Soybean is threatened by many pathogens that negatively affect this crop's yield and quality, e.g., different Fusarium species that cause wilting and root rot diseases. Fusarium root rot (FRR) in soybean can be caused by F. graminearum and other Fusarium spp. that are associated with Fusarium head blight (FHB) in cereals. Therefore, it was important to enquire whether Fusarium pathogens from soybean can cause disease in wheat, and vice versa. Here, we investigated the Fusarium root rot complex in Manitoba (Canada) from symptomatic plants, using both culture- and molecular-based methods. We developed a molecular diagnostic toolkit to detect and differentiate between several Fusarium spp. involved in FHB and FRR, then we evaluated cross-pathogenicity of selected Fusarium isolates collected from soybean and wheat, and the results indicate that isolates recovered from one host can infect the other host. Trichothecene production by selected Fusarium spp. was also analyzed chemically using LC-MS in both soybean (root) and wheat (spike) tissues. Trichothecenes were also analyzed in soybean seeds from plants with FRR to check the potentiality of trichothecene translocation from infected roots to the seeds. All of the tested Fusarium isolates were capable of producing trichothecenes in wheat spikes and soybean roots, but no trichothecenes were detected in soybean seeds. This study provided evidence, for the first time, that trichothecenes were produced by several Fusarium spp. (F. cerealis, F. culmorum and F. sporotrichioides) during FRR development in soybean.
Fusarium graminearum is a toxigenic plant pathogen that causes Fusarium head blight (FHB) disease on cereal crops. It has recently shown to have cross-pathogenicity on noncereals (i.e., Fusarium root rot [FRR] on soybean) in Canada and elsewhere. Specific detection and differentiation of this potent toxigenic, trichothecene-producing pathogen among other closely related species is extremely important for disease control and mycotoxin monitoring. Here, we designed a PCR restriction fragment length polymorphism protocol based on the DNA sequence of the translational elongation factor 1α (TEF1α) gene. A unique restriction site to the enzyme HpaII is only found in F. graminearum sensu stricto strains among different Fusarium strains in the F. graminearum species complex (FGSC) and other Fusarium spp. associated with FHB in cereals and FRR in soybean. Partial amplification of the TEF1α gene with newly designed primers mh1/mh2 generated a 459-bp PCR fragment. Restriction digestion of the generated fragments with the HpaII enzyme generated a unique restriction pattern that can rapidly and accurately differentiate F. graminearum sensu stricto among all other Fusarium spp. A primer pair (FgssF/FgssR) specific to F. graminearum sensu stricto also was designed and can distinguish F. graminearum sensu stricto from all other Fusarium spp. in the FGSC and other closely related Fusarium spp. involved in FHB and FRR. This finding will be very useful for the specific detection of F. graminearum sensu stricto for diagnostic purposes as well as for the accurate detection of this pathogen in breeding and other research purposes.
The aim of this study is to assess the effect of extracts of Nerium oleander, Eucalyptus chamadulonsis and Citrullus colocynthis against bacterial spot disease of tomato and to investigate the induction of resistance by tomato (Solanum lycopersicum) in order to promote a sustainable management system. The antibacterial activity of aqueous and ethanol plant extracts was tested against Xanthomonas axonopodis pv. vesicatoria, isolate PHYXV3, in vitro and in vivo. The highest antibacterial activity in vitro was obtained with C. colocynthis, N. oleander and E. chamadulonsis, respectively. In vivo, ethanol extracts of N. oleander and E. chamadulonsis were more effective than aqueous extracts in reducing pathogen populations on tomato leaves. Under greenhouse conditions, application of the plant extracts at 15% (v/v) to tomato plants significantly reduced disease severity and increased the shoot weight of ‘Super Marmande’ tomato. In most cases, plant extracts significantly increased total phenol and salicylic acid content of tomato plants compared to either healthy or infected ones. In addition, C. colocynthis and E. chamadulonsis extracts significantly increased peroxidase activity while only E. chamadulonsis increased polyphenol oxidase after infection with the causal agent. The results indicated that the plant extracts showed promising antibacterial activity and could be considered an effective tool in integrated management programs for a sustainable system of tomato bacterial spot control.
