Grafting Eggplant onto Tomato Rootstock to Suppress Verticillium dahliae Infection: The Effect of Root Exudates

Verticillium wilt (caused by Verticillium dahliae), a soilborne disease, often causes significant reductions of yield in eggplant (Solanum melongena L.) production where crop rotation is limited. Rootstock replacement through grafting is considered an effective method to control this disease. This 2-year study investigated the eggplant yield, resistance to verticillium wilt, and allelochemicals in root exudates of eggplant grafted onto a tomato rootstock. Both disease incidence and disease severity on grafted eggplant were markedly lower than those of nongrafted eggplants. Fifteen days after V. dahliae inoculation, grafted eggplants did not exhibit any infection, whereas the disease incidence and disease severity index of the nongrafted eggplants were 68.3% and 37.8% in 2006 and 66.7% and 36.3% in 2007, respectively. Twenty-five days after inoculation, disease incidences on grafted eggplants were only 8.1% and 9.5% in 2006 and 2007, respectively, but those of the nongrafted eggplants increased to 100%. As a result, early yield, total yield, and average fruit weight were significantly increased by grafting when inoculated with V. dahliae in 2006 and 2007. Mycelium growth of V. dahliae was inhibited by the root exudates of grafted eggplants. In contrast, the root exudates of nongrafted eggplants enhanced the mycelium growth. The gas chromatography–mass spectrometry analysis revealed that the composition in the root exudates released by grafted eggplants differed not only from the nongrafted eggplants, but also from the tomato rootstock plants. Ten chemical classes were isolated and identified in root exudates of grafted eggplants. Carbazoles, amines, azulene, and fluorene were only detected in the grafted eggplants. The relative contents of ester compounds were the highest in the root exudates from the grafted eggplant followed by derivatives of benzene, whereas the relative contents of benzene derivatives were much higher than that of the ester compounds in the root exudates from the nongrafted eggplant and tomato rootstock. Eggplant (Solanum melongena L.) is a major vegetable crop worldwide. Considerable yield loss under continuous monocropping in commercial eggplant production is often caused by soilborne diseases, particularly verticillium wilt (caused by Verticillium dahliae). Approximately 30% to 40% of the reduction in Chinese eggplant production has been ascribed to verticillium wilt (Wang et al., 2005). Symptoms of verticillium wilt on eggplant include yellow–bronze-wilted areas, mainly between the leaf veins, and vascular discoloration, which may result in stunted plant growth (O’Brien, 1983) and significant yield loss (Bletsos et al., 1999). Verticillium dahliae infects eggplants through the roots and overwinters in the form of microsclerotia in the soil. Control of verticillium wilt by chemical fungicides has not been very successful, especially on infected plants. In addition, because of the environmental concerns posed by chemical pesticides, proactive cultural practices are often recommended, including crop rotation (Garber, 1973), solarization (Morgan et al., 1991), resistant cultivars (Kalloo, 1993), and grafting eggplant onto resistant rootstocks (Lockwood et al., 1970; Oda, 2007). Many reports have suggested that using appropriate rootstocks can control the eggplant verticillium wilt effectively (Bletsos et al., 2003; Wang et al., 2004; Zhao et al., 1997; Zhou et al., 1997). Although wild species, including Solanum torvum Sw. and Solanum sisymbriifolium Lam., have been demonstrated as potential eggplant rootstocks with high resistance to verticillium wilt (Alconero et al., 1988; Bletsos et al., 2003; Zhou et al., 1997), using resistant tomato (Solanum lycopersicum L.) rootstocks was also shown to be effective (Lockwood et al., 1970; Wang et al., 2003b; Zhou and Wang, 2002). Production of wild eggplant rootstock seedlings can be very challenging as a result of low germination rate of seed, poor emergence, and slow early growth, whereas such issues may be resolved by the use of selected tomato rootstocks. Among other mechanisms for resistance of grafted eggplants to V. dahliae, the allelopathic suppression exhibited by the root exudates from the grafted eggplants has been suggested to play a unique role (Zhou et al., 2001). Root exudates from grafted eggplants were shown to directly inhibit the spore germination and mycelium growth. It is well known that allelochemicals can be released through root exudation. Allelochemicals released from plants are primarily secondary metabolites, which can exhibit positive or negative effects on recipient plants or microorganisms (Rice, 1984). The synthesis and exudation of allelochemicals, along with increased overall production of root exudates, is typically enhanced by stress conditions that the plant encounters (Pramanik et al., 2000). Root exudates also represent one of the largest direct inputs of plant chemicals into the rhizosphere environment (Bertin et al., 2003). Allelochemicals in root exudates can influence plant growth through their impact on the rhizosphere microbial community. It is hypothesized that the compositions and contents of allelochemicals in root exudates after grafting may lead to enhanced disease resistance of eggplants to verticillium wilt. However, to date, little is known with respect to the allelochemicals in root exudates of grafted eggplant. Elucidating the effect of root exudates on disease resistance of grafted eggplants would open a possible avenue for understanding vegetable grafting mechanisms and could facilitate rootstock selection. The objectives of this study were to investigate the suppression of V. dahliae by root exudates of grafted eggplants and further identify the allelochemical Received for publication 30 June 2009. Accepted for publication 13 Oct. 2009. This work was financially supported by the National Natural Science Foundation of China (30771469), the National 863 of China (2004AA247010), the 11th Five-Year Key Support Programs for Science and Technology Development of China (2008BADA6B02), and the Innovation Group of Higher Education of Liaoning Province (2009T087), China. We thank Drs. William M. Stall and Edward E. Carey for reviewing the manuscript. To whom reprint requests should be addressed; e-mail [email protected]. 2058 HORTSCIENCE VOL. 44(7) DECEMBER 2009 constituents of grafted eggplants using tomato as the rootstock. Materials and Methods Plants, cultural conditions, and experimental design. Xi’anlu, a commercially available eggplant cultivar, was used as the scion, whereas Lydl, a local tomato cultivar with identified resistance to verticillium wilt, was used as the rootstock. The experiment was carried out in a nonheated plastic greenhouse during 2006 and 2007 in the Vegetable Crops Experimental Station at the Shenyang Agricultural University. Scion and rootstock seeds were sown into sterilized medium consisting of 3 perlite:2 peatmoss:1 vermiculite (by volume) on 8 Feb. and 10 Mar. 2006 and 13 Feb. and 17 Mar. 2007, respectively. When the rootstock seedlings reached fourto five-leaf stage (13 Apr. 2006 and 23 Apr. 2007), grafting was carried out using the cleft method (Wang et al., 2003a). After 17d healing (1 May 2006 and 10 May 2007), the grafted eggplant seedlings were transplanted into 20.5-cm diameter containers with the same sterilized substrate mixture as used for seedling germination and cultured in the greenhouse. Nongrafted eggplant and tomato plants were also grown to use as controls. The plants were grown under natural lighting in the greenhouse. Pots were placed on the ground where the ground soil was a sandy loam. Each plant was supplied with 500 mL Hoagland nutrient solution twice a week. The treatments were arranged in a randomized complete block design with three replications. Each replication consisted of 10 plants per treatment in both years. Disease evaluation and yield. Five d after transplanting, a spade was vertically inserted into the rhizosphere 3 cm from the base of the stem for injuring the roots in each pot, and then the V. dahliae spore suspension (1 · 10/ mL) was artificially injected into the rhizosphere along the same path to infect the roots. The V. dahliae isolate from infected eggplants was cultured in a 1-L glass jar containing 300 mL potato dextrose medium for 7 d at 25 C. The inoculum suspension was adjusted to 1 · 10 spores/mL with sterile distilled water. Each plant from the grafted eggplant, nongrafted eggplant and tomato treatments were inoculated with 100 mL of the pathogen suspension. With the onset of verticillium wilt symptoms, disease incidence and disease severity were evaluated every 5 d using the following calculations: