First Report of Bacterial Leaf Spot Disease of Broussonetia papyrifera Caused by Pseudomonas syringae pv. actinidiae in China

Abstract

Broussonetia papyrifera (paper mulberry) is a deciduous tree with a number of uses and is native to northeastern Asia. Because of its fast-growing nature and high tolerance to dust, smoke, and high temperatures, paper mulberry is regarded as an important and economically-valuable component of a biologically diverse community and is used extensively in several areas including medicine, animal husbandry, paper making, weaving, afforestation and light industry (Mei et al. 2016). From June to August of 2019, symptoms on paper mulberry trees were observed in Shiniushan village, Sanhua town, Xishui County, Hubei province of China. Typical symptoms on leaves included small, angular, brown spots surrounded by yellow haloes. These spots coalesced into necrotic areas. The incidence was around 30%, which threatened the survival and reduced the yield of paper mulberry. In order to identify the causal pathogenic organism, leaf samples from 10 different infected trees were collected every two weeks and isolations made over three months. Several circular, flat, granulated colonies with entire margins were isolated on King’s B medium (KB). The biochemical and physiological characteristics of thirty typical strains were tested and listed as following: gram negative, aerobic, rod shaped, and non-fluorescent on King’s B medium; positive for carbohydrate utilization (sucrose, glucose, fructose and arabinose), levan production, hypersensitive on tobacco, potato and tomato; and negative for oxidase, arginine dehydrolase, tyrosinase and urease activity, gelatin liquefaction, and reduction of nitrate. Psa pathovar-specific primers PsaF1/PsaR2 (280bp product ) identified two representative strains as Psa (Rees-George et al. 2010). BLAST analysis further confirmed that the 16S rDNA region amplified by primers 27F/1492R (NCBI accession nos. MT472100 and MT472101) shared 99.84% and 99.77% identity with the Psa type strain ICMP 18884 (CP011972) respectively (Weisburg et al. 1991). For ten typical strains, pathogenicity was confirmed by spraying a bacterial suspension (108 cfu/mL) onto fifty one-year seedlings of B. papyrifera, five seedlings repetitions for each strain. Symptoms of infection similar to those observed initially in the field were detected within 7 days after incubation at 25°C with 80–85% humidity. No symptoms were observed on control plants. The pathogen was re-isolated from symptomatic leaves and re-identified as Psa by morphological characteristics and sequencing. To our knowledge, this is the first report of Psa causing bacterial leaf spot disease on B. papyrifera, China. Psa has been reported as a pathogen causing bacterial canker of kiwifruit worldwide, resulting in severe economic losses to kiwifruit growers (McCann & Li, 2017). As a host of Psa, B. papyrifera may be a source of inoculum for nearby kiwifruit orchards, and consequently effective control measures should be taken to manage this disease. Funding: This study was supported by the National Natural Science Foundation of China (31701974; 31901980), Science and technology program funded by Wuhan Science and Technology Bureau (2018020401011307). References: Mei et al. 2016. Eur J Plant Pathol. 145: 203. McCann & Li et al. 2017. Genome Biol Evol. 9: 932. Rees-George et al. 2010. Plant Pathol. 59: 453 Weisburg et al. 1991. J Bacteriol. 173: 697.

Genomic Variation and Host Interaction among Pseudomonas syringae pv. actinidiae Strains in Actinidia chinensis ‘Hongyang’

“…Undoubtedly, genetic transfer among Psa populations would further accelerate its diversity with potential changes in ecological fitness, virulence, and host range. In fact, it has been recently reported that Psa can cause leaf spot disease on Broussonetia papyrifera (an economic tree species) grown nearby kiwifruit orchards [ 29 ], suggesting a possible expansion of the Psa host range.…”

Mitigation of Emergent Bacterial Pathogens Using Pseudomonas syringae pv. actinidiae as a Case Study—From Orchard to Gene and Everything in Between

“…Aside from the accurate identification of the bacterial pathogen, sufficient knowledge of its life cycle is of utmost importance to anticipate potential disease outbreaks and plan the most efficient management strategies to contain its local and global dissemination. In the case of Psa, its main hosts are the species from the genus Actinidia, with very few reports of its presence in non-kiwifruit plants [12,37,38]. As with other pathogenic pseudomonads, Psa infection begins with bacteria entering the host plant through natural openings (e.g., stomata, lenticels, broken trichomes and hydathodes), natural wounds (e.g., wind damage) and artificially made wounds (e.g., pruning, and girdling activities) [12,39,40].…”

“…For example, Psa was found living epiphytically on Cryptomeria japonica, often used as shelterbelts in kiwifruit orchards in New Zealand, surviving up to 14 days when artificially inoculated [123]. Psa was also isolated from Setaria viridis, Alternanthera philoxeroides, Paulownia tomentosa, and Broussonetia papyrifera, which inclusively presented the same characteristic disease symptoms [37,38]. Several nonhost plants collected in proximity to infected kiwifruit plants, including Calystegia sylvatica and Capsella bursa-pastoris, have also shown potential for harboring Psa [12].…”

Seeing the forest for the trees: Use of phages to treat bacterial tree diseases

“…mori (bacterial blight of white mulberry), and pv. persicae (bacterial decline of stone fruit) (Alippi et al, 2013; Cunty et al, 2015; Green et al, 2009; Krawczyk & Łochyńska, 2020; Li et al, 2020a; Rodrigues et al, 2017; Scortichini et al, 2012; Vanneste et al, 2013; Young, 2014). The pv.…”