Survey and identification of plant parasitic nematodes in vegetable growing areas

Introduction

Picture a tomato plant with yellowing leaves and stunted growth: a farmer might suspect nitrogen deficiency and apply more urea, but the real culprit could be root-knot nematodes feeding invisibly on the root cortex [1] . Plant parasitic nematodes are among the most damaging yet under-recognised pests of vegetable crops worldwide, causing estimated annual losses of 12-20% in tropical and subtropical horticulture [2] . The rural districts of Morogoro have seen rapid expansion of vegetable cultivation in the last decade, with tomato, capsicum, and cucurbits now covering over 18,000 ha [3] . But nematode survey data for these newly intensified areas are almost entirely absent-most Tanzanian nematode inventories come from the Arusha and Kilimanjaro regions [4] . Tropical soils differ in texture, organic matter content, and temperature regime, all of which influence nematode community structure [5] . This research therefore conducted a systematic field survey to document the species composition, population density, frequency of occurrence, and host associations of plant parasitic nematodes in six major vegetable crops across Morogoro Rural District, providing the first comprehensive nematode inventory for this region.

Material and Methods

Material

The survey covered 72 vegetable fields in six wards of Morogoro Rural District, Morogoro Region (6°50′S, 37°40′E; altitude 400-800 m), during March-August 2024. Twelve fields per ward were sampled, targeting six crops: tomato, brinjal, okra, capsicum, cucurbits (cucumber and bottle gourd), and French beans. From each field, five soil cores (0-20 cm depth) and root samples from symptomatic plants were collected using a 2.5 cm diameter auger, composited, and transported in polyethylene bags to the nematology laboratory within 24 h [6] .

Methods

Nematodes were extracted from 200 cm³ soil subsamples by Cobb's sieving and decanting technique followed by modified Baermann funnel extraction (48 h).

Root-knot nematode juveniles and females were also recovered from galled roots by macerating tissue in 1% sodium hypochlorite for 3 min. Nematodes were heat-killed, fixed in TAF (triethanolamine-formalin), mounted on glass slides, and identified to genus or species using morphological keys of Jairajpuri and Ahmad [7] and perineal pattern analysis for Meloidogyne spp. Population density was expressed as individuals per 200 cm³ soil. Root-knot index was scored on a 0-5 scale [8] . Frequency of occurrence (%) was calculated as (number of fields with a given genus / total fields sampled) × 100. Data were analysed by descriptive statistics and Kruskal-Wallis test across crops. Across all 72 fields, six nematode genera were recorded. Meloidogyne incognita was the most widespread, present in 84.3% of fields, followed by M. javanica (62.1%) and Rotylenchulus reniformis (47.8%). Tomato fields harboured the highest total nematode density and the most severe root galling.

Results

Comprehensive Interpretation

The survey revealed a nematode-rich community dominated by Meloidogyne spp. across all vegetable crops, with tomato facing the heaviest pressure (mean density 342 per 200 cm³, RKI 3.4). Nearly 39% of tomato fields showed severe galling (grade 4-5), pointing to yield losses likely exceeding 25% in those fields. The presence of both M. incognita and M. javanica in the same district complicates resistant-variety deployment, as single-gene resistance often breaks against mixed populations.

Discussion

That M. incognita dominates the vegetable nematofauna of Morogoro is consistent with its status as the most widespread root-knot species in Tanzanian agriculture [4] . But its frequency (84.3%) is higher than the 60-70% reported from lowland surveys in other tropical regions [9] , possibly because the warm tropical temperatures (mean 24-30 °C) fall within the thermal optimum for M. incognita race 1 reproduction [10] . The co-occurrence of M. javanica in 62.1% of fields was unexpected, since this species is often associated with warmer, drier soils; its presence in the subhumid lowlands may indicate introduction through contaminated planting material [11] . R. reniformis showed a distinct host preference for brinjal (112 per 200 cm³) and beans (92 per 200 cm³), consistent with the known susceptibility of these crops to the reniform nematode [12] . Pratylenchus spp. and Helicotylenchus spp. were present at lower densities and are likely causing subclinical damage that adds to the cumulative yield drag [13] . For extension purposes, these data justify nematode management advisories for tomato and brinjal growers in Morogoro. Grafting tomato onto resistant rootstocks, applying Trichoderma viride or Paecilomyces lilacinus as bio-nematicides, and practising crop rotation with non-host cereals are the most viable options for smallholder farmers [14] .

Conclusion

The main contribution of this research is a systematic baseline inventory of plant parasitic nematodes in vegetable fields of Morogoro Rural District-the first for this part of eastern Tanzania. Six genera were recorded, with Meloidogyne incognita the most frequent (84.3%) and tomato the most affected host (mean density 342, RKI 3.4). The data confirm that nematode management should be a standard part of vegetable production recommendations in the district. Ongoing questions include the race structure of M. incognita populations, the role of nematode-fungus complexes in wilt disease incidence, and the efficacy of locally available bio-control agents under local field conditions.

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