Reaction of melon genotypes to Meloidogyne incognita and Meloidogyne javanica 1

Melon (Cucumis melo L.) is one of the most economically important crops in Brazil, especially in the Northeast region, where favorable climatic conditions allow it to be grown year-round (Pires et al. 2013). China is the world’s leading producer of muskmelon, with an annual average of 17.5 million tons (FAO 2012). Brazil is ranked tenth, with an estimated production of 565.9 thousand tons, in 2013, and its Northeast region reaching 537.4 thousand tons (IBGE 2013). With marked growth in recent years, the cultivation of this cucurbit species has attracted attention. Muskmelon has advantages over other melons, underscoring its good commercial value and profitability. Moreover, it contains a high level ABSTRACT RESUMO

Melon is one of the most economically important crops cultivated in Brazil, especially in the Northeast region.However, as its cultivation increases, phytosanitary problems arise, including those caused by nematodes, which are most effectively controlled using resistant cultivars.This study aimed at assessing the reaction of muskmelon genotypes, in terms of resistance to Meloidogyne incognita and M. javanica infestation.The experiment was conducted under greenhouse conditions using a completely randomized design, in a 2 x 15 factorial scheme, with six replications.A total of 15 muskmelon genotypes were evaluated and the 'Santa Cruz Kada' tomato was used as a susceptible control.The total number of eggs and juvenile nematodes in the roots and the reproduction factor were used to assess the genotype reaction.None of the genotypes was resistant to M. incognita.Eight genotypes were classified as resistant to M. javanica and promising for use in melon breeding programs.

