Studies on heterosis for pest and disease resistance in Yardlong Bean [Vigna unguiculata (L.) Walp. subsp. sesquipedalis]

Introduction

Yardlong bean [Vigna unguiculata (L.) subsp. sesquipedalis], commonly grown for its long, tender pods, is an important warm-season legume vegetable valued for its adaptability, high productivity and nutritional contribution of protein, dietary fibre, vitamins and minerals. Despite its importance, productivity is severely constrained by insect pests and diseases that damage foliage, pods and the vascular system of the plant. The pod borer (Maruca vitrata) is among the most destructive pests of yardlong bean and cowpea, causing damage to flowers and pods that can substantially reduce marketable yield; genotypic variation in susceptibility to this pest has been documented under field screening in yardlong bean by Ahmed et al. (2018) [2] and among cowpea genotypes by Ambavane et al. (2026) [3] . The aphid (Aphis craccivora) is another major pest causing direct feeding damage and transmitting viral diseases; differential aphid susceptibility among yardlong bean and cowpea genotypes has been reported by Benchasri et al. (2012) [4] , while a wide range of genotypic response, from resistant to highly susceptible, has been recorded among wild and cultivated cowpea accessions by Souleymane et al. (2013) [14] . Sclerotium wilt, caused by Sclerotium rolfsii, is an important soilborne disease of cowpea and related legumes; freedom from basal stem rot caused by this pathogen has been reported among cowpea genotypes by Tanimu et al. (2018) [15] , and the broader variability available within cowpea germplasm for resistance to Sclerotiumincited stem rot and damping-off has been documented by Adandonon et al. (2006) [1] . Anthracnose, caused by Colletotrichum gloeosporioides, and cowpea mosaic virus are also of concern; resistance to anthracnose among yardlong bean genotypes has been reported by Merin et al. (2022) [10] , while a wide range of genotypic reaction to a seed-and aphid-borne mosaic virus has been documented among cowpea genotypes by Manjunatha et al. (2017) [8] . The exploitation of heterosis and the identification of resistant parental combinations through diallel mating designs offer an economical and sustainable route to improving pest and disease resistance in yardlong bean. In the present investigation, six genetically diverse yardlong bean parents were crossed in a full diallel mating design excluding selfing to generate thirty F₁ hybrids, which were evaluated together with the parents and a commercial check hybrid for resistance to Sclerotium wilt, anthracnose, cowpea mosaic virus, pod borer and aphid, and standard heterosis was estimated over the check hybrid to identify promising resistance sources.

Materials and Methods

Experimental Site

The present investigation was conducted at PAJANCOA&RI, Karaikal during 2025-26, under field conditions favourable to the natural occurrence of the target pests and diseases.

Experimental Materials

Six genetically diverse yardlong bean parents (P₁-P₆) were crossed in a full diallel mating design excluding selfing, resulting in thirty F₁ hybrids including all reciprocal combinations. A commercial check hybrid was included as the standard check for estimation of standard heterosis.

Experimental Design and Crop Management

The experiment consisted of thirty F₁ hybrids, six parental lines and one check hybrid, evaluated under field conditions in randomised block design with 2 replications. Recommended agronomic practices were followed throughout the crop growth period, and no insecticide, fungicide or virucide protection was applied during the observation period so as to permit reliable natural-condition screening of resistance.

Screening for Pests and Diseases

All six parents and thirty crosses of yardlong bean, along with the check hybrid, were screened for incidence of pests and diseases under field conditions.

Sclerotium wilt (Sclerotium rolfsii)

The individual plants in each genotype were scored using a 0-4 scale as proposed by Fery and Dukes (2002) [5] , where:

No wilting 1 Slight or partial wilting 2

Dead plant The per cent index of wilt intensity was calculated as proposed by Mayee and Datar (1986) [9]: [12] , and per cent disease index was calculated accordingly. The reaction of each genotype was classified as follows. Highly susceptible (HS)

Rating

Cowpea mosaic virus

A 0-60 scale developed by Shoyinka et al. (1997) [13] for numerical scoring of black-eye cowpea mosaic virus (BlCMV) was used, and percentage disease index was calculated using the same formula given above. The reaction of each genotype was classified as follows.

Pod borer (Maruca vitrata)

The total number of pods was counted at each harvest from all tagged plants, and per cent pod damage was calculated following Jackai and Singh (1988) [6] :

Per cent pod damage = (Total number of damaged pods / Total number of pods) × 100

Infestation was scored using the following scale.

