Powdery mildew caused by Sphaerotheca sp. is the most serious disease of Rosa roxburghii cultivation. In this study, the foliar application of chitosan induced Rosa roxburghii Tratt. against Sphaerotheca sp. and its effects on the disease resistance, growth, yield, and quality of R. roxburghii were investigated. The results show that the foliar application of 1.0%~1.5% chitosan could effectively control Sphaerotheca sp. of R. roxburghii with the inducing control efficacy of 69.30%~72.87%. The foliar application of 1.0%~1.5% chitosan significantly (p < 0.01) increased proline, soluble sugar, flavonoids, superoxide dismutase (SOD), and polyphenoloxidase (POD) activities of the R. roxburghii leaf and decreased its malonaldehyde (MDA), as well as reliably enhanced its photosynthetic rate and chlorophyll. Moreover, the foliar application of 1.0%~1.5% chitosan notably improved single fruit weight, yield, vitamin C, soluble solid, soluble sugar, total acidity, soluble protein, flavonoids, and SOD activity of R. roxburghii fruits. This study highlights that chitosan can be used as an ideal, efficient, safe, and economical inductor for controlling powdery mildew of R. Roxburgh and enhancing its resistance, growth, yield, and quality.

Powdery mildew, caused by Sphaerotheca sp., annually causes severe losses in yield and quality in Rosa roxburghii production areas of southwest China. In this study, the role of the co-application of allicin and chitosan in the resistance of R. roxburghii against powdery mildew and its effects on growth, yield and quality of R. roxburghii were investigated. The laboratory toxicity test results show that allicin exhibited a superior antifungal activity against Sphaerotheca sp. with EC50 value of 148.65 mg kg−1. In the field, the foliar application of allicin could effectively enhance chitosan against powdery mildew with control efficacy of 85.97% by spraying 5% allicin microemulsion (ME) 100–time liquid + chitosan 100–time liquid, which was significantly (p < 0.01) higher than 76.70% of allicin, 70.93% of chitosan and 60.23% of polyoxin. The co-application of allicin and chitosan effectively enhanced the photosynthetic rate and chlorophyll of R. roxburghii compared with allicin, chitosan or polyoxin alone. Moreover, allicin used together with chitosan was more effective than allicin or chitosan alone in enhancing R. roxburghii plant growth and fruit yield as well as improving R. roxburghii fruit quality. This work highlights that the co-application of allicin and chitosan can be used as a green, cost-effective and environmentally friendly alternative strategy to conventional antibiotics for controlling powdery mildew of R. roxburghii.