Plant nematodes (PPNs) have been documented as economically important pests of yam in different parts of the world with Pratylenchus spp. and Meloidogyne spp. being the most widespread and destructive pests in Asia, causing significant yield losses.
1. Introduction
Yam belongs to the genus
Dioscorea (family; Dioscoreaceae) consisting of about 644 species with underground tubers
[1]. The three most common cultivated species include
D.
rotundata,
D.
alata, and
D.
cayenensis [2], while
D.
esculenta,
D.
nummularia,
D.
bulbifera,
D.
pentaphylla,
D.
transversa,
D.
japonica,
D.
trifida,
D.
dumetorum, and
D.
opposita are also of economic importance.
Dioscorea opposita is the only species cultivated in temperate areas and is commonly grown in China and Japan
[3]. Yam is an important staple food crop in tropical and subtropical areas of Africa, Asia, South America, the Pacific, and the Caribbean region
[4,5][4][5]. It is the fourth most important tuber crop after potato, cassava, and sweet potato globally, with West Africa accounting for about 92% (67 MT) of the total output annually
[6]. Though China’s yam output was not included in the FAO statistics, it is currently a significant and isolated center of yam domestication
[7]. In China, approximately 5 to 6 MT of yam are produced every year through the planting of diverse species in most yam-producing areas such as Jiangsu and Shandong provinces
[7,8][7][8].
In China, consumption of yam is on the increase due to the recent belief that, nutritionally, its reserves of micronutrients, minerals, and proteins can be used to address potential deficiencies
[7,9][7][9]. Additionally, its production brings in income to the country
[5,7][5][7]. However, yam is susceptible to numerous diseases, particularly bacterial dry rot (
Corynebacterium spp.), anthracnose (
Colletotrichum spp.), viruses, and nematodes
[4,10][4][10]. It is highly susceptible to plant-parasitic nematodes (PPN) especially the yam nematode (
Scutellonema bradys), root-lesion nematodes (RLN;
Pratylenchus spp.), and root-knot nematodes (RKN;
Meloidogyne spp.). Plant-parasitic nematodes interacts with other plant pathogens, resulting in increased damage caused by other diseases and affecting global food supplies
[4,5][4][5]. Yam infected with RKN has been reported to show galls and “crazy root” syndrome on tubers, altering tuber appearance and reducing tuber quality.
Pratylenchus spp. and
S.
bradys also contribute to dry rot disease and tuber surface cracking, which in turn influence the production and quality of tuber
[4,11,12][4][11][12]. Additionally, tubers with nematode symptoms are less appealing and so have poor market value
[4,11,12][4][11][12]. The association of yam to PPN has been reported in China with RKN responsible for the loss of several cultivars
[4,5][4][5]. About 24–80% yield reduction in yam in China is associated with RKN (
M.
arenaria), while about 30–100% is attributed to RLN (
P.
coffeae)
[4].
Stunt nematodes of the genus
Merlinius Siddiqi, 1970 include 34 nominal species
[13]. These nematodes are migratory ectoparasites of a wide range of plants globally and are ubiquitous in agricultural settings. The accurate identification of these stunt nematodes from both rhizosphere soil and root samples is critical for successful disease management
[13,14,15][13][14][15]. The majority of species attack and feed on plant roots and underground plant parts, resulting in low to moderate yield losses in several crops, including rice (
Oryza sativa L.), wheat (
Triticum spp.), maize (
Zea mays), and potato (
Solanum tuberosum L.)
[13,15][13][15]. Allen
[14] first described a species of this genus,
Merlinius brevidens, as a major pathogen of grasses in the USA and several other countries
[13,15][13][15]. Although this species is not among the species considered most important to yam (
Dioscorea spp.), some of them can become destruction agents when populations exceed economic threshold levels.
