In a study on pathogenicity of Meloidogyne incognita an inoculum level of 10 juveniles (J2)/plant/pot was detrimental to the growth and development of cucumber, indicating it as a pathogenic threshold level of M. incognita during 2022 and 2023. Root, soil and total nematode population of the cucumber increased progressively with an increase in inoculum levels from 10 to 10,000 J2/plant. Nematode reproduction rate decreased with an increase in inoculum levels and it was maximum in the level of 10 J2/plant and minimum in 10,000 J2/plant against root-knot nematode in cucumber.

Pathogenicity, cucumber, M. incognita, inoculum level.

Cucumber is a semi-tropical vegetable and grows best under polyhouse conditions. In polyhouse under stable environmental, with favourable conditions, optimum nutrients, less pest-disease infestation and the cucumber plant grows speedily. The optimum temperature for better development of fruits is 14-20 C. It requires mild climate and it does well under polyhouse conditions. In India protected cultivation area is around 30000 ha which contributes 0.23 per cent of horticulture cropped area (Pal et al., 2020). In India, cultivation of cucumber is noted at least 3000 years ago and during 100 B.C. in China and remained the largest cucumber producer in the world with 61,949 tonnes/ha production volume per year and the major cucumber cultivating states in India are Haryana, Karnataka, Madhya Pradesh, Tamil Nadu, Andhra Pradesh, Telangana, Assam, Uttar Pradesh, Bihar, Jammu & Kashmir (Anonymous, 2021). India contributes an average annual production of 1922.3 million tonnes from an area of 139.7 hectares (Anonymous, 2024). In Gujarat annual production of cucumber is 177.67 million tonnes from an area of 12.28 hectares (Anonymous, 2024). 

Like other field and polyhouse crops, cucumber is also prone to attack by several biotic and abiotic stresses. Among the different biotic stresses, diseases incited by plant parasitic nematodes viz., Meloidogyne spp., Rotylenchulus reniformis, Heterodera spp., Pratylenchus spp., Hoplolaimus spp. and Helicotylenchus spp. are conjoined with vegetable crops in India. Nematodes being ubiquitous browsers and hidden enemy cause discernable damage to the crop because of longer duration with deeper root system. Moreover, in addition to their own pathogenetic effects, they also play a crucial role in predisposing the host to attack by other pathogenic microorganisms like fungi, bacteria, viruses etc. which in turn create more aggravated situation (Sasser and Jenkins 1975). Plant parasitic nematode causes approximately 21.00 per cent losses in crops, it's amounting to Rs.102,039.79 million (1.58 billion USD) annually in India. Due to favourable environmental conditions in field and polyhouse, multiplication of insect pests and pathogens including plant parasitic nematodes are very high which leads to significant reduction in the quantity and quality of cucumber fruit production (Ismail et al., 2012; Bhati and Baheti, 2020a). 

Out of these nematodes, root-knot nematodes are sedentary endoparasitic nematodes which complete their life cycle inside the plant tissues. Otherwise, the common symptoms of the infestation with root-knot nematodes are stunting, yellowing and wilting, but the major symptom is the gall formation in plant roots (Khalil, 2013). It is the most important and causing great damage to crops grown under polyhouses including cucumber (Rao et al., 2015; Bhati and Baheti 2020b). Root-knot nematodes, Meloidogyne spp. is responsible for 12.00 per cent estimated losses in cucumber crop with monetary loss of Rs. 110.46 million per annum in open field (Kumar et al., 2020). This dreadful pest poses a serious problem because of its higher reproductive potential and subsequent number of generations. The information on the losses caused by the root-knot nematodes in cucumber crop under protected conditions is still limited and their management strategy should be taken into account. 

The pathogenicity of Meloidogyne spp. in which inoculated cucumber plants with known population of root-knot nematodes, with a new to assess the extent of damage and disease symptoms through quantifiable parameters, such as gall formation, root weight, plant height and overall yield. The Meloidogyne incognita has detrimental effect on the sustainable production of cucumber production. It is a significant pest that significantly reduces cucumber plant growth and causes yield losses of 66.23 percent (Tamireddy et al., 2022). This investigation will provide critical insights into the nematode-host interaction dynamics, contributing to the development of effective management strategies in cucumber production within protected environments. So, it is crucial to conduct a study on the pathogen density levels against a susceptible host under controlled conditions and it helps in the making proper decisions for the integrated management of pathogen (Lalhruaitluanga and Hazarika 2020).

