First report of Rotylenchulus reniformis infecting turmeric in Vietnam and consequent damage

Soil samples were collected following a grid pattern method from a turmeric growing area in the Central Highlands of Vietnam. At 10 sampling sites a visual inspection of plant symptoms was combined with the collection of four samples around a clump of turmeric, after removing the detritus layer to create a bulk sample (collection of 40 soil samples using a core (5 × 25 cm) resulted in 10 bulk samples). Percentage of yellowing leaves, dry rot rhizomes, and rhizome weight for each clump of turmeric were recorded (Coyne et al., 2018). Permanent slides of nematodes were made following Nguyen et al. (2019a). Morphological identification was done based on Robinson et al. (1997) and Dasgupta et al. (2011). For molecular identification, D2-D3 regions of 28 S rDNA and COI mtDNA sequences were amplified and analyzed following Nguyen et al. (2019b). The correlations between the density of R. reniformis and any plant damage (yellowing leaves and dry rot rhizome), as well as rhizome weight over all sampling sites was analyzed using SPSS version 25 after checking data assumptions (IBM Corp, 2017).

The inspection of soil samples from the turmeric growing area revealed a high frequency of R. reniformis (90%) at a relatively high density (up to 480 nematodes/100 ml of soil). Morphological characterizations of R. reniformis in this study are in agreement with the description of R. reniformis by Robinson et al. (1997) and Dasgupta et al. (2011), including body length of immature females (409 ± 21 (380−430) µm), stylet length (16.5 ± 0.8 (15−17) µm), vulva position (71 ± 2 (69−74)%), and presence of males. The D2-D3 of 28 S rDNA (accession number: MT225542) and COI mtDNA (accession number: MT232760, MT232761) sequences of our population were respectively 99.5 and 100% similar to the most closely-related D2-D3 of 28 S rDNA sequences (accession number: KT003743-KT003745, HM131858) and COI mtDNA sequences (accession number: MK908054, MK908055, LC348942−LC348948, KT003730) of R. reniformis from GenBank.

A strong correlation between the density of R. reniformis and rhizome weight was found (R_S = −0.841, Spearman correlation test, significance level = 0.002). The relation of nematode density and damage (yellowing leaves and dry rot rhizome) appeared to be more fluctuating (Figure 1). However, a highly significant correlation between the density of R. reniformis and the amount of plant damage (yellowing leaves and dry rot rhizomes) was found (R_S  = 0.64, Spearman correlation test, significance level = 0.046 and R = 0.781, Pearson correlation test, significance level = 0.008, respectively) (Table 1).

Figure 1:

Nair (2019) reported that plant-parasitic nematodes such as M. incognita, Pratylenchus coffeae, and Radopholus similis can cause stunting, discolouration, and rotting of mature rhizomes in turmeric − symptoms that are indeed similar to those recorded in this study (Figure 2). It is well known that R. reniformis is one of the most devastating plant-parasitic nematodes in the world (Robinson et al., 1997; Jones et al., 2013), even at densities between 0.1 to 5 nematodes/1 cm³ of soil (Gaur and Perry, 1991). Furthermore, R. reniformis is reportedly able to deplete the roots of certain plants and facilitate the infection of different harmful fungi (Lele et al., 1978; Taha and Kassab, 1980; Tchatchoua and Sikora, 1983). Remarkably, for turmeric, Haider et al. (1998) found that R. reniformis caused a significantly higher growth reduction compared to the infamous damaging root-knot nematode M. incognita. In our study, the relatively high density of R. reniformis (up to 480 nematodes/100 ml of soil) and the significant correlations between R. reniformis and the level of plant damage (yellowing leaves and dry rotten rhizomes) indicate the clear involvement of R. reniformis in causing plant damage and yield loss of turmeric in the Central Highlands of Vietnam. Although R. reniformis has already been widely reported in several other crops in Vietnam such as banana and coffee (Nguyen and Nguyen, 2000; Trinh et al., 2009), these reports were unsubstantiated by molecular data or damage assessment (Nguyen and Nguyen, 2000; Trinh et al., 2009). The present study provides the first report of R. reniformis associated with turmeric in Vietnam with the support of molecular data, which is vitally necessary given the presence of cryptic species in Rotylenchulus (Singh et al., 2020, in review). Furthermore, the observed relation of nematode density to rhizome biomass and disease symptoms in this study confirms the importance of this nematode as a pest menace to turmeric. Despite the many experiments that have been done in lab conditions to estimate the impact of R. reniformis on crops over the world (Robinson et al., 1997), damage assessments using different approaches under field conditions are also of crucial importance to provide real-life, up-to-date information and datasets that will contribute in a meaningful way to forming a basis for nematode management strategies both in Vietnam and worldwide.

Figure 2: