Annals of Applied Biology ISSN 0003-4746 RESEARCH ARTICLE Damage functions and thermal requirements of Meloidogyne javanica and Meloidogyne incognita on watermelon M. López-Gómez 1 , A. Gine 2 , M.D. Vela 3 , C. Ornat 2 , F.J. Sorribas 2 , M. Talavera 4 & S. Verdejo-Lucas 1,5 1 IRTA, Barcelona, Spain 2 DEAB-UPC, Barcelona, Spain 3 Centro de Chipiona, IFAPA, Cadiz, Spain 4 Centro Camino de Purchil, IFAPA, Granada, Spain 5 Present address: Centro de La Mojonera, IFAPA, 04745 Almería, Spain Keywords Citrullus lanatus; equilibrium density; reproduction rate; root-knot nematode; tolerance limit. Correspondence Soledad Verdejo-Lucas, Centro de La Mojonera, IFAPA, Camino de San Nicolás no. 1, 04745 La Mojonera, Almería, Spain. E-mail: soledad.verdejo@juntadeandalucia.es Received: 20 December 2013; revised version accepted: 17 July 2014. doi:10.1111/aab.12154 Abstract The relationship between the initial (P i ) and final (P f ) population densities of Meloidogyne javanica and yield of watermelon, Citrullus lanatus, cv. Sugar Baby were determined in pot and field experiments. In the pots, the maximum reproduction rate of the nematode was 14, and the equilibrium density was 49 400 eggs/100 cm 3 of soil. Yield data represented as fresh top weight fitted the Seinhorst damage function (P < 0.001, R 2 = 0.7), and the minimum relative yield (m) was 0.65 at P i ≥ 3200 eggs/100 cm 3 of soil and the tolerance limit (T ) 74 eggs/100 cm 3 . In the field experiments (2011 and 2012), the maximum reproduction rate was 73 and 70, and the equilibrium density 32 and 35 second-stage juveniles (J2)/100 cm 3 soil. Yield data represented as fruit weight fitted the Seinhorst damage function in 2011 (P < 0.001, R 2 = 0.92) and the m- and T -values were 0.63 and 20 J2/100 cm 3 of soil, respectively. Meloidogyne incognita and M. javanica needed similar length of time for development to egg-laying females and life cycle completion at 24.4 ∘ C. Introduction Root-knot nematodes (RKN), Meloidogyne spp., are major limiting factors for growing vegetables worldwide (Sikora and Fernandez, 2005). Several crops within the family Cucurbitaceae are severely damaged by RKN, includ- ing watermelon, Citrullus lanatus, (Thunb.) Matsum. & Nakai (Thies and Levi, 2003; Pofu et al., 2011). In Spain, watermelon is cultivated in about 18 600 ha with an annual production of 782 000 t, of which 48% are pro- duced under protected cultivation (MARM, 2010). The estimated economic losses in south-eastern Spain due to RKN on watermelon were €45 1940 under protected cultivation (Talavera et al., 2012). In this area, chemicals are frequently used for controlling soil-borne pathogens and nematodes. Currently, no commercial watermelon cultivars are resistant to Meloidogyne spp. (Thies & Levi, 2007). Therefore, understanding the host–parasite rela- tionship on watermelon is necessary for predicting yield losses and modelling the population dynamics of the nematode. Heavy root galling was observed after inoculation of watermelon with Meloidogyne arenaria, Meloidogyne incognita or Meloidogyne javanica (Winstead & Riggs, 1959). By contrast, Edelstein et al. (2010) found no galling or low gall indexes in plants inoculated with M. javanica and M. incognita, respectively. An estimation of the growth potential of the RKN population in a given crop will provide information on the host suitability and, in turn, on the crop tolerance to the nematode. In susceptible crops, there is a negative relationship between the initial nematode population density and the reproduction rate (Ferris, 1986). Crop yield losses are influenced primarily by preplanting population densities, and the damage potential of the nematode to the crop has been described by mathematical models (Seinhorst, 1965). Yield losses and damage-function models have been reported for several cucurbit crops (Ornat et al., 1997; Ploeg & Phillips, 2001; Webster et al., 2001; Kim & Ferris, 2002). However, data on the relationship between RKN preplanting populations and watermelon yield are scarce. As far as we are aware, only two reports refer to Ann Appl Biol (2014) 1 © 2014 Association of Applied Biologists