Pathogenicity of Meloidogyne incognita on cucumber to determine threshold level in pots were carried out at Department of Nematology, B. A. College of Agriculture, Anand Gujarat Agricultural University, Anand, Gujarat during 2022 and 2023. Earthen pots of 15 cm diameter were washed and disinfected using 4% formaldehyde (Formalin 40 EC) solution. After drying, the pots were filled with steam sterilized soil (1.5 kg/pot). Three seeds of root-knot susceptible cucumber variety were sown in each pot. On germination, plants were thinned down to one plant/pot. Second stage juveniles (J2) were extracted from the respective egg masses of Meloidogyne incognita inoculated on artificially in the rhizosphere of each plant by removing soil around plant stem with the help of forceps. The inoculum levels were tested 0, 10, 100, 1000 and 10,000 J2 of M. incognita on cucumber plant. The uninoculated plants served as untreated control. Each treatment was repeated four times in Completely Randomized Design. Plants were watered regularly. Plants were uprooted carefully (After 45 days of inoculation). The roots were washed with water, observations on plant growth characters and nematode multiplication were recorded and analyzed.

Significant differences in vine length, fresh shoot and root weights, root-knot index (RKI) and root nematode population as well as soil and total nematode population were observed for various inoculum levels of M. incognita (Table 1-3). 

The data presented in Table 1 revealed significant differences among the treatments during the year 2022, 2023 and in pooled analysis for vine length. The year were significant and T×Y interaction were non significant. The results revealed that with increase in levels of inoculum, there was progressive decrease in vine length.

During 2022, significant reduction in vine length (58.82) of cucumber was observed at initial inoculum level of 10 Juveniles (J2) per plant. The lowest vine length (26.08) was recorded in the level of 10,000 J2 per plant. The uninoculated plants (control) showed highest vine length (166.09). This was significantly different from 10 J2 per plant (142.98). Maximum vine length (177.20) was recorded in uninoculated (control) and it was statistically at par with 10 J2 per plant (166.40). Inoculum level of 100 J2 per plant (62.20) and 1000 J2 per plant (49.20) statistically at par with each other in the year of 2023.Similar trends observed in pooled over years. There was significant reduction in vine length at an inoculum level of 10 J2 per plant and above.  

Maximum fresh shoot weight (93.90) was observed in uninoculated plants (control) and it is differed from 10 J2 per plant (80.80). Initial inoculum level of 10 J2 of M. incognita per plant significantly reduced the fresh shoot weight as compared to its lower inoculum levels. The inoculum level 100 J2 per plant (69.60) and 1000 J2 per plant (61.90) were significantly at par with each other. Minimum fresh shoot weight was recorded in 10,000 J2 per plant during the year 2022. Same trend recorded in 2023 and in pooled analysis. The T×Y interaction was non significant indicating there was no environmental effect on treatment during both the years.

The data on fresh root weight Table 2 indicated exactly the same trend as observed in fresh shoot weight per plant during both the years and in pooled analysis indicating initial inoculum of 10 J2/plant as damaging to fresh root weight in individual as well as in pooled analysis. The inoculum level 100 J2/plant and 1000 J2/plant were found at par with each other. There was significant reduction in fresh root weight which was observed from an inoculum of 10,000 J2/plant and above as compared to control in 2022, 2023 and pooled analysis.

With regard to root-knot index (Table 2) all the treatments significantly differed from each other. It increased with an increase in inoculum levels. The data on root-knot index (5.00) revealed significant differences among the treatments during 2022, 2023 and pooled over years. The year effect was significant while T×Y interaction was non significant. Root-knot index was found to increase with increase in initial inoculum level in both the years and pooled over years. Significantly highest root-knot index was observed in 10,000 J2 per plant as compared to their respective lower levels during 2022 and 2023. The inoculum level 100 and 1000 J2 per plant was differed from each other during both the years and pooled over years.

Regarding reproduction of M. incognita, there was increase in number of females, number of egg masses and number of eggs per egg mass with increase in inoculum levels (Table 3). All the three attributes were found maximum in the inoculum level of 10,000 J2/plant. Differences among the treatments were found significant in 2022, 2023 and pooled over years. As expected, there was no females in control plants as nematodes were not inoculated. Similarly, during 2022, 2023 and in pooled analysis, the differences among the treatments were significant for number of eggs per 5 g roots. 

