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The main edible part of the carrot is fresh taproot; so the foremost emphasis of our investigation is to produce healthy and good quality carrot as well as to control the root-knot nematode, M. incognita using eco-friendly tactics. The results proclaimed that all the tested cultivars of carrot behaved differently to nematode infestation i.e. root-knot index, egg masses/root, eggs/egg mass, nematode population and reproduction factor. The carrot cultivars are categorized on the basis of root-knot index. The results also revealed that, none of the carrot cultivar was found to be highly resistant or immune against the root-knot nematode, M. incognita. However, the cultivar Golden Rosy was observed with least root-knot index (1.2) which indicates the resistant reaction. Maximum root-knot index (5.0) was recorded in cultivar Kamini indicating the greatest nematode attack was experienced by it. The cultivar Kamboj, Surbhi, Super Red and Pearl Red were found moderately susceptible (MS); Rose Red, Noorie, Lali, Sindhuri, and Selection 80 were found susceptible (S) to root-knot nematode. Two cultivars Red King and Desi Red with root-knot indices (1.8 and 2.0) displayed moderately resistant (MR) reaction against the nematode.
The variations in the severity of gall formation in different cultivars may be due to the number of nematodes penetrated into the roots and their stabilization. It is evident from the finding that the resistance or susceptibility of cultivars to root-knot nematodes denoted by the presence or absence of galls on roots of tomato plants. However, significant differences in the number of galls represent the levels of susceptibility (Castagnone-Sereno, 2006; Jaiteh et al., 2012). The reduced rates of nematode reproduction, egg masses and consequently, low nematode population densities than that of a susceptible one demonstrated the resistance in host plants against nematodes (Khan, 1994; Ansari et al., 2018). Another salient property of resistance to root-knot nematode is the impact on galls development commonly linked to compatible interactions of nematode-host plants (Garcia et al., 1996). Resistance during screening experiments can only be evaluated by development of galls in most susceptible plants infested with root-knot nematodes (Fassuliotis, 1979). Hirunsalee et al., (1995) observed that gall formation and nematode multiplication on the roots of plant were supported by susceptible and tolerant cultivars while inhibition was observed in resistant ones.
In addition to galls, the nematode also provoked a significant increase in the incidence of other undesirable characteristics on the infected carrots. The most prominent was sudden and localized constrictions with twisting and distortion in carrot. The second is the emergence of thumb-like branch of roots or forked roots. The highly susceptible cultivar showed maximum forking as it exhibit highest susceptibility to M. incognita followed by susceptible cultivars. Whereas least forking was displayed by the moderately resistant cultivars as they experienced comparatively less nematode attack. However, forking, galling, twisting and cracking type symptoms were completely unseen in the resistant cultivar. The forking, twisting and cracking completely damage and distorted the roots in terms of their shape, length, weight and appearance. Besides the above symptoms, the formation of root hairs also affects the root length of cultivars. Comparatively larger number of egg masses was obtained from the carrot cultivars; Noorie, Lali, Sindhuri, and Selection 80. From the roots of highly susceptible cultivar Kamini highest number of egg masses was recorded which revealed that maximum number of juveniles penetrated the roots and completed their life cycles in a successful manner.
On the other hand, cultivar Golden Rosy allowed only a limited number of juveniles of M. incognita to penetrate the roots, leading to maturity as it is confirmed by number of egg masses and reproduction factor. Similarly the maximum eggs were recorded on highly susceptible cultivar roots compared with the susceptible, moderately susceptible, moderately resistant and/or resistant cultivars. Formation of galls over the roots of susceptible cultivar is the primary symptom of root-knot nematode infection. According to Cousins and Walker (1998) root-knot nematode eggs developed poorly on resistant cultivars compared to susceptible ones. Also, the authors reported that total number of eggs represents the population of nematodes that reached reproductive maturity, and therefore provide one measure of resistance. Screening of individual seedlings for nematode resistance helps in elimination of susceptible plants prior to field plantation which results in the conservation of breeders’ nursery stocks.
It was investigated from this study that, the highly susceptible cultivar (Kamini) recorded with the highest nematode population/250 g soil which was significantly different from all the cultivars studied in the pots and uninoculated control. Nematode population density can be ascribed to the disease severity and size of the root-knot galls which varies among the different tested cultivars. The increased population density and size of the root galls might be due to penetration of large number of nematodes favored by the highly susceptible cultivar and finally stabilized them to form the giant cells. Similarly, Huang (1986) reported that resistance is associated to reduced rate of nematode penetration, development, egg formation and enhanced plant growth which ultimately results in a low nematode population density with the fact that a few nematodes could completed their life cycle. According to El-Sherif et al., 2007, roots of susceptible cultivars are found to be more prone to root-knot nematode activities and encourage reproduction and survival of juveniles. Therefore, more juveniles were observed on susceptible cultivars compared to the resistant ones. The host type is also a key factor in the development of J2s (Davide, 1980). The potential development of juveniles was observed on susceptible host whereas in resistant hosts the development can be inhibited (Nelson et al., 1990).
Resistance and susceptibility of hosts to phytopathogenic nematodes affects the reproductive capacity of nematodes (Trudgill, 1991). The present reports showed significant differences in reproduction factors of M. incognita on all the carrot cultivars.
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