Objective To investigate the biological characteristics of Colletotrichum fioriniae on Rhododendron delavayi in Baili Azalea Nature Reserve of Guizhou Province, and to screen effective fungicides for disease control.

Method The mycelial growth rate method was applied to explore the biological characteristics of C. fioriniae on R. delavayi. The indoor toxicity of pathogen was determined using 10 fungicides, including trifloxystrobin·tebuconazole, pyraclostrobin, difenoconazole, propiconazole, bromothalonil, dithianon, tetramycin, ethylicin, cnidiadin and ningnanmycin. Combined toxicity of two fungicides with high antifungal activities and different toxicological mechanisms to C. fioriniae was assessed in different mixture ratio.

Result The pathogenic fungi could grow under 5−35 ℃ and pH 5−11. The optimum temperature was 25 ℃ and the optimum pH was 8. The best carbon sources were glucose and soluble starch. Peptone was the best nitrogen source for the pathogen growth and the growth rate reached the maximum on PSA medium. The results of the indoor toxicity test indicated that all of the 10 fungicides inhibited the mycelial growth of the pathogen to some extent. Among 10 fungicides, trifloxystrobin·tebuconazole, pyraclostrobin, tetramycin, difenoconazole, and propiconazole had better inhibitory effect with EC₅₀ of 0.102, 0.118, 1.107, 1.202 and 2.101 mg/L, respectively, followed by cnidiadin with EC₅₀ of 6.803 mg/L. The combination of tetramycin and difenoconazole with different mixture ratio showed synergistic inhibiting effect on the pathogen compared with single fungicide. The optimal mixture ratio was 7∶3 with co-toxicity coefficient (CTC) of 584.56, which was obviously higher than those of other mixture ratios. Both the mixture ratio of 6∶4 and 8∶2 had CTC above 500, being next to the best ratio.

Conclusion The growth of C. fioriniae is significantly affected by temperature, pH, culture medium, carbon and nitrogen sources. The combination of tetramycin and difenoconazole in different mixture ratios has obvious synergistic toxicity, and the 7∶3 mixture ratio can be selected for field control experiments.