Aphid (Aphis craccivora)

Aphid infestation was recorded from all plants and scored on a 0-9 scale devised by Litsinger et al. (1977) [7] . Damage intensity was calculated as: > 500 Large continuous colonies Reaction classification for pod borer and aphid damage intensity followed the scale proposed by Mayee and Datar (1986) [9] , presented in section 2.4.1 above.

Estimation of Standard Heterosis

Standard heterosis was estimated over the check hybrid using the following formula:

For resistance traits, lower values indicate superior resistance; negative standard heterosis therefore indicates desirable performance relative to the check hybrid, whereas positive values indicate inferior (higher-incidence) performance. Standard heterosis was not computable for anthracnose or cowpea mosaic virus because the check hybrid recorded zero incidence for both traits (division undefined).

Statistical Analysis

Data were recorded from five randomly selected competitive plants of each genotype [in each replication] and analysed following standard procedures for field-screening data [Panse and Sukhatme, 1985] [11] . Standard heterosis over the check hybrid was estimated following Turner (1953) [16] .

Results and Discussion

The reaction of six parents and thirty F₁ hybrids against major insect pests and diseases -Sclerotium wilt, anthracnose, cowpea mosaic virus, pod borer and aphidwas evaluated under field conditions and is presented in Table 1.

Pod Borer

Pod borer infestation varied considerably among the parents and hybrids. Among the six parents, pod damage ranged from 18.1 to 33.2 per cent; P₃ recorded the lowest damage (18.1%), followed by P₁ (19.4%), while P₄ recorded the highest infestation (33.2%). Among the thirty hybrids, pod damage ranged from 15.3 to 32.8 per cent. The hybrid P₁ × P₃ recorded the lowest pod damage (15.3%), followed by P₃ × P₁ (16.4%), P₁ × P₆ (17.2%) and P₅ × P₃ (18.6%), all exhibiting a moderately resistant reaction. The highest pod damage was observed in P₂ × P₆ (32.8%), followed by P₄ × P₆ (32.1%), P₂ × P₅ (31.6%) and P₄ × P₅ (30.8%), categorised as susceptible. Most remaining hybrids showed a moderately susceptible reaction (21.9-30.2%). Relative to the check hybrid (31.6%), P₁ × P₃, P₃ × P₁, P₁ × P₆ and P₅ × P₃ exhibited desirable standard heterosis of -51.58, -48.10, -45.57 and -41.14 per cent, respectively, indicating substantially superior resistance to pod borer over the check. A comparable range of genotypic variation for pod borer resistance in yardlong bean has been reported by Ahmed et al. (2018) [2] , while a similar spread of resistance reactions among cowpea genotypes screened under field conditions for the same pest was documented by Ambavane et al. (2026) [3] , lending support to the genotypic differences in pod borer damage observed among the present set of parents and hybrids.

Aphid

Aphid damage intensity among the parents ranged from 8.0 to 14.6 per cent. Parent P₃ recorded the lowest intensity (8.0%), followed by P₁ (8.5%), P₅ (9.1%) and P₆ (9.4%), all categorised as highly resistant, while P₂ (14.0%) and P₄ (14.6%) recorded the highest intensity and were categorised as resistant. Among the hybrids, aphid damage intensity ranged from 6.7 to 14.4 per cent. The hybrid P₁ × P₃ recorded the lowest intensity (6.7%), followed by P₃ × P₁ (7.2%), P₁ × P₆ (7.6%) and P₅ × P₃ (8.2%), all highly resistant. The highest intensity was observed in P₂ × P₆ (14.4%) and P₄ × P₆ (14.1%), categorised as resistant. Relative to the check hybrid (13.9%), P₁ × P₃, P₃ × P₁, P₁ × P₆ and P₅ × P₃ exhibited desirable standard heterosis of -51.80, -48.20, -45.32 and -41.01 per cent, respectively. A parallel finding of differential aphid susceptibility among yardlong bean and cowpea genotypes screened against Aphis craccivora was reported by Benchasri et al. (2012) [4] , and a similarly wide range of genotypic response, extending from resistant to highly susceptible reactions, was recorded among wild and cultivated cowpea accessions by Souleymane et al. (2013) [14] , both supporting the reliability of visual aphid-incidence scoring as an effective screening approach in the present investigation.

