Pathological characteristics of soil-borne diseases in cucurbitaceous crops caused by Fusarium species and Meloidogyne incognita

Abstract

however, no vascular wilt and stem and root rot symptoms were induced by RKN. Pathological interrelations of two soil-borne diseases in cucurbits caused by FI and RKN were characterized by DI, GI and EI in inoculation tests using the lower (LD) and higher inoculum densities (HD) of FI and M. incognita. Virulence of FI determined by DI at 4 weeks after inoculation was mostly in the higher order of F. proliferatum F6, F5 and F. oxysporum f. sp. melonis (FOM) or F. oxysporum f. sp. niveum (FON) with no significant differential interactions among the cucurbits or RKN co-infection. Significant increases of DI due to RKN coinfection were noticed in watermelon and oriental melon (attributed to F. proliferatum with LD), suggesting the DI increase due to RKN coinfection may depend upon the virulence of FI relative to RKN aggressiveness to the cucurbits. In the cucurbits with the coinfection of FI and RKN, GI and EI were mostly reduced logarithmically with the increase of DI, largely more in EI than GI, in all crops except for shintosa. Microscopic examination of the root tissues showed histopathological features characteristic to infection types, which was visualized in relations of the formation of defense structures such as tyloses and xylem mucilage, development of giant cells and their cytoplasmic statuses with the values of DI, GI and EI, respectively. All of these results suggest the current prevalence of the fungal disease in the oriental melon may be potentially from the infection of other Fusarium species such as F. proliferatum alone or in combination with RKN that produces seemingly wilting caused by severe stem rotting, for which the synergistic effect in the increase of the disease severities occurred favorably to DI and adversely to GI and EI depending on the fungus-nematode relationships. These findings will be helpful to develop control strategies of the soil-borne disease complex based on their pathological characteristics.

The root-knot nematode (Meloidogyne incognita) and fusarium wilt fungi (Fusarium spp.) are important soil-borne pathogens causing sever damages to greenhouse-grown cucurbitaceous crops in Korea. Especially the fusarium wilt is still prevalent in oriental melon greenhouses even though shintosa (Cucurbit maxima x C. moschata) resistant to fusarium wilt has been widely used as a stock for grafting the oriental melon cultivars for their propagation. In this study, the four cucurbitaceous crops (oriental melon, cucumber, shintosa and watermelon) were inoculated with Fusarium spp. and root-knot nematode (M. incognita) (RKN) alone and in combination with each other. Diseases severities were examined by the degrees of the fungal disease severity index (DI) caused by FI and gall index (GI) and eggmass index (EI) on the cucurbits caused by RKN. Twenty-seven Fusarium isolates (FI) obtained from oriental melon fields were identified based on their morphological characteristics and by the analyses of elongation factor-1 alpha gene (EF-1α) and internal transcribed spacer (ITS) rDNA sequences as 6 Fusarium species (8 isolates of F. oxysporum, 8 F. commune, 5 F. proliferatum, 3 F. equiseti, 2 F. delphinoides, and 1 F. andiyazi) that were coincided with 6 EF-1α sequence-based phylogenetic clades, respectively, suggesting the former three Fusarium species are prevalent in the oriental melon fields. Among these, the isolates of F. proliferatum and F. oxysporum showed the average DI of ~1.0 and ≥2.5 on oriental melon and the average DI of around 0.5 and 1.4 on shintosa, respectively, suggesting F. proliferatum may be pathogenic and virulent to the both cucurbitaceous crops, although it induced stem or root rots but not authentic fusarium wilt. All the cucurbitaceous crops were highly susceptible to the root-knot nematode with GI≥ 2.0 and EI≥3.5