Importance of Molecular Data to Identify Fungal Plant Pathogens and Guidelines for Pathogenicity Testing Based on Koch’s Postulates

Fungi are an essential component of any ecosystem, but they can also cause mild and severe plant diseases. Plant diseases are caused by a wide array of fungal groups that affect a diverse range of hosts with different tissue specificities. Fungi were previously named based only on morphology and, in many cases, host association, which has led to superfluous species names and synonyms. Morphology-based identification represents an important method for genus level identification and molecular data are important to accurately identify species. Accurate identification of fungal pathogens is vital as the scientific name links the knowledge concerning a species including the biology, host range, distribution, and potential risk of the pathogen, which are vital for effective control measures. Thus, in the modern era, a polyphasic approach is recommended when identifying fungal pathogens. It is also important to determine if the organism is capable of causing host damage, which usually relies on the application of Koch’s postulates for fungal plant pathogens. The importance and the challenges of applying Koch’s postulates are discussed. Bradford Hill criteria, which are generally used in establishing the cause of human disease, are briefly introduced. We provide guidelines for pathogenicity testing based on the implementation of modified Koch’s postulates incorporating biological gradient, consistency, and plausibility criteria from Bradford Hill. We provide a set of protocols for fungal pathogenicity testing along with a severity score guide, which takes into consideration the depth of lesions. The application of a standard protocol for fungal pathogenicity testing and disease assessment in plants will enable inter-studies comparison, thus improving accuracy. When introducing novel plant pathogenic fungal species without proving the taxon is the causal agent using Koch’s postulates, we advise the use of the term associated with the “disease symptoms” of “the host plant”. Where possible, details of disease symptoms should be clearly articulated.

Leaf Spot Caused by Boeremia linicola on Siberian ginseng in China

Siberian ginseng (Eleutherococcus sessiliflorus (Rupr. & Maxim.) S. Y. Hu, Araliaceae), is a perennial medicinal plant that is widely cultivated in China. Leaf spot was observed in 2- and 3-year-old Siberian ginseng in Zuojia County (126°05′23.2″E, 44°03′09.5″N), northeast China, in August 2019. Polygonal or irregular black spots ranging from 2 to 9 mm in diameter were found on infected leaves, and each leaf had dozens of spots. The green color around the lesions gradually faded. As the disease progressed, the spots withered and multiple lesions merged into large disease spots, causing leaf wilting (Fig. 1). More than 38% of plants in one 25-ha field were infected in 2019. Fifteen diseased leaves were collected from those plants and cut into 5-mm pieces. The pieces were surface-disinfected by immersion in 1% NaOCl for 2 min and then rinsing twice with sterile distilled water. The leaf pieces were placed on acidified potato dextrose agar (PDA, pH 4.7) in Petri plates, and incubated in the dark at 25°C. Nineteen isolates were obtained and all were purified from a single spore in water agar. Isolate CWJ7 was randomly selected for identification and pathogenicity testing. The colonies on PDA were olivaceous gray to olivaceous black, velvet, with dense hyphae and a scalloped or irregular margin. The reverse side was gray-black and surrounded by tawny halos. The conidia were aseptate and variable in shape and dimension: piriform, columnar, drop-shaped, dumbbell-shaped or oval, measuring 4.90 (7.03) 9.50 × 2.10 (2.78) 3.40 µm (n=100), and chlamydospores were absent. Black pycnidia (132.2–241.5 µm in diameter) appeared after 7 days. The pathogen was initially identified as Phoma or Phoma-like (Boerema et al. 2004). Further confirmation was also determined by sequencing the nuclear ribosomal internal transcribed spacer region (GenBank accession no. MT912950), 28S ribosomal RNA gene (MT912968), and genes encoding β-tubulin (MT920618), the second largest subunit of RNA polymerase II (MT920619) and translation elongation factor (MT946526) (de Hoog and Gerrits van den Ende 1998; Rehner & Samuels 1994; Liu et al. 1999; Vilgalys & Hester 1990), and Blast searches showed 90%–100% homology with GU237754, GU237938, KT389780, KT389575, and KY484705, respectively. In a phylogenetic analysis combining all loci, CWJ7 and the type strains of Boeremia linicola clustered in one group (Fig. 2). Based on its morphological characteristics and phylogenetic analysis, isolate CWJ7 was identified as B. linicola as revised in 2019 (Jayawardena et al. 2019). Healthy 2-year-old plants were used for pathogenicity testing. The leaves of nine potted plants (one plant per pot, three plants per replicate) were spray-inoculated with a suspension of conidia (1×105 spores/ml) from colonies on PDA for 7 days and cultured for 48 h under continuous black light. Nine plants were sprayed with sterile water as the control. This experiment was repeated twice. All plants were cultured in a greenhouse (25°C, 12-h photoperiod, 78% relative humidity). Clear plastic bags were used to maintain high humidity. After 7 days, the inoculated plants showed lesions on the leaves, similar to those observed in the field. The control plants remained symptomless. The pathogen was reisolated and identified by sequencing. This is the first report of B.linicola causing Siberian ginseng leaf spot, and a new record of this species in China. This disease poses a threat to production and management strategies should be developed.