RESEARCH NOTE
Nematodes are difficult to control, since each situation requires a careful analysis of the best method to use (Ferraz 1992).Peixoto et al. (1999) report that preventive measures are the most effective in controlling these phytoparasites.These include crop rotation, use of plants toxic to nematodes, soil solarization and application of chemical products.However, the authors state that these measures are laborious, burdensome and often inefficient.The use of resistant plants, therefore, is the most economical and efficient method to control root-knot nematodes in cucurbits, including muskmelon crops (Ito et al. 2014).
In recent years, a number of studies have evaluated sources of resistance to gall-forming nematodes in muskmelon.Santos et al. (1999) studied the resistance of muskmelon genotypes to M. incognita and observed that 2 of the 54 genotypes assessed exhibited resistance to the pathogen.Ito et al. (2014) evaluated sources of cucurbit resistance to M. incognita and M. javanica and found that the 'Gaúcho Redondo' genotype was resistant to M. javanica.However, none of the genotypes is resistant to both species.
Therefore, developing muskmelon genotypes with simultaneous resistance to M. incognita and M. javanica is important for commercial crops, especially for areas where these phytoparasites occur.M. incognita and M. javanica may occur simultaneously.Thus, the present study aimed at assessing the reaction of muskmelon genotypes, in terms of resistance to M. incognita and M. javanica infestation.
The experiment was conducted from March to June 2015, under greenhouse conditions, in Jaboticabal (21º14'05"S, 48º17'09"W and altitude of 614 m), São Paulo State, Brazil, where the average minimum and maximum temperatures were 17 ºC and 27 ºC, respectively.
The nematode species used in the study were M. javanica and M. incognita.Nematode identification was confirmed in the Nematology Laboratory of the Universidade Estadual Paulista Júlio de Mesquita Filho.The perineal pattern was used according to Taylor & Netscher (1974) and the morphology of the labial region in males in accordance with Eisenback et al. (1981).After identification, M. javanica and M. incognita were respectively multiplied in okra (Abelmoschus esculentus L. Moench) and cotton (Gossypium hirsutum L.) susceptible hosts.The seedlings used to multiply the inoculum were maintained in a separate greenhouse.Around 120 days after inoculation, second-stage eggs and juveniles (J 2 ) of the nematode species were extracted from the root systems of their respective host plants.
Inocula were prepared using the technique developed by Hussey & Barker (1973), with changes introduced by Bonetti & Ferraz (1981) and Oliveira et al. (2009).The population of eggs and juveniles in suspension was estimated using a Peters counting camera, under a photonic force microscope, with subsequent concentration adjustment to 1,000 eggs and J 2 mL -1 .Then, 5,000 eggs and the J 2 of each species were inoculated by applying 5 mL of the suspension per plant.
The experimental design was completely randomized, in a 2 x 15 factorial scheme, with six replications and experimental units of one plant.Muskmelon and tomato seedlings were produced in expanded 128-cell polystyrene trays, in a greenhouse equipped with a sprinkler irrigation system.Two seeds were placed in each cell filled with Bioplant ® substrate, with subsequent thinning to obtain high quality seedlings.
Seedlings were transplanted to plastic pots with 2 L of previously autoclaved substrate (120 ºC, 1 atm, 1 hour) when two leaves sprouted, approximately 25 days after sowing.The substrate was composed by a mixture of soil, sand and hardened cow manure (1:1:1).During transplanting, the suspension containing eggs and J 2 was inoculated separately for each species in the substrate.After 60 days of nematode inoculation, plants were analyzed by separating them into shoots and roots.
To eliminate soil excess, roots were removed from the pots and washed.They were then processed Reaction of melon genotypes to Meloidogyne incognita and Meloidogyne javanica e-ISSN 1983-4063 -www.agro.ufg.br/pat-Pesq.Agropec.Trop., Goiânia, v. 46, n. 1, p. 111-115, Jan./Mar.2016 to extract eggs and nematodes in other development stages (Hussey & Barker 1973).The final population of each suspension, derived from the individually processed root systems, was determined by counting eggs and J 2 with the aid of a Peters counting camera, under a photonic force microscope.This population was used to determine the reproduction factor (RF), given by the ratio between the initial (IP) and final population (FP) of inoculated eggs (RF = FP/IP), where plants with RF < 1 were considered resistant to the nematodes and those with RF ≥ 1 were considered susceptible to them (Oostenbrink 1966).
The data were log transformed and submitted to analyses of variance for muskmelon genotypes and nematode species and their interaction.Means were compared by the Scott-Knott test (p < 0.01), using the AgroEstat software (Barbosa & Maldonado Júnior 2010).
There was a significant difference between muskmelon genotypes and nematode species by the F-test, at 1 %, which may be an indication of genotype variability for the characteristics assessed (Table 1).
The inoculations performed with M. javanica and M. incognita were considered efficient, since there was species multiplication in the 'Santa Cruz Kada' tomato, a susceptible host plant for both species.In the tomato plant, the reproduction factor and total number of second-stage eggs and juveniles were respectively 11.7 and 58,457, for M. incognita, and 25.8 and 129,057, for M. javanica.
Concerning the muskmelon genotypes reaction to M. incognita, there was a significant difference between the materials, for the RF variable.Even though genotypes differed, all of them were considered susceptible to M. incognita, with RF ranging from 2.52 to 9.22 (Table 1).Paiva et al. (2004) assessed 30 muskmelon genotypes (Cantaloupe cultivar), selecting eight lines that showed resistance to M. incognita.According to (a) R = resistant; S = susceptible.Analyses were conducted using log transformed data. (b) Means followed by the same upper case letter in the column and lower case letter in the line do not differ according to the Scott-Knott test (p ≤ 0.01). (c) Susceptibility control.** Significant at 1 %.Liu et al. (2015) reported a new rootstock from the Cucumis putulatus species resistant to M. incognita and Fusarium wilt, which causes disease in the main cucurbits, including the muskmelon.Anwar & McKenry (2010) studied the incidence and reproduction of M. incognita in genotypes of several plant species (Momordica charantia, Cucumis sativus, Cucurbita argyrosperma, Luffa cylindrica, Citrullus lanatus).They used the reproduction factor to determine the susceptibility and found that M. incognita is highly aggressive in cucurbit genotypes, as observed in the present study.
To date, in Brazil, there is no muskmelon genotype with multiple resistance to M. javanica and M. incognita.Thus, resistant genetic material can be used in breeding programs aimed at developing cultivars resistant to root-knot nematodes.

Table 1 .
Average total number of eggs and J 2 and reproduction factor for Meloidogyne javanica and M. incognita in 15 muskmelon genotypes and the susceptible control 'Santa Cruz Kada' tomato.Luffa cylindrica (sponge gourd), Benincasa hispida (winter melon) and Trichosanthes cucumerina (snake gourd)].However, none of the genotypes assessed showed resistance to both species of root-knot nematodes, corroborating the results obtained here.