Plant diseases are often thought to be caused by one species or even by a specific strain. Microbes in nature however mostly occur as part of complex communities and this has been noted since the time of van Leeuwenhoek. Interestingly, most laboratory studies focus on single microbial strains grown in pure culture; we were therefore unaware of possible interspecies and/or inter-kingdom interactions of pathogenic microbes in the wild. In human and animal infections, it is now being recognized that many diseases are the result of multispecies synergistic interactions. This increases the complexity of the disease and has to be taken into consideration in the development of more