Relationship Between the Density of Ditylenchus destructor in Soil at Harvest Evaluated Using Real-Time PCR and Sweetpotato Disease Incidence

Sweetpotato stem rot nematode (Ditylenchus destructor) causes the most devastating disease affecting sweetpotato production in China. The objectives of this study were to (i) establish a quantification method using real-time PCR for D. destructor of sweetpotato; (ii) analyze the effect of D. destructor density at harvest on the percentage of disease incidence in sweetpotatoes; and (iii) evaluate the effect of soil physical properties on disease incidence. Populations of D. destructor isolated from 28 different production areas in Henan Province exhibited identical sequences, and then real-time PCR-specific primers (PRNf and PRNr) were designed. The primer set demonstrated a 100% match with the sequences of D. destructor from three regions in China but did not align perfectly with different species within the genus Ditylenchus isolated from nine other countries. There were two and thirteen base mismatches with the closely related species D. africanus and D. gallaeformans, respectively. The log-transformed number of inoculated nematodes (x) to soil was highly correlated with the corresponding Ct value (y), resulting in the quantification curve (y = −1.0859x + 32.025, R 2 = 0.9866, P < 0.01). This quantification curve was utilized to evaluate the densities of D. destructor in wheat fields that were not cultivated with sweetpotato and in fields under continuous sweetpotato cultivation. The results showed no detection in the wheat fields, and the densities ranged from 0 to 13,300 ± 1,020 individuals per 20 g of dry soil. At harvest, the D. destructor density established a negative correlation with soil pH (y = −1.7431x + 12.568, R 2 = 0.6028, P < 0.05). When soil pH values were between 5.6 and 6.8, and the density of D. destructor at harvest was less than 160 ± 30 per 20 g of soil, the percentage of disease incidence at harvest was 0. A significant correlation was established between D. destructor density and the severity of sweetpotato disease. The results indicate that the specific primer set developed in this study allows for the accurate and rapid quantification of D. destructor density in the soil, as well as the prediction of disease incidence in sweetpotato.