First Report of Black Spot Caused by Neoscytalidium dimidiatum on Sisal in Guangxi, China

Sisal (Agave sisalana Perrine) is an important hard fiber crop that is widely planted in Guangxi, Guangdong, Hainan, Yunnan, and Fujian provinces, China. In July 2019, a new leaf disease of sisal with a disease incident of about 36% was found in Guangxi (Fig.1a~d). The oval or circular black lesions were 2.3 cm to 15.9 cm in length and 1.6 cm to 5.5 cm in width on both sides of the diseased leaves. The central part of the lesions was slightly hollow. The lesions continuously enlarged and ultimately penetrated the leaves. Reddish brown and dark mucus was secreted from the lesions. The junction of lesions and healthy parts was reddish brown to yellow. The diseased leaf fiber and mesophyll tissues were reddish brown and necrotic. Fresh leaf yield was reduced about 30% by the disease, and fiber quality was significantly compromised every year in Guangxi. Six kinds of fungi distinguished by their morphology, size and color of the colonies were isolated from diseased leaf tissues of 60 sisal plants sampled from five different farms in Guangxi. Isolate JMHB1 was isolated at a rate of 95.67%. The isolate JMHB1 was initially white with dense and hairy aerial mycelium, gradually turning dark grey to olive green on PDA (Fig. 2). Conidia, arthrospores, and chlamydospores were observed on PDA in culture (Fig. 3). The conidia formed arthric chains, disarticulating, cylindrical-truncate, oblong-obtuse to doliiform, colorless and transparent, zero- to one-septate, and averaging 4.4 to 13.8 µm × 2.2 to 5.6 µm (n=100). Arthrospores were short columnar, pigmented and transparent, single or formed arthric chains, averaging 5.5 to 17.9 µm × 2.1 to 3.5 µm (n=100). Chlamydospores were dark brown, round or oval, averaging 4.5 to 9.6 µm × 4.5 to 8.6 µm (n=100). Pathogenicity testing was conducted by inoculating 3-year-old healthy sisal plants with PDA plugs (5 × 5 mm) on which the fungus had grown for 5 days. Nine healthy plants were wounded on the leaves with a sterile needle, and mycelial plugs were placed on the wounds, covered with sterile moist cotton, and wrapped with parafilm. Nine control plants were wounded and treated with PDA plugs as the negative control. The test was repeated three times. All treated plants were kept in a greenhouse at ~28 ℃ and 40% RH. After 5 days, only leaves inoculated with isolate JMHB1 showed lesions similar to symptoms observed in the field (Fig.1e~f). The fungus was re-isolated from all nine diseased plants, and no symptoms were observed on the leaves of control plants. Molecular identification of the fungus was made by PCR amplification of the internal transcribed spacer (ITS) region of rDNA, EF1-α gene and β-tubulin gene using primers ITS1/ITS4 (White et al. 1990), EFl-728F/EF1-986R (Carbone and Kohn 1999), TUB2Fd/TUB4Rd (Aveskamp et al. 2009) respectively. The ITS (MT705646), EF1-α (MT733516) and β-tubulin (MT773603) sequences of JMHB1 were similar to the ITS (AY819727), EF1-α (EU144063) and β-tubulin (KF531800) sequences of the epitype of Neoscytalidium dimidiatum (CBS 499.66) with 100%, 99.65% and 99.02% identity, respectively. Based on pathogenicity testing, morphological characteristics, and molecular identification, the pathogen of sisal causing black spot was identified as N. dimidiatum (Penz.) Crous & Slippers (Crous et al. 2006). To our knowledge, this is the first report of black spot caused by N. dimidiatum on sisal in China. Sisal is the main economic crop in arid and semi-arid areas that is widely planted in several provinces of southern China. The serious occurrence of the disease caused by N. dimidiatum has greatly affected the development of sisal industry and local economic income in China. Identification of the pathogen of the disease is of great significance to guide disease control, increase farmers' income and promote the development of sisal industry. References: Aveskamp, M. M., et al. 2009. Mycologia, 101: 363. https://doi.org/10.3852/08-199. Carbone, I., and Kohn, L. M. 1999. Mycologia, 91:553. https://doi.org/10.1080/00275514.1999. 12061051. Crous, P. W., et al. 2006. Stud. Mycol. 55:235. https://doi.org/10.3114/sim.55.1.235. White, T. J., et al. 1990. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, Page 315. doi.org/10.1002/mrd.1080280418. Supplemental photographs: Fig. 1 Symptoms of sisal black spot disease a, b, c, d showed symptoms in the field, e and f were symptoms after inoculating Neoscytalidium dimidiatum JMHB1. a, c, and e were the front of the lesions, b, d, and f were the back of the lesions. Fig. 2 Primary colony (a) and old colony (b) of Neoscytalidium dimidiatum JMHB1 Fig. 3 Arthrospores (a), conidia and chlamydospores (b) of Neoscytalidium dimidiatum JMHB1

Isolation and pathogenicity testing of avian reticuloendotheliosis virus from layer chickens in China

Reticuloendotheliosis virus (REV) can cause runting, immunosuppression, acute reticulum cell neoplasia, and chronic lymphoid tumors in a variety of domestic and wild birds. We diagnosed a case of reticuloendotheliosis with obvious tumors in liver and kidney. We isolated and sequenced the virus and performed pathogenicity testing of the REV strain. Immunohistochemistry and PCR confirmed that the diseased layer chickens were infected with REV. The strain, named BJ1503, was successfully isolated from the case by inoculation of tissue homogenates onto chicken embryo fibroblasts. The length of the proviral REV genome is 8,293 nucleotides. The isolate had 99.7% identity with REV-HA9901 (AY842951.1), which was isolated from Jiangsu, China, in 1999. The chickens infected with REV-BJ1503 had depressed weight gain and lymphoid atrophy. Our findings suggest that REV isolate BJ1503 was phylogenetically close to the earlier strain found in China, with minor variations, and the virus was associated with severe production problems.

A Diagnostic Guide for Basil Downy Mildew

Downy mildew, caused by the oomycete pathogen Peronospora belbahrii, is one of the most important diseases affecting sweet basil worldwide. Field- and greenhouse-grown basil may be affected, and crop losses are observed as the reduction of marketable leaves during both the production and postharvest handling stages. As an obligate biotroph, P. belbahrii cannot be cultured and maintained without live plant tissue, which may complicate efforts to diagnose and identify the causal agent. Thus, the goal of this diagnostic guide is to outline the appropriate methods required to identify basil downy mildew based on the symptoms of the disease and signs of the pathogen. Additionally, methods for pathogen identification, pathogen isolation, storage of single-sporangium cultures on live plants, and pathogenicity testing are described in detail.

Development of rice conidiation media for Ustilaginoidea virens

Rice false smut, caused by the ascomycete Ustilaginoidea virens, is a serious disease of rice worldwide. Conidia are very important infectious propagules of U. virens, but the ability of pathogenic isolates to produce conidia frequently decreases in culture, which influences pathogenicity testing. Here, we developed tissue media with rice leaves or panicles that stimulate conidiation of U. virens. Generally, rice leaf media more effectively increased conidiation than panicle media, and certain non-filtered tissue media were better than their filtered counterparts. Among the tested media, the Indica rice leaf medium with 0.06 g/ml of Wanxian 98 leaf was most efficient for inducing conidiation, and it was also usable for conidiation-defective isolates. Although the conidia induced in rice tissue media were smaller, they were able to germinate on potato sucrose agar medium and infect rice normally. This method provides a foundation for the production of conidia in U. virens that will be widely applied in the pathogenicity testing as well as in genetic analyses for false smut resistance in rice cultivars.

Black Rot of Sweetpotato: A Comprehensive Diagnostic Guide

Black rot of sweetpotato (Ipomoea batatas) has been considered one of the most historically devastating diseases of the crop. The pathogen, Ceratocystis fimbriata (Ellis and Halst), is able to infect a variety of hosts including morning glory (Ipomoea sp.), coffee (Coffea sp.), and mango (Mangifera indica) over a wide geographic range. The slow-growing nature of the pathogen can lead to difficulty in isolating and maintaining cultures of the fungus. Thus, the objective for this diagnostic guide is to provide information about effective techniques for pathogen isolation, identification, storage, and pathogenicity testing as well as describe the host and geographic range, taxonomy, and disease in sweetpotato.