[13,16][13][16]. They are mostly recognized for the damage they caused to grasses and cereals through feeding on epidermal cells, resulting in root and foliage stunting, leaf yellowing, defoliation, and wilting
[16]. Morphological identification of
Merlinius brevidens is characterized by the absence of areolated lateral fields; six longitudinal incisures; female tail sub-cylindrical (c’ = 2.4–4.6) with rounded terminus; and robust stylet
[17,18][17][18]. Males are rarely seen and unlikely to be required for reproduction. However, the substantial intraspecific diversity of key diagnostic features makes morphological identification of this genus challenging, and its evolutionary relationships with other genera in the same family are complex, making it problematic and contentious
[16,17,18][16][17][18]. To elucidate the identity and evolutionary relationships of the species in this genus and related genera, morphological and molecular analyses are required
[13,17,18][13][17][18].
Plant-parasitic nematodes infecting yam must be identified accurately and promptly to develop appropriate management methods that will reduce losses. This is particularly important for yam due to the diversity of nematode species found in different yam-growing regions across the world
[4,11,19][4][11][19]. Accurate nematode identification enables the differentiation of regulated and non-controlled nematode pests, as well as the exclusion of species under quarantine or regulatory measures. The prevalence and geographical distribution of PPNs in Asia were reviewed by Bridge et al.
[11] who reported that eight species (
P.
coffeae,
M.
incognita,
M.
arenaria,
M.
hapla,
Radopholus similis,
Paratrichodorus porosus,
Rotylenchulus reniformis, and
Helicotylenchus dihystera) were found damaging yam. In China, only three species of PPNs (
P.
coffeae,
M.
incognita, and
M.
arenaria) have been reported in association with yam
[4]. However, many other species were not reported and verified with appropriate molecular analysis. The current availability of molecular methods may aid in the identification of nematodes by providing tools for species differentiation. Therefore, ribosomal DNA sequences from partial 18S, ITS regions, and the D2-D3 expansion segments of the 28S, as well as mitochondrial DNA (mtDNA) sequences, have proven to be useful diagnostic tools for the characterization and establishment of phylogenetic relationships within PPNs, particularly when morphological features may cause doubt in interpretation
[14,15][14][15]. Integrative taxonomy, which combines molecular methods with morphology and morphometry methods to diagnose the identified species, is critical for accurate nematode identification.
2. Survey
The 110 bulk soil samples and 48 tubers obtained from nine locations in Jiangxi and Shandong provinces of China yielded 16 species (four species were found in tubers) of PPNs (
Table 21 and
Table 32). Data pooled for all farming communities showed that
Pratylenchus (depicted by the species
P.
coffeae) had the highest MPD, in both tuber (MPDs of 415 individuals/5 g) and soil (MPD = 291 indiv./100 mL) samples, followed by
Meloidogyne (
M. incognita,
M. hispanica, and
M. ethiopica), and
Rotylenchulus (depicted by the species
R.
reniformis) (
Figure 1,
Figure 2 and
Figure 3).
Merlinius was the fourth in soil samples, according to MPDs (
Figure 1), and the most commonly recorded genus with ectoparasitic feeding behavior. The least prominent genus in terms of MPDs in tubers was
Ditylenchus and in soil
Hirschmanniella mucronata (
Figure 1 and
Figure 2).
Figure 1. Mean population densities (MPD) and frequencies of occurrence (FO) of plant-parasitic nematodes species recovered from yam rhizosphere in Jiangxi and Shandong provinces of south-east China.
Figure 2. Mean population densities (MPD), and frequencies of occurrence (FO) of plant-parasitic nematodes species recovered from roots/tubers of yam in nine localities of Jiangxi and Shandong provinces across south-east China during 2020/2021.
Figure 3. Dry rot (A) and Galls (B) on yam tubers caused by Pratylenchus coffeae and Meloidogyne spp.
Table 21. Mean population densities (MPD), and frequencies of occurrence (FO) of plant-parasitic nematodes species isolated from 100 mL of rhizosphere soil sampled from yam plants in nine locations of Jiangxi and Shandong provinces across south-east China during 2020/2021.
|
|
|
Province |
|
|
|
Jiangxi (n = 65) |
|
Shandong (n = 45) |
|
MPD |
FO (%) |
|
MPD |
FO (%) |
- |
- |