data not shown).

A large proportion of the variation in tuber damage and nematode densities was explained by cultivar differences within sites (>33%, Table 2). Correlations also indicated strong positive associations between P. sudanensis and cracks on the tubers (r = 0.98, P < 0.002) and between Meloidogyne spp. and tuber galling (r = 0.95, P, <0.009). However, densities of P. sudanensis and Meloidogyne spp. were highly divergent (r = 0.753, P < 0.05), as was the association between tuber galling and cracks (r = -0.79, P < 0.034) (Table 3).

Following principal components analysis of nematode densities and tuber damage, the components prin 1 and 2 accounted for 79 and 20%, respectively, of total variation, of which nematodes densities and associated damage contributed substantially to prin 1 (Table 4). The pairwise comparision of the least square means of prin 1 suggests that cv Ndaggu Nziba is the most nematode susceptible cultivar (Table 5). Ndaggy Nziba (LS mean = 1.74) was more susceptible than cv Nandigoya (P<0.05), cv Masebe (P < 0.023), cv Kyetutumula (P < 0.049) or cv Ndaggu Nganda (P < 0.008).

DISCUSSION

In this study, P. sudanensis was the most common species associated with yam in the main yam growing areas of Uganda; followed by Meloidogyne spp. This contradicts earlier reports (Mudiope et al., 1998; Coyne et al., 2003), which found Meloidogyne spp. to be the most frequent nematodes on yam in Uganda. This was due to the previous exclusion of central and northern Uganda (Coyne et al., 2003). The current study suggests that P. sudanensis is more widespread than previously reported (Coyne et al., 2003). The study also provides for the first time, strong evidence of the association between P. sudanensis densities and cracked (damaged) yam tubers. The importance of P. sudanensis to yam production and pathogenicity to specific species (and cultivars) of Dioscorea has yet to be determined.

In West Africa, S. bradys is a serious production constraint of yam, the field and particularly during storage (Bridge, 1973; Bridge et al., 2005); while P. coffeae is a primary namatode pest of yam in the Pacific region (Bridge, 1988) and Central America (Acosta and Ayala, 1976). Meloidogyne javanica has been identified as the primary nematode species on yams in Uganda (Mudiope et al., 1998), thus supporting observations from an earlier study (Coyne et al., 2003). In the current study, however, it is interesting that the presence of Meloidogyne spp. and related galling damage were negatively correlated with P. sudanensis densities and tuber cracks. This suggests interspecific competition between the two nematodes. Similar competition on yam was observed between S. bradys and P. cofeae in Puerto Rica (Acosta and Ayala, 1976) and French West Indies (Castagnone-Sereno and Kermarrec, 1988), where P. coffeae was seen to out-compete or exclude S. bradys. Although dual infections resulted in overall greater tuber damage (Acosta and Ayala, 1976), infection by one dominant species is more commonly observed (Castgnone-Sereno and Kermarrec, 1988).

Yam species and cultivars are known to vary in susceptibility to both Meloidogyne spp. and S. bradys. However, there is limited information on differences in susceptibility to Pratylenchus spp. (Bridge et al., 2005).

The significant differences observed in susceptibility to P. sudanensis among yam cultivars, offer a possibility that sources of resistance are available in some indigenous cultivars. The distribution of cultivars also appears quite varied within Uganda, which may possibly be a result of different levels of pest or disease resistance, resulting from when yam was more prevalent. The cv Ndaggu Nganda (D. alata) and cv Makunyi (D. bulbisiana) appear to have some resistance to both P. sudanensis and Meloidogyne spp., and supported relatively low densities of these nematodes species. Greater densities of P. sudanensis occurred in the tuber than in roots, indicating that P. sudanensis is likely to cause damage during storage, similar to S. bradys and P. coffeae; and likely to reduce the viability and usefulness of infected tubers for use as seed material Bridge et al., 2005.

ACKNOWLEDGEMENTS

This study was made possible by a grant from the Deutscher Akademischer Austausch Dienst (DAAD), Germany, and IITA Fellowship Award.

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