African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 6, Num. 4, 1998, pp. 367-375

J.W. Kimenju, S.W. Waudo¹, A.W. Mwang'ombe, R.A. Sikora² and R.P. Schuster²

Department of Crop Protection, P. O. Box 29053, Nairobi, Kenya
¹Department of Botany, P. O. Box 43844, Nairobi, Kenya
²Department of Phytopathology in Soil Ecosystems, Nussallee 9, 53115 Bonn, Germany

(Received 21April, 1998; accepted 1 October, 1998)

The distribution and impact of lesion nematodes (Pratylenchus spp.) in Kenyan maize producing areas were evaluated. Soil and root samples were taken from 120 farms in three districts of Kenya, namely Kakamega, Machakos and Trans Nzoia. Mean populations of Pratylenchus spp. (P. zeae and P. brachyurus) in 5 g roots were 280, 131, and 6 in Kakamega, Machakos and Trans Nzoia districts, respectively. Lesion nematodes were recovered from 73.3% of the samples. P. zeae and P. brachyurus had overall incidences of 72.5 and 6.7%, respectively. Greenhouse tests were conducted to determine the effects of P. zeae on growth of seven maize genotypes used in Kenya, Dryland composite 1, Katumani composite, and hybrids 511, 512, 614, 625 and Pwani. Numbers of P. zeae extracted from 5 g roots were significantly (P< 0.05) different and ranged between 6230 in hybrid 614 and 10970 in Katumani composite. The nematode caused significant (P < 0.05) reduction in root weight of Katumani composite, Dryland composite 1, hybrid 511 and hybrid 512. Nematode infection significantly (P < 0.05) reduced shoot weight of Pwani hybrid and height of Dryland composite 1.

Key Words: Maize genotypes, Katumani, Zea mays

La distribution et l'impact des nématodes (Pratylenchus spp) dans les zones de production de maïs au Kenya ont été évalués. Le sol et les échantillons de racines ont été obtenus de 120 fermes dans trois districts du Kenya (Kakamega, Machakos et Trans Nzoia). Les populations moyennes de Pratylenchus spp. (P. Zeae et P. brancyurus) de 5g de racine étaient respectivement de 280, 131, et 6 dans les districts de Kakamega, Machakos et Trans Nzoia. Les lésions de nématodes ont été retrouvées dans 73,3% des échantillons. Les P. zeae et P. brachyurus ont respectivement d'incidences générales de 72,5 et 6,7%. Les tests de serre ont été conduits de manière à déterminer les effets de P. Zeae sur la croissance de sept génotypes de maïs utilisés au Kenya; les composites 1 du milieu sec, ceux katumani, et les hybrides 511, 512, 614, 625 et Pwani. Plusieurs P. zeae extraits de 5g de racine était d'une façon significative (p<0,05) différents et variés entre les composites 6230 d'hybride 614 et 10970, de katumani. Les nématodes ont causé une sérieuse réduction (p<0,05) du poids de la racine dans le composite Katumani, le composite 1 du milieu sec, l'hybride 511 et l'hybride 512. L'infection de la nématode a significativement réduit (p<0,05) le poids de pousse de l'hybride Pwani et la taille du composite 1 du milieu sec.

Mots Clés: Génotypes de maïs, Katumani, Zeae mays

Maize (Zea mays L.) is the staple food to over 90% of Kenyans and provides 25% of Kenyans' employment pool (Anon., 1990). Plant-parasitic nematodes are known to cause up to 50% yield loss on maize in Kenya, with lesion nematodes being rated as the most important (Hollis, 1962; Adeniji et al., 1979; Bridge, 1994). Continuous cropping, a common practice in the Kenyan small-scale farming, can favour nematode population build-up above the injurious threshold (Maqbool and Hashmi, 1986). However, information on distribution and injurious thresholds of the nematodes is lacking.

