Plant parasitic nematodes are ever-present and incidental with plant growth and crop production, occurring on just about every crop or plant known.
Nematodes are mostly microscopic and thus unseen; and the symptoms of nematode infection are difficult to determine in the ï¬eld, as these are often nonspeciï¬c.
Yam farmers are very much aware of the physical damage that nematodes cause to the tubers, but are mostly completely unaware of what causes the damage. In the field, nematodes reduce crop vigor and performance, leading to lower yields. They cause significant damage to the tubers, resulting in deformed, unsightly tubers or tubers with cracked and flaking skin that conceals an underlying rot. Such symptoms have an immediate and direct impact on the marketability of tubers, but they also relate to reduced crop productivity. Infected tubers, when unwittingly used as planting material, due to low, unnoticeable levels of infection, affect the ability of seed to produceâ€”or even to germinate.
A wide range of nematodes are associated with yam, but only two â€˜typesâ€™ are of concern: root-knot nematodes (Meloidogyne spp.), which are evident by the disfigurement they cause to tubers (Figs. 1 and 2), and lesion nematodes (Scutellonema bradys, Pratylenchus spp.), which result in â€˜dry rotâ€™ and cracked tubers. Infected tubers can also develop erratically growing roots, referred to as â€˜crazy rootsâ€™ (Fig. 1A).
Meloidogyne spp., are an especially damaging group of pests for numerous crops, which are becoming an increasingly serious problem on yam, likely due to gradual intensification of cropping practices. For example, a recent 2013 survey by IITA in Nigeria found that approximately a quarter of all harvested tubers have some level of tuber damage by Meloidogyne spp. (Fig. 1B).
The same survey also discovered, for the first time, a particularly aggressive species, M. enterolobii, infecting yam, among a number of other Meloidogyne species, which may occur simultaneously in the same field, and on the same plant. This has implication in breeding for resistance, and requires that the screening process takes into account the variety of species affecting the crop, such as is undertaken at IITA. Once infected by Meloidogyne spp., tubers become galled and disfigured. Symptoms will vary depending on conditions, nematode species, and yam variety. Tubers normally look â€˜knobbledâ€™ due to the development of galls on the surface, the severity of which depends on the level of infection.
Farmers do not understand how this disfigurement occurs, believing it to be a supernatural occurrence in some cases. Tuber galling damage will not generally develop further once harvested. During storage, galled tubers lose weight and deteriorate much faster than healthy tubers. Galled tubers used for seed will (if they survive) result in the development of more heavily damaged (galled) tubers at harvest.
Dry rot, caused by lesion nematodes, results from their feeding action as they â€˜migrateâ€™ from cell to cell, destroying them as they pass through the yam tissue. This damage occurs first in the subsurface tissue, just below the tuber skin, moving deeper with time. A relatively healthy looking skin can also often mask the underlying damage, which may not be visible until the surface is damaged or cut back (e.g., Fig. 2A), to reveal the brown, discolored, necrotic tissue beneath. Surface cracking is also a typical symptom of lesion nematode infection, but the relation between the nematodes and cracking is less clear-cut, with cracked tubers occurring in the absence of nematodes in tubers.
The species of nematodes responsible for dry rot and cracking on yam is intriguing for several reasons. One is that S. bradys is a key species, which feeds as an endoparasite, while the remaining species in the genus are better known as ectoparasitic feeders, remaining in the soil and feeding on the outside of roots. The other is that at least two species of Pratylenchus (P. coffeae and P. sudanensis) also cause the same symptoms as S. bradys. The species distribution and occurrence is geographically related; the more research we undertake, the more we unravel the story. We have determined the center of origin for S. bradys, for example, as the central Nigeria/Benin area, although the nematode has since been distributed around the globe on infected yam tubers. At harvest, severely affected tubers may be obvious, but lower levels of infection may go unnoticed. However, during storage the nematodes continue to feed, which is a major consequence of lesion nematode infection. Heavy infections result in the complete deterioration of tubers, while less damaged tubers can, to some extent, be consumed after removing the rotted sections. Low levels of infection may not be detected, and can contribute to the disease cycle when tubers are used as seed material.
A particular characteristic of yam is that the nematode problem is perpetuated through the use of infected planting material. Heavily damaged tubers are obvious, and are not generally used as seed, and usually consumed at home. Less visibly damaged tubers may be sold in the market or stored, for sale or consumption later or for use as a planting material in the next season. The use of poor quality planting material thus serves to maintain the disease cycle, by returning inoculum from the store back to the field. This adversely affects crop establishment, yield, and storability of harvested tubers, ensuring a continued and negative impact on quality, especially of highly susceptible cultivars. It is likely even, that nematode damage, in particular, has been a major contributing factor to the loss of some traditional (susceptible) varieties.
To overcome nematode problems on yam, a key area of focus is to target the seed system, and farmer awareness and understanding of the problem. This has become an increasingly important focus for IITA and its partners in recent years. It also forms the central pillar for a large IITA-led yam project. By generating and maintaining sustainable healthy seed systems, farmers will have greater access to seed material that will result in more productivity that will be less likely disfigured at harvest. In-field infection will occur, especially by root-knot nematodes, but damage will be less severe and yields higher.