Sclerotium Wilt

Sclerotium wilt incidence was generally low among the parents and hybrids. Among the parents, P₁, P₃, P₅ and P₆ recorded no wilt incidence and were categorised as immune, while P₂ (18.9%) was resistant and P₄ (20.4%, the highest among parents) was moderately resistant. Among the thirty hybrids, sixteen -P₁ × P₃, P₁ × P₅, P₁ × P₆, P₂ × P₁, P₂ × P₃, P₂ × P₆, P₃ × P₁, P₃ × P₅, P₃ × P₆, P₄ × P₁, P₄ × P₃, P₄ × P₅, P₅ × P₁, P₅ × P₃, P₆ × P₁ and P₆ × P₃recorded no wilt incidence and were immune. Wilt incidence among the remaining hybrids ranged from 13.4 (P₃ × P₂) to 20.1 per cent (P₄ × P₆). Relative to the check hybrid (19.6%, resistant), the immune hybrids, including P₁ × P₃, P₁ × P₆, P₃ × P₁ and P₅ × P₃, exhibited the maximum possible desirable standard heterosis of -100 per cent. A comparable proportion of cowpea genotypes showing complete freedom from basal stem rot caused by Sclerotium rolfsii was reported by Tanimu et al. (2018) [15] under field screening conditions, while the broader variability available within cowpea germplasm for resistance to Sclerotium-incited stem rot and damping-off has similarly been documented by Adandonon et al. (2006) [1] , reinforcing the presence of usable resistance sources for this disease within the parental material used in the present study.

Anthracnose

No anthracnose incidence was observed in any of the six parents and thirty hybrids throughout the experimental period; all entries recorded zero disease incidence and were classified as immune, and standard heterosis could not be computed as the check hybrid also recorded zero incidence. This finding is in agreement with the resistance reaction reported for selected yardlong bean genotypes screened against Colletotrichum gloeosporioides by Merin et al. (2022) [10] , who similarly identified genotypes completely free from anthracnose infection under field screening, further supporting the reliability of visual disease-reaction scoring as an effective first-level screen for anthracnose resistance in this crop.

Cowpea Mosaic Virus

None of the parents or hybrids exhibited symptoms of cowpea mosaic virus infection during the crop growth period; all entries recorded zero incidence and were categorised as highly resistant, and standard heterosis could not be computed for the same reason as above. A similarly wide range of genotypic reaction to a seedand aphid-borne mosaic virus, extending from immune to highly susceptible, was reported among cowpea genotypes screened against Bean common mosaic virus by Manjunatha et al. (2017) [8] , lending indirect support to the freedom from mosaic virus infection observed among the present parents and hybrids and to the value of continued monitoring of such material across seasons before a conclusive resistance status can be assigned.

Overall Performance

Across all five traits evaluated, hybrids P₁ × P₃, P₁ × P₆, P₃ × P₁ and P₅ × P₃ exhibited superior and consistent performance, recording the lowest pod borer and aphid damage together with immunity to Sclerotium wilt, and were also immune to anthracnose and cowpea mosaic virus along with every other entry in the trial. Unlike traits where the check hybrid remains unsurpassed, these four hybrids exceeded the check hybrid for all three quantifiable resistance traits, recording desirable (negative) standard heterosis ranging from -41.01 to -51.80 per cent for pod borer and aphid, and -100 per cent for Sclerotium wilt. These hybrids therefore represent genuinely promising sources of combined pest and disease resistance for yardlong bean breeding, rather than merely the best among an inferior set.

Conclusion

The present investigation demonstrated considerable genetic variability among the parental lines and F₁ hybrids of yardlong bean for resistance to pod borer, aphid, Sclerotium wilt, anthracnose and cowpea mosaic virus. Unlike traits in which hybrids merely approach a resistant check, the hybrids P₁ × P₃, P₁ × P₆, P₃ × P₁ and P₅ × P₃ clearly surpassed the check hybrid for pod borer, aphid and Sclerotium wilt resistance, recording desirable standard heterosis of up to -51.80 per cent, and were immune to anthracnose and cowpea mosaic virus along with all other entries. These four hybrids may therefore be regarded as valuable parental materials for future breeding programmes aimed at developing yardlong bean cultivars with combined resistance to major insect pests and diseases, subject to validation across seasons and locations.

Limitation

The present investigation was conducted under natural field conditions. Although natural infestation provided adequate pest and disease pressure for evaluating resistance, the intensity and distribution of insect pests and pathogens may vary across environments and seasons. The resistance observed under natural field conditions should therefore be validated through multilocation and, where appropriate, artificial-inoculation trials before the identified hybrids are used in resistance breeding programmes or commercial cultivation. The study also did not evaluate yield, quality or other agronomic traits, which would need to be considered jointly with resistance in any final parental selection.

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