‘Piel de Sapo’ Breeding Lines Tolerant to Melon Vine Decline

Abstract

Melon vine decline (MVD) results in severe economic losses in melon crops (Cucumis melo L.) throughout the Mediterranean area and in the United States. MVD is a complex disease produced by many soilborne pathogens such as Monosporascus cannonballus Pollack et Uecker, Acremonium cucurbitacearum Alfaro-Garcia, W. Gams et Garcia-Jimenez, Macrophomina phaseolina (Tassi) Goidanich, and Rhizopycnis vagum DF Farr (Bruton et al., 1995; Chilosi et al., 2008; Fita et al., 2007a; Garcia-Jimenez et al., 2000). Occurrence and pathogenicity of fungi associated with MVD is variable depending on the climate area and the cultural practices (Aegerter et al., 2000; Bruton et al., 1999; Makam et al., 2005, Martyn, 2007). Despite this complex situation, M. cannonballus is the pathogen most frequently associated with severe outbreaks of MVD. It has great virulence and produces severe root damage in artificial inoculation alone or combined with other fungi, comparable to that found in naturally infested roots (Armengol et al., 2003; Beltran, 2006; Fita et al., 2008; Iglesias et al., 2000a). Also, realtime polymerase chain reaction has demonstrated that the roots of genotypes reported to be resistant to MVD have lower levels of M. cannonballus in comparison with susceptible genotypes (Pico et al., 2008). M. cannonballus causes brown discoloration of the roots, which can evolve into rot and even lead to necrosis of the whole root system. Affected roots are unable to supply the vine with the necessary nutrients and water, leading to plant wilt and collapse. Vine decline and/or collapse are usually observed in the late season when the level of water required by fruit increases (Fita et al., 2007a). The occurrence and intensity of these aboveground symptoms are highly dependent on the environmental conditions and cultural practices such as temperature, fruit load, water management, and so on. Consequently, disease assessment has been specifically centered on the extent of root damage. This methodology requires root extraction from the soil, which makes breeding programs highly difficult and time-consuming because all the plants must first of all be artificially pollinated and then, at the adult stage, evaluated by their root systems (Fita et al., 2007a). The limited number of sources of resistance to MVD reported to date (Cohen et al., 1996; Crosby, 2001; Esteva and Nuez, 1994; Wolff and Miller, 1998) is also a major obstacle in the development of resistant cultivars. One of the most promising accessions for its tolerance to MVD was ‘Pat 81’, which belongs to the subsp. agrestis of C. melo (Pitrat, 2008). ‘Pat 81’ displayed very low levels of vine collapse in field assays and exhibited root lesions that were less widespread and less severe than susceptible cultivars after artificial inoculations with M. cannonballus and A. cucurbitacearum (Dias et al., 2002, 2004; Iglesias et al., 2000a, 2000b, 2000c). In addition, ‘Pat 81’ develops a highly branched root system with long laterals and a high regeneration ability, which has proven to be crucial in overcoming the disease. The performance of melon plants against MVD is highly dependent on two factors: the intrinsic tolerance to the fungus infection and the architecture of the root system before and after infection (Fita et al., 2006, 2008).‘Pat 81’ has these two features and has proven to be a satisfactory rootstock for the grafting of melons in the battle against MVD (Fita et al., 2007b). For these reasons, it was selected as a donor parent to breed melon lines resistant to MVD (Dias, 2003). The ‘Piel de Sapo’ melon is very popular in the Mediterranean area, especially in Spain, and is becoming increasingly important in the European market. To cover the market demand during winter, Brazil is also producing this melon type. Unfortunately, ‘Piel de Sapo’ melons are highly susceptible to MVD, which seriously limits output in the main producing areas of the world.