All the inoculum levels were significantly differed from each other for number of eggs/egg mass for both the years as well as in pooled number of eggs/egg mass were increased with increase in inoculum level. Maximum eggs/egg mass was recorded in inoculum level of 10,000 J2 per plant.

With respect to soil nematode population, it increased correspondingly with an increase in the levels of nematode inoculum. All inoculum levels were differed significantly from each other. Maximum soil nematode population (15488) was recorded in the level of 10,000 J2/plant. It was minimum in the level of 10 J2/plant (37) followed by the level of 100 J2/plant (124) during 2022 (Table 3). Same trend observed in 2022 was reported for 2023 and pooled. The year effect was significant, while the Y × T interaction was non significant.

Data on total population (Table 3) exhibited exactly the same trend as noticed for soil population, with maximum reproduction rate in initial inoculum level of 10 J2 per plant followed by 100, 1000 and 10,000 J2 per plant (Table 3). Total nematode population increased with an increase in the levels of nematode inoculum. Control plants which were not inoculated with nematodes had obviously no nematodes recorded during 2022, 2023 and pooled over years. Hence, reproduction rate progressively increases with decrease in nematode inoculum levels. Reproduction rate was maximum (74.1 times) in the level of 10 while it was minimum (2.81 times) in the highest level of 10,000 J2 per plant. This might be due to the destruction of root system by the parasitism of root-knot nematodes which led the competition for food and nutrition among the developing nematode within the root system and also due to inability of juveniles to find out new infection sites subsequent generation. The high rate of multiplication at low levels of inoculum, on the other hand, could possibly be due to the positive factors like abundance of food, lack of competition and the ability of host to support their levels of population. Thus, it is concluded that an inoculum level of 10 and above levels of M. incognita J2/plant proved detrimental to the growth and development of cucumber (Table 3). An initial inoculum level of 10 J2 of M. incognita per plant was found damaging to the cucumber as it significantly reduced the plant growth characters viz., vine length, fresh shoot and root weight.

The foregoing results revealed that in most of the plant growth characters, an inoculum level of 10 J2/plant and above of M. incognita was found to be damaging to cucumber. No reports are available in the literature about the effect of M. incognita on cucumber. The results reported by Lalhruaitluanga and Hazarika (2020); Pankaj and Siyanand (1990); Venkatesan  (2009). They found 100 J2/plant inoculum level was damaging in pumpkin, round melon and bittergourd, respectively. Reduction in rate of reproduction with increase in initial inoculum at 1000 J2/plant inoculum level in bitter gourd and smooth gourd was observed and similar results as reported by Gupta et al. (1995). Singh and Patel (2012); Giribabu et al. (2008) reported reduction in growth and development of bottle gourd at 100 and above inoculum levels due to M. incognita. They observed such reduction at 45 days after inoculation (DAI) in case of vine length as well as fresh shoot and root weights. Similar result with respect to vine length as well as fresh shoot and root weights per plant and nematode multiplication were akin to above results.

Table 1: Pathogenicity of root-knot nematodes, M. incognita in cucumber: Effect on vine length, fresh shoot weight and fresh root  weight and root-knot index during 2022 and 2023.

Table 2: Pathogenicity of root-knot nematodes (M. incognita) in cucumber: Effect on number of females, egg masses and eggs/egg mass during 2022 and 2023.

Table 3: Pathogenicity of root-knot nematode (M. incognita) on cucumber: Effect on soil, total population and reproduction rate during 2022 and 2023.

This study establishes the pathogenic threshold levels of Meloidogyne incognita and M. javanica pt.1 in cucumber, highlighting their impact on plant growth and nematode reproduction. Future research should focus on determining the economic threshold level (ETL) through field trials, screening and developing nematode-resistant cucumber varieties, and exploring grafting techniques with resistant rootstocks. Integrated Nematode Management (INM) strategies, including biocontrol agents, organic amendments, and safe chemical nematicides, should be optimized for sustainable control. Molecular studies on virulence genes and nematode-host interactions will aid in early detection and resistance breeding. Additionally, AI-based decision-support tools and sensor-based monitoring in polyhouses can enhance precision nematode management in protected cultivation.