First Report of Meloidogyne enterolobii Infecting Cassava (Manihot esculenta) Resulting in Root Galling Damage in Africa.

Abstract

Meloidogyne enterolobii is a highly polyphagous tropical species of root knot nematode. It has been recorded to be causing major damage to a range of economically important crops and is increasingly recorded from new locations. The morphological similarity and overlap of characteristics with other commonly occurring species, especially M. incognita, has confused its diagnosis using morphometrics. Cassava (Manihot esculenta) is an important crop across the tropics, including Africa, where it is among the most important root and tuber crop for food security. Cassava can be heavily infected by root knot nematodes, which can incur heavy production losses. The main species known to affect cassava are M. incognita and M. javanica (Coyne and Affokpon, 2018). With the exception of one report of M. enterolobii morphologically identified from cassava roots during a survey in Brazil (Rosa et al., 2014), there is no record with molecular confirmation of it infecting the crop. In the absence of any molecular or isozyme confirmation, diagnosis of M. enterolobii is difficult to determine. In the current study, the species responsible for substantial galling damage (Fig. 1A) on several cassava roots growing in Ibadan, Nigeria (7°22'39″ N; 3°54'21″ E) were diagnosed. DNA isolated from juveniles recovered using a modified Baermann method (Hooper, 1986) from these roots was used for PCR amplification of the mitochondrial Nad5 using primer pair, NAD5F2 (5'-TATTTTTTGTTTGAGATATATTAG-3') and NAD5R1 (5'-CGTGAATCTTGATTTTCCATTTTT-3'). The 515 bp PCR DNA product was sequenced on both strands (GenBank Accession No. MW965454) and found to be 100% identical to M. enterolobii with those in the DNA sequence database (KU372358, KU372359) (Janssen et al., 2016; Kolombia et al., 2017). In addition, M. incognita was also recovered from the galled roots and identified using the same primers (GenBank Accession No. MW965455) indicating a combined species infection (Fig. 2). Cultures of M. enterolobii, developed from single egg masses were maintained on tomato plants and used to assess infection on cassava in 10 L pots filled with steam sterilized loam soil in the screenhouse. Cassava cv. IITA-TMS-IBA070593 cuttings planted in June, 2018 and repeated in April, 2019 were inoculated with 1,000 juveniles per pot at three weeks after planting, and then maintained for four months before quantifying the nematode densities in both roots and soil. Nematode reproduction factor (RF), calculated from total nematode densities (n=8) from soil and roots was as high as 44.3, compared to uninoculated controls. Molecular diagnostics of M. enterolobii, as above, confirms unequivocally the host status of cassava to this nematode. This study reports for the first time the infection of cassava by M. enterolobii under field conditions in Africa and for the first time demonstrates the host suitability of cassava to this nematode (Fig. 1B). M. enterolobii is among the most commonly occurring root-knot nematode species in West Africa (dos Santos et al., 2019). It is therefore anticipated that M. enterolobii has long been infecting, especially in West Africa, but has been overlooked due to its morphological similarity with M. incognita. Given the high reproductive ability of M. enterolobii on cassava and its highly aggressive nature on a range of crops, it is likely that it is causing, or will result in, high levels of losses on cassava in Africa.