Control efficacy of different fungicides against kiwifruit soft rot caused by Botryosphaeria dothidea

Abstract: 【Objective】Kiwifruits (Actinidia chinensis Planch.) are becoming increasingly popular because of their nutritional and delicious properties. However, kiwifruit is highly perishable after harvest and is susceptible to infection by various pathogenic microorganisms, especially Botryosphaeria dothidea. Kiwifruit soft rot, caused by B. dothidea, spreads rapidly in confined spaces. A single diseased fruit easily contaminates others, causing a whole box of fruits to soften or rot quickly. In addition, fruits that appear healthy may also turn out to be decayed after their skins are peeled. Therefore, the significance of kiwifruit soft rot in the value chain is often underestimated. Currently, chemical control is the most effective approach for kiwifruit soft rot prevention and healing. Nevertheless, there is limited knowledge on fungicides that can be applied in the growth and storage periods to prevent kiwifruit soft rot. The lack of scientific evidence for the preventive use of fungicides has led to insufficient recommendations, widespread misuse and abuse of these chemicals. Therefore, we aimed at identifying high-efficiency, low-toxicity and low-residue fungicides for kiwifruit soft rot prevention and control during the growth and storage periods. 【Method】Indoor toxicity tests. Indoor toxicity effects of tebuconazole, prochloraz, fluazinam, propiconazole, imazalil sulfate, pyraclostrobin, flusilazole and azoxystrobin on B. dothidea were assessed via the mycelial growth inhibition method. For evaluation of in vitro preventive and curative activities of prochloraz, azoxystrobin, pyraclostrobin·fluxapyroxad, trifloxystrobin·tebuconazole, imazalil sulfate and fludioxonil against kiwifruit soft rot, the fruits were treatment in preventive (applied 24 and 72 h pre-inoculation) and curative (applied 24 and 72 h post-inoculation), preventive as well as curative effects evaluated after kiwifruits had softened. In 2020, to the screening filed trials of flusilazole, pyraclostrobin·fluxapyroxad, azoxystrobin, trifloxystrobin·tebuconazole was conducted in the modern agricultural research and development base of Sichuan Agricultural University. The fungicides were applied using a spray on 20^th April 2020 (after shedding), 20^th May 2020 (young fruit stage), 24^th June 2020 (fruit expansion stage) and 24^th July 2020 (30 days before harvest). Collected fruits from each treatment and stored at 25°C, the disease index incidence and control efficiencies were calculated after 15 days of storage. For storage trials, the fruits were soaked in trifloxystrobin·tebuconazole, prochloraz, fludioxonil, imazalil sulfate and myclobutanil for one min, air dried and stored. The disease index incidence and control efficiencies were calculated after 90 days of storage. After 90 days of storage, the kiwifruit samples treated with the most effective fungicide and water were taken for quantitative analysis and detection of fungicide residues. The national food safety standard GB 2763-2019 guidelines on maximum residue limits of fungicides in food were used to assess the residual safety of fungicides. 【Results】In this study, based on the results of indoor toxicity and in vitro control efficacy assessments, we selected different fungicides for subsequent field and storage efficacy tests and identified three fungicides with great potential for control of kiwifruit soft rot. Prochloraz (a.i. 97%) had the strongest inhibitory activities against B. dothidea mycelial growth whereas pyraclostrobin (a.i. 97.5%) had the weakest inhibitory activities. The EC₅₀ values of prochloraz (a.i. 97%) and pyraclostrobin (a.i. 97.5%) were 0.1349 μg·ml^-1 and 9.9987 μg·ml^-1, respectively. In vitro, trifloxystrobin·tebuconazole had preventive effects of 78.15% whereas prochloraz had curative effects of 89.38%. During growth in field trials, four fungicide applications were effective against kiwifruit soft rot. Among them, three fungicides had better control efficacies on the disease after applying them four times at different growth stages. Trifloxystrobin·tebuconazole successfully controlled kiwifruit soft rot with control efficacies of 92.30%, compared with other fungicides (p＜0.05). It was followed by pyraclostrobin·fluxapyroxad and azoxystrobin with control efficacies of 80.90% and 76.46%, respectively, whereas flusilazole had the least control efficacy of 32.84%. After fruit immersion in all five fungicides before storage, prochloraz exhibited the highest control efficacy of 95.98%, followed by myclobutanil and trifloxystrobin·tebuconazole with control efficacies of 91.14% and 86.30%, respectively. None of the fungicides were associated with phytotoxic effects on the kiwifruits after treatment, suggesting that they were safe. When kiwifruits were stored for 90 days, residual levels of tebuconazole, trifloxystrobin and prochlorazin water control were 0.015 mg/kg, 0.011 mg/kg and ＜0.01 mg/kg, respectively, whereas tebuconazole and trifloxystrobin residues in triclostrobin·tebuconazole treatment were 0.45 mg/kg and 0.22 mg/kg, respectively. Fungicide residues in the three treatments were significantly low in kiwifruits than the MRL for grapes and no pesticide residues exceeded the standard. 【Conclusion】Taken together, this is the first study to comprehensively assess the control effects of different fungicides on kiwifruit soft rot in the field and during storage. We found that alternate applications of trifloxystrobin·tebuconazole and pyraclostrobin·fluxapyroxad in the field during the growth period and soaking postharvest fruits in prochloraz before storage are potential useful and effective measures for kiwifruit soft rot management. Moreover, fruit damage during harvesting and transportation should be avoided as much as possible to effectively prevent and control postharvest kiwifruit soft rot. 

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