Bacterial antagonists are effective as an alternative to synthetic bactericides in the control of potato soft rot. The use of bioagents reduces the application of synthetic bactericides, which are harmful to humans and the environment. However, the mechanisms of some bioagents, such as some fungi and bacteria, are not yet understood. This paper reviews the current situation of potato soft rot, biological controls, antagonistic bioagents and their mechanisms, application strategies and future directions in today’s agriculture. These mechanisms include mycoparasitism, competition, rhizosphere colonisation, synthesis and release of metabolites. Bioagents increased the defensive system of plants by increasing the antioxidants genes, such as superoxide dismutase, peroxidase (POD) and catalase (CAT), and eventually increased the plant growth and yield production.
Potato soft rot, caused by the pathogenic bacterium Lelliottia amnigena (Enterobacter amnigenus), is a serious and widespread disease affecting global potato production. Both salicylic acid (SA) and proline (Pro) play important roles in enhancing potato tuber resistance to soft rot. However, the combined effects of SA and Pro on defense responses of potato tubers to L. amnigena infection remain unknown. Hence, the combined effects of SA and Pro in controlling newly emerging potato soft rot bacteria were investigated. Sterilized healthy potato tubers were pretreated with 1.5 mM SA and 2.0 mM Pro 24 h before an inoculation of 0.3 mL of L. amnigena suspension (3.69 × 107 CFU mL−1). Rotting was noticed on the surfaces of the hole where the L. amnigena suspension was inoculated. Application of SA and Pro with L. amnigena lowered the activity of pectinase, protease, pectin lyase, and cellulase by 64.3, 77.8, 66.4 and 84.1%, and decreased malondialdehyde and hydrogen peroxide contents by 77.2% and 83.8%, respectively, compared to the control. The activities of NADPH oxidase, superoxide dismutase, peroxide, catalase, polyphenol oxidase, phenylalanine ammonia-lyase, cinnamyl alcohol dehydrogenase, 4-coumaryl-CoA ligase and cinnamate-4-hydroxylase were increased in the potato tubers with combined treatments by 91.4, 92.4, 91.8, 93.5, 94.9, 91.3, 96.2, 94.7 and 97.7%, respectively, compared to untreated stressed tubers. Six defense-related genes, pathogenesis-related protein, tyrosine-protein kinase, Chitinase-like protein, phenylalanine ammonia-lyase, pathogenesis-related homeodomain protein, and serine protease inhibitor, were induced in SA + Pro treatment when compared with individual application of SA or Pro. This study indicates that the combined treatment of 1.5 mM SA and 2.0 mM Pro had a synergistic effect in controlling potato soft rot caused by a newly emerging bacterium.
Walnut (Juglans sinensis L.) is an important economic tree. Its fruit are rich in omega-3 essential fatty acids, which are valuable nutritionally (Cheon et al, 2013). In March 2019, severe branch blight of walnuts (cv. Qingxiang) were observed in two fields in Ganquan Town, Gansu Province, China (N 33°56'/E105°44'). The incidence was about 3% among 10,000 walnuts. Disease symptoms included fusiform or oval black lesions gradually expanded on the branches, blight and dieback of branches, reddish brown dead branch bark with lots of black small spots (pycnidia), and defoliation. To isolate pathogen, 30 diseased tissues collected from different trees were disinfected with 0.1% HgCl solution for 1 min, rinsed in sterilized water 3 times, and placed on potato dextrose agar (PDA) at 25℃ in the dark. About three days later, 4 fungal isolates (3-3, 3-6, 3-9 and H3) with similar morphological characteristics were obtained. Their colonies, with regular margin on OA, 6.1~6.8 cm diam. after 7 days, were loose, greenish olivaceous to olivaceous grey, velvety, floccose to woolly. The conidia (n=60) were 4.77 to 8.84 μm long (mean = 6.88 μm; SD ± 0.91 μm) × 1.71 to 3.89 μm wide (mean = 2.81 μm; SD ± 0.53 μm), cylindrical, ellipsoidal to oblong, hyaline and aseptate. Pycnidia (n=25) were 76.66 ~ 132.86μm diam. (mean = 102.93 μm; SD ± 12.15 μm), variable in shape and size, mostly globose to subglobose. These characteristics were similar to B. exigua var. exigua (Boeremia et al, 2004). Pathogenicity tests of four isolates were performed 3 times on 5 healthy 2-3 years old walnuts (cv. Qingxiang). Plants were wounded by insect needle No.6 and inoculated with 5 mm mycelium block grew on PDA medium or PDA medium as control and kept moist in climatic cabinet (> 85% RH, 25°C). After 3 days, oval black lesions were occurred on branches and gradually expanded, but control was asymptomatic. And original isolates were re-isolated from these diseased shoots. Genomic DNA of four isolates were extracted, and the internal transcribed spacer (ITS), β-tubulin (tub2) and RNA polymerase II second largest subunit (rpb2) gene were amplified and sequenced using the primers ITS1/ITS4, Btub2Fd/Btub4Rd and RPB2-5F2/fRPB2-7CR (White et al, 1990; Woudenberg et al, 2009; Chen et al, 2015), respectively. Sequences were deposited in GenBank (accession no. ITS: MT154621, MT154622, MT154623, MT154624; tub2: MT223481, MT223482, MT223483, MT223484; rpb2: MW448152, MW448153, MW459982, MW459983), and compared with available sequences in NCBI. Results showed that ITS of four isolates have 100% sequence identity to Boeremia spp., tub2 and rpb2 have 100% sequence identity to B. exigua var. exigua (GenBank accession no. MN983734, MN983315) and B. exigua var. linicola (GenBank accession no. MN983785, MT920619). According to host specificity (Boeremia et al, 1976). A 106 conidium/mL spore suspension of four isolates or sterile water were inoculated on stem base of two-month old flax seedlings, placed in climatic cabinet (> 85% RH, 25°C) for moisturing and repeated three times. After two weeks, all inoculated plants still were asymptomatic, indicated that four isolates aren’t B. exigua var. linicola. Therefore, they were identified as B. exigua var. exigua based on morphological characteristics, molecular analysis and pathogenicity tests. To our knowledge, this is the first report of B. exigua var. exigua causing walnut branch blight worldwide, which will provide further guidance for prevention and control of walnut branch blight.