Although nematicides, crop rotation and bare fallow are effective in nematode management, they are inappropriate on low value crops such as maize (Kerry, 1987; Stirling, 1991; Sikora, 1992). Host resistance is a viable alternative because it is cheap and poses no technical difficulties to the farmer (Trudgill, 1991). In spite of those positive attributes, no information is available on maize resistance to lesion nematodes in Kenya. This study was, therefore, conducted with the objectives of (i) determining the occurrence and distribution of lesion nematodes associated with maize and (ii) characterizing reactions of Kenyan maize cultivars to the most prevalent species of lesion nematodes in Kenya.

Nematode survey. Soil and root samples were collected from 40 farms randomly selected from each of three districts, namely Kakamega, Machakos and Trans Nzoia in Kenya (Fig. 1). The sampling procedure was adopted from Dropkin (1980). Twenty soil and root samples were taken from the rhizosphere of maize plants in each farm. The soil samples from each farm were thoroughly mixed to form a composite sample. One kilogramme of soil was drawn from the composite sample and placed in a polythene bag. Roots were also bulked and placed in a separate bag. Nematodes were extracted from three 200-cm3 soil subsamples using the sieving and filtering method (Flegg, 1967). Nematodes were extracted from three 5-g root subsamples using the maceration and filtration technique (Fallis, 1943). Nematodes that emerged after 3 days were collected in beakers and fixed in hot formalin-acetic acid before temporary mounts were made (Hooper, 1970). Species identification was based on morphological characteristics using the synoptic key formulated by Handoo and Golden (1989). A 1-ml aliquot of a well mixed nematode suspension was placed in a counting slide and the nematode population was determined under a compound microscope. Counting was repeated in three aliquots to improve accuracy.

Multiplication of Pratylenchus zeae Graham. A monoxenic culture of P. zeae, originally isolated from Kenya, was obtained from Prof. R.A. Sikora, University of Bonn, Germany. The nematode culture was multiplied in petridishes on maize root tissue grown on Murashige and Skoog (1962) growth medium supplemented with 2% sucrose and 1% agar as a solidifying agent (Amin, 1994). The pH of the medium was adjusted to 6.5 with 1M KOH before the medium was autoclaved.

Maize seeds were surface sterilized by immersion in 95% alcohol for 1 minute and then in 2.5% aqueous sodium hypochlorite for 20 minutes (Amin, 1994). The seeds were subsequently rinsed in seven series of sterile tap water. One seed was placed on the growth medium in each petridish and incubated at 23 ^oC for 7 days. The shoot was aseptically cut at its base and discarded, leaving the roots. Inoculation was done by transferring a block of medium from a 2-month-old culture onto the fresh roots aseptically. The dishes were sealed with parafilm to minimise water loss and incubated at 23 ^oC in darkness for 2 months.

Inoculum preparation. Two-month-old P. zeae cultures were used as a source of inoculum. Nematodes were extracted from both roots and medium by a modified Baermann funnel technique (Hooper, 1990). The roots and medium were thinly spread on double milk filters supported on a sieve standing in a 20-cm-diameter dish. Water was carefully added to the dish to just cover the filters and was left for 12 hours in darkness to allow the nematodes to move through the filters into the water. The nematode suspension was concentrated by draining off excess water through a 0.02 mm-pore sieve. Residues on the sieve were poured into a beaker. Nematode density was determined by taking the average of nematodes in three 1-ml aliquots of the nematode suspension. The concentration of the suspension was adjusted to 1000 nematodes per ml by decanting or adding water.

Maize susceptibility to Pratylenchus zeae. The (DLC1) maize genotypes used were Dryland composite 1, Katumani composite and the hybrids 511 (H511), 512 (H512), 614 (H614), 625 (H625) and Pwani. Pots of 15 cm diameter were filled with a 1:2 (v/v) sterile loam soil:sand mixture. Maize seeds were pre-germinated on moist filter papers in petridishes. Nematode inoculation was done by spreading 10000 P. zeae, suspended in 10 ml of water, at the bottom of the planting hole before a 5-day-old seedling was transplanted into each pot. Each maize genotype had its own control which was not inoculated with the nematode. Treatments were replicated five times and arranged in a completely randomized design in a glasshouse. Plants were watered on a daily basis. Fertilization was done weekly by pipetting 10 ml of 0.3% Basfoliar (BASF) solution into each pot.

Nematode damage was assessed 90 days after planting by comparing fresh root and shoot weights and height of inoculated plants with their respective controls. Nematode populations were determined by extraction from two 100-cm³ and two 5g root subsamples as described above. The experiment was repeated once. Data were subjected to analysis of variance and differences between means were evaluated for significance using Duncan's multiple range test or Student's test (Steel and Torrie, 1980).

Nematode Survey. The major symptoms of nematode damage in maize fields were stunting and chlorosis, which occurred in patches. Small black lesions, characteristic of lesion nematode infection, were observed in 60, 49, and 30% of samples collected from Kakamega, Machakos and Trans Nzoia districts, respectively. The lesions were frequently accompanied by root pruning and rot, especially on samples from Kakamega district.

Number of lesion nematodes varied significantly (P < 0.05) between divisions (Table 1). The mean populations of Pratylenchus species recovered from 5 g roots were 280, 131, and 6 in Kakamega, Machakos and Trans Nzoia districts, respectively (Table 1). The corresponding populations in 200 cm3 soil were 99, 154 and 2. Pratylenchus spp. were recovered from 38, 33 and 17 samples out of 40 samples from Machakos, Kakamega and Trans Nzoia districts, respectively (Table 2). Two lesion nematode species, Pratylenchus zeae and P. brachyurus, were recovered from root samples with occurrence frequencies of 72.5 and 6.7%, respectively. Mixed populations of P. zeae and P. brachyurus were present in 3 and 4 root samples from Kakamega and Trans Nzoia districts, respectively. P. brachyurus occurred alone in one of the samples obtained from Trans Nzoia district but the nematode was absent in samples collected from Machakos district (Table 2).

TABLE 2. Frequency of occurrence of Pratylenchus zeae and P. brachyurus recovered from soil and maize roots collected from 120 farms in three districts in Kenya

*40 samples per district

Susceptibility of maize cultivars to Pratylenchus zeae. Nematode infection had a negative effect on plant growth (Table 3). Reduction in height was least, 0.4%, in H614 and greatest, 10.6%, in H512 (Table 3). The reduction in height was significant (P < 0.05) in DLC1. The reductions in shoot and root weights were up to 11.8 and 27.4%, respectively. The lowest and highest reductions in shoot weights were recorded in H614 and Pwani hybrid, respectively. Differences in root weights between nematode-infected plants and their respective controls were significant (P< 0.05) in H511, H512, DLC1 and Katumani composite (Table 3). Pwani hybrid and DLC1 had the lowest and highest reductions in root weight, respectively (Table 3).

Numbers of P. zeae, in 100 cm3 of soil, supported by different maize cultivars were significantly (P<0.05) different 90 days after inoculation (Table 3). Katumani composite, DLC1 and H625 supported significantly (P< 0.05) higher numbers of the nematode than H511, H512, H614 and Pwani hybrid. Nematode populations in roots were significantly (P<0.05) higher in Katumani composite than in all the other genotypes.

Nematode-treated plants were consistently shorter than non-treated plants (Table 4). Reductions in plant height varied between 0.5 and 11.4%, with no significant (P< 0.05) differences between nematode-infected plants and their respective controls (Table 4). Nematode-treated plants were consistently lighter than their respective controls, with reductions of up to 10.9 and 29.7% in shoot and root weights, respectively (Table 4). Root weight reductions were significant (P<0.05) in cultivars Katumani composite, H511 and H512. H511 and Pwani hybrid had the lowest and highest shoot weight reductions, respectively (Table 4).

There were significant (P< 0.05) differences among cultivars in numbers of P. zeae recovered from soil and roots. Cultivar Katumani composite supported a higher nematode population in roots than hybrids 614 and 625 (Table 4). Nematode number in soil supported by cultivars DLC1, H625 and Katumani composite were significantly (P<0.05) higher than those associated with the other cultivars (Table 4).

