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

Abstract: 【Objective】Kiwifruit (Actinidia chinensis Planch.) is becoming increasingly popular because of its nutritional and delicious properties. However, kiwifruit is highly perishable after harvest and is susceptible to infection by various pathogenic microorganisms, especially Botryosphaeria dothidea. The 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 decay after their skins are peeled. Therefore, the loses caused by the kiwifruit soft rot is often underestimated. Currently, chemical control is the most effective approach for kiwifruit soft rot prevention and control. Nevertheless, there is limited knowledge on fungicides that can be applied during 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.【Methods】Indoor toxicity tests. Indoor toxicity effects of the 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 the prochloraz, azoxystrobin, pyraclostrobin ·fluxapyroxad, trifloxystrobin ·tebuconazole, imazalil sulfate and fludioxonil against kiwifruit soft rot, the fruits were given preventive (applied 24 and 72 h pre-inoculation) and curative (applied 24 and 72 h post-inoculation) treatments, the effects were evaluated after kiwifruits had softened. In 2020, the filed trials of the flusilazole, pyraclostrobin ·fluxapyroxad, azoxystrobin, trifloxystrobin ·tebuconazole were conducted at the modern agricultural research and development base of Sichuan Agricultural University. The fungicides were applied using a spray on 20th April 2020 (after shedding), 20th May 2020 (young fruit stage), 24th June 2020 (fruit expansion stage) and 24th July 2020 (30 days before harvest). The fruits were collected from each treatment and stored at 25 ℃, the disease index incidence and control efficiencies were calculated after 15 days of storage. For the storage trials, the fruits were soaked in the solutions of 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 controlling kiwifruit soft rot. The prochloraz (a.i. 97%) had the strongest inhibitory activities against B. dothidea mycelial growth whereas the pyraclostrobin (a.i. 97.5%) had the weakest inhibitory activities. The EC50 values of the prochloraz (a.i. 97%) and pyraclostrobin (a.i. 97.5%) were 0.134 9 μg ·mL- 1 and 9.998 7 μg ·ml- 1 , respectively. In vitro, the trifloxystrobin · tebuconazole had preventive effects of 78.15% whereas the prochloraz had curative effects of 89.38%. During growth period in field trials, four fungicide applications were effective against the kiwifruit soft rot. Among them, three fungicides had better control efficacies on the disease after applying them four times at different growth stages. The trifloxystrobin·tebuconazole successfully controlled the kiwifruit soft rot with control efficacies of 92.30%, compared with other fungicides (p<0.05). It was followed by the pyraclostrobin · fluxapyroxad and azoxystrobin with control efficacies of 80.90% and 76.46%, respectively, whereas the flusilazole had the least control efficacy of 32.84%. After fruit immersion in the five fungicides before storage, the prochloraz exhibited the highest control efficacy (95.98%), followed by myclobutanil and trifloxystrobin ·tebuconazole (91.14% and 86.30%). None of the fungicides were associated with the phytotoxic effects on the kiwifruits after treatments, suggesting that they were safe. When kiwifruits were stored for 90 days, the residual levels of tebuconazole, trifloxystrobin and prochlorazin water control were 0.015 mg · kg-1 , 0.011 mg · kg-1 and<0.01 mg · kg-1 , respectively, whereas tebuconazole and trifloxystrobin residues in triclostrobin · tebuconazole treatment were 0.45 mg · kg-1 and 0.22 mg · kg-1 , respectively. The fungicide residues in the three treatments were significantly lower in kiwifruits than the MRL for grapes and no pesticide residues exceeded the standard.【Conclusion】The alternate applications of the trifloxystrobin · tebuconazole and pyraclostrobin · fluxapyroxad in the field during the growth period and soaking postharvest fruits in the prochloraz solution before storage should be potentially useful for effectively controlling the kiwifruit soft rot. More-over, fruit damage during harvesting and transportation should be avoided as much as possible to effectively prevent and control postharvest kiwifruit soft rot.