Potato (Solanum tuberosum) is one of the most economically important crops in China, containing carbohydrates, protein, fiber, numerous vitamins and minerals, and is a heart healthy food (Raidl, 2020). Potato infected by Fusarium spp. exhibits quality and yield decline, and even death. In infected plants, the upper leaves exhibit chlorosis, the lower leaves wither and the vascular bundles of stems and tubers turn yellow, and then tan to brown. In August 2018, symptomatic potato stems and roots were collected from Zhangye city, Gansu province, China. Diseased stem tissues were surface sterilized with 75% alcohol for 30 s, and then rinsed in sterile water. The tissue pieces were placed on potato dextrose agar (PDA) and incubated at 25°C in darkness. Fusarium-like colonies were consistently isolated and three monoconidial isolates were obtained. Isolate 3SMJ-2 was selected as a representative for morphological characterization, molecular analysis, and pathogenicity tests. 3SMJ-2 was inoculated in PDA liquid medium, grown on a shaker for 7 days at 25℃ to obtain a mix suspension of hypha fragments and spores (107 spores/mL). Healthy potato plants, named “Xin Daping” and were planted in pots (17 cm diameter by 12 cm) filled with 2L of sterile soil per pot. After 8 weeks, the plants were inoculated with the inoculum or distilled water. Then they were incubated in growth chambers at 25°C under a 12-h/12-h day/night potato period with 90% relative humidity for 24 h. For each treatment, 3 pots were inoculated. After 50 days, 100% of the inoculated potato plants exhibited wilt symptoms similar to those in the field but the control plants were symptomless. A Fusarium identical to strain 3SMJ-2 was re-isolated from symptomatic potato plants to fulfilling Koch’s postulates. Morphological characteristics of the re-isolated strain were identical to the original isolate, which confirmed pathogenicity of strain 3SMJ-2 originally isolated from the potatoes. Colonies of 3SMJ-2 were white with short conidiophores, a few microconidia and sickle-shaped macroconidia (25.2 to 42.9× 3.1 to 4.6 µm) (n = 60) with 4~7 septa, and mostly 5 septa, after cultivated on PDA in an incubator at 25℃ for 14 days. Spherical terminal or intercalary chlamydospores were observed on the mycelium. Strain 3SMJ-2 was identified preliminarily as Fusarium sp. based on morphological characteristics (Leslie et al., 2006). Genomic DNA was extracted from 3SMJ-2 using the OMEGA Fungal DNA kit according to the manufacturer’s protocol. The internal transcribed spacer (ITS), translation elongation factor 1-alpha (TEF) and RNA polymerase II second largest subunit (RPB2) were amplified using ITS1/ITS4 (White et al., 1990), Ef728M/Tef1R (Stępień et al., 2012) and 5F2 /7cR (O'Donnell et al., 2007), respectively. After sequencing by Beijing TSINGKE Biological Technology Co., Ltd., 3 fragments of approximately 519 bp, 587 bp and 1059 bp from the strain 3SMJ-2 were deposited in GenBank as MN420681, MW561963 and MW561964. The ITS, TEF and RPB2 sequences were 100%, 100% and 99.8% identical to those of F.equiseti (KY365589, KF499577, and MH582110). Based on the pathogenicity tests, morphological characteristics and molecular analyses, we identified the strain 3SMJ-2 as F. equiseti, the pathogen causing Fusarium wilt on potato in Zhangye City. Although, F. equiseti has been reported to cause root rot of cowpea (Li et al., 2017) and sugar beet (Cao et al., 2018) in China. To our knowledge, this is the first report confirming F. equiseti causing potato wilt in China. Potato is an economically important crop in Gansu and the occurrence of the new disease caused by F. equiseti on potato needs to be properly managed to reduce yield loss.
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