TABLE 3. Effect of 10,000 Pratylenchus zeae per plant on growth parameters of seven maize cultivars and population of the nematode 90 days after inoculation

Cultivars: H = hybrid, DLC1 = Dryland composite 1, Katumani = Katumani composite, Pwani = Pwani hybnd.
Control = nematode-free
Data are means of five replications
*The difference between inoculated plants and the control is significant (P £ 0.05)in the respective growth parameter by Student's test
^aNematode numbers followed by the same letter(s) within the rows are not significantly (P* 0.05) different according to Duncan's multiple range test

TABLE 4. Effect of 10,000 pratylenchus zeae per plant on growth parameters of seven maize cultivars and population of the nematode 90 days after inoculation

Cultivars: H = hybrid, DLC1 = Dryland composite 1, Katumani = Katumani composite, Pwani = Pwani hybrid.
Control = nematode-free.
Data are means of five replications.
*The difference between inoculated plants and the control is significant (P£0.05) in the respective growth parameter by Students test.
^aNematode numbers followed by the same letter within the rows are not significantly (P£0.05) different according to Duncan's multiple range test

The occurrence of P. zeae and P. brachyurus in maize fields in Kakamega, Machakos and Trans Nzoia districts in Kenya supports previous reports that the two species are among the most common lesion nematodes associated with maize in the tropics (De Waele and Jordaan, 1988; Swarup and Sosa-Moss, 1990). Maize is known to be a good host of the two lesion nematodes (Corbett, 1976; Fortuner, 1976). Pratylenchus zeae dominated over P. brachyurus both in incidence and abundance. De Waele and Jordaan (1988) reported that P. zeae was widely distributed and outnumbered P. brachyurus in maize fields in South Africa. Dominance of P. zeae over P. brachyurus may be attributed to its relatively high reproductive rate and tolerance to environmentally related stress (Olowe and Corbett, 1976).

The high incidence of P. zeae in Machakos district was unexpected because the district experiences frequent droughts. Pratylenchus thornei and P. penetrans are known to become anhydrobiotic when moist soil dries out gradually (Glazer and Orion, 1983; Townshend, 1984). The same mechanism could enable P. zeae to survive during the long dry spells that occur between seasons, especially in Machakos district. The presence of weed hosts is also known to influence the density of lesion nematodes in maize fields (Egunjobi, 1974).

Reductions in shoot and root weights and height of nematode-infected plants and the high nematode populations supported by maize cultivars H511, H512, H614, H625, DLC1, Katumani composite and Pwani hybrid, indicate that the cultivars are susceptible to P. zeae. The dark lesions and root pruning caused by P. zeae are further indicators of susceptibility of the maize cultivars to this nematode. These symptoms are similar to those associated with other lesion nematode species (Ogiga and Estey, 1975; Zirakparvar, 1980; Waudo and Norton, 1986; Swarup and Sosa-Moss, 1990). According to Swarup and Sosa-Moss (1990), small black lesions are diagnostic symptoms of lesion nematode infection. Susceptibility of maize cultivars to lesion nematodes is widely reported (Egunjobi, 1974; Norton and Hinz, 1976; Zirakparvar, 1980; Waudo and Norton, 1983 and 1986). Results from this study also confirm a previous report by Bridge (1994) that P. zeae is the important nematode on maize in Kenya.

The significant reduction in root weight of cultivars DLC1, H511, H512 and Katumani composite, and insignificant reduction in root weight of cultivars H614, H625 and Pwani hybrid indicated differences in susceptibility of these genotypes to P. zeae. It calls for an extensive screening programme to identify cultivars with higher levels of resistance or tolerance to P. zeae. Subsequent incorporation of lesion nematode-resistance in maize cultivars would help to alleviate the losses caused by the nematodes and, hence, contribute towards attainment of food security in Kenya.

Although it is difficult to extrapolate results obtained in the greenhouse to those obtainable in the field, it is possible that P. zeae infection would result in more severe damage under field conditions due to added stress from abiotic and biotic factors (Meagher and Chambers, 1971; Egunjobi, 1974; Sikora, 1987) that were absent in the greenhouse environment.

The authors are grateful to the German Academic Exchange Service (DAAD) for financial support to this work. Support from the University of Nairobi and University of Bonn is greatly acknowledged.

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