In Oct. 2019, soybean plants (Glycine max) (cv. 24-10RY, R7 growth stage) with dry rot, necrosis, reddish-brown lesions, and small black fruiting bodies in linear rows were collected from fields in Manitoba (Carman, St. Adolph, Dauphin), Canada. The pods and seeds were shrivelled, small and some seeds were covered with whitish mycelium. Symptoms began as brown lesions, which darkened, elongated, causing wilt of the above stems then plant death. Microscopy showed that the fruiting bodies were pycnidia. Symptomatic stems were cut into 1-2 cm pieces and seeds surface-sterilized in 0.5% NaOCl, rinsed twice in sterilized H2O, air-dried on sterilized filter paper, and plated on PDA medium amended with 100 mg/L streptomycin sulfate at room temperature with 12-h fluorescent light/12-h dark for 3 days. The emerging hyphae were transferred using the hyphal tip method to new PDA petri dishes and incubated for 21 days (room T°). Mycelia of 20 isolates were dense, white and floccose with occasional green-yellow areas. Black stromata in concentric patterns or scattered as large masses were visible on the cultures’ back. Pycnidia formed solely or aggregated after 4-5 weeks of incubation on PDA. Alpha conidia emanated from pycnidia in creamy-to-yellowish drops and were hyaline, non-septate, ellipsoid to fusiform, and biguttulate. The average length and width of Alpha conidia were 5.5 μm and 1.5 μm, respectively (n = 30). No perithecia were seen. The cultures’ morphology was consistent with Phomopsis longicolla’s description (Hobbs et al., 1985). Seven isolates were selected for molecular characterization to confirm their identity by amplifying the ITS region with universal primers ITS4/ITS5 (White et al. 1990). All PCR amplicons were analyzed by electrophoresis through 1.5 % agarose gels and the size of PCR amplicons estimated using 1-kb plus DNA ladder (Thermo Fisher Sci., ON, Canada). PCR amplicons (~650 bp) were purified and sequenced in two directions by Psomagen Inc. (Rockville, MD, USA). ITS sequences were identical for all isolates, and GenBank searches (BLASTn: Altschul et al. 1990) confirmed species identity. ITS sequences (accessions MW466183-MW466189) were deposited in GenBank and matched the type sequence of Diaporthe longicolla strain ATCC 60325 (accession NR_144924) from G. max in USA with identities = 473/475 (99.6%) and gaps = 0/475 (0%). To confirm the pathogenicity of the seven isolates, the stems of V4-stage (four open trifolilates) soybean plants (cv. 24-10RY) were excised using a sterile scalpel. Mycelial plugs (9 mm in diameter) from 1-week-old culture of each isolate were placed over the wounded stems (Abdelmagid et al., 2019). Sterile PDA plugs were used on control plants. Six plants were used per isolate and control. Plugs of both treatments were wrapped with parafilm to avoid drying. The plants were incubated in a humidity chamber for 4 days and then in a greenhouse at 24:16°C day/night, 13:11-h light/dark cycle, and 70-80% relative humidity, and were irrigated as needed. Symptoms similar to those observed in the field were seen on the stems and seeds of all artificially-infected plants approx. 8 weeks after inoculation. Pods and seeds of inoculated plants were shrivelled and small. No symptoms were observed on control plants. Diaporthe longicolla was re-isolated only from the diseased plants and seeds. To our knowledge, this is the first report following Koch’s postulates to identify the causal pathogen of soybean pod and stem blight and seed decay in Western Canada. This will be instrumental in determining the causes of stem decay and contribute in properly dealing with soybean seed issues in Western Canada in the future.
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