Tree peony black spot (TPBS), mainly caused by Alternaria suffruticosae, is a common leaf disease on the ornamental peony, which poses a great threat to the flower buds in the current year and the flowering quality in the next year. However, there is only one fungicide registered for the control of this disease, difenoconazole. In order to avoid the severe problem of pathogen resistance caused by long-term use of difenoconazole, it is necessary to screen more chemical fungicides for the prevention and control of TPBS. In this study, the biological activities of flutolanil, phenamacril, pyraclostrobin, and boscalid on mycelial growth, conidial germination, germ tube elongation, and sporulation quantity of A. suffruticosae were determined, and the field control efficacy was tested to evaluate the preventive and therapeutic activities. Difenoconazole was used as a control simultaneously. The results showed that pyraclostrobin had the strongest inhibitory effects on the conidial germination, mycelium growth, germ tube elongation, and sporulation quantity, with the average EC₅₀ values of 0.0517, 0.5343, 0.0008, and 0.8068 μg/ml, respectively. The inhibitory activity of flutolanil on the four developmental stages of A. suffruticosae was weaker than that of the other three fungicides. Compared with flutolanil, boscalid, the other succinate dehydrogenase inhibitor, had more strong inhibitory effects on the mycelial growth and sporulation quantity, with the average EC₅₀ values of 3.8603 and 1.4760 μg/ml, respectively. Phenamacril had a moderate inhibitory level and had more inhibitory activity on conidial germination and germ tube elongation, with the average EC₅₀ values of 31.5349 and 5.2597 μg/ml, respectively. All of the four fungicides had no significant effects on the shape of spores and germ tubes. The control fungicide difenoconazole had the strongest inhibitory activity on mycelial growth, and the average EC₅₀ value was only 0.3297 μg/ml. However, its inhibitory activity on the other three growth stages was not high. In the field trials, pyraclostrobin had high control efficacy on TPBS even at low concentrations, reaching a minimum of 62.6293%, which was higher than that of difenoconazole. The other three fungicides had higher control efficacy at high concentrations but decreased significantly at low concentrations. Considering the dosage and control efficacy, pyraclostrobin was the first choice for the control of TPBS. Pyraclostrobin is the preferred alternative fungicide to difenoconazole for the prevention and control of TPBS in production.

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, R² = 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, R² = 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.