Host-status and sensitivity of twenty-two sweet potato lines to meloidogyne incogita and meloidogyne javanica and potential mechanisms of resistance

Abstract

Most thermophilic crop-producing regions are replete with root-knot (Meloidogyne species) nematodes especially M. incognita, M. javanica and M. enterolobii. Meloidogyne species are a serious threat on various sweet potato (Ipomoea batatas L.) cultivars. Previously, internationally M. incognita was viewed as being a more aggressive Meloidogyne species than M. javanica, whereas in South Africa the opposite had been correct. Additionally, in most cropping systems, M. incognita and M. javanica occur as mixed population densities and were previously managed using fumigant nematicides, which were later withdrawn from the agrochemical markets due to their negative impact on human and environmental health. Following their withdrawal, the use of resistant plant genotypes served as an alternative strategy in managing high nematode population densities. However, after the withdrawal of fumigant nematicides, another root-knot nematode, namely, M. enterolobii, emerged as a major threat in various crops, breaking the renowned Mi resistant genes in various crops. The objectives of this study were to: (1) determine whether the reproductive potential (RP) and relative susceptibility (rsa) values of M. incognita, M. javanica and M. enterolobii each on sweet potato test lines and cultivars would be below unity and then (2) investigate whether non-host status in sweet potato lines would have post-infectional nematode resistance mechanism to M. incognita, M. javanica and M. enterolobii. To achieve Objective 1, three greenhouse experiments were conducted, and all 19 sweet potato line cuttings were inoculated at 14 days after transplanting with the same level of 250 eggs and second stage-juveniles (J2) of M. incognita, M. javanica and M. enterolobii in each trial. In each trial, nematode-plant relations were assessed using RP and rsa, each indicator suggesting that the lines were expressing either host-status or non-host-status. In Objective 2, two sweet potato lines, namely, nematode resistance, namely, pre- or post-infectional nematode resistance mechanism. The two lines had each features which suggested the existence of post-infectional nematode resistance mechanism, .consistently characterised by the existence of (1) necrotic spots, (2) poorly developed giant cells, (3) proliferation of rootlet interference, (4) absence of root galls and (5) non-detectable J2 in roots. In conclusion, the two sweet potato lines were added to the existing sweet potato cultivars with post-infectional nematode resistance to thermophilic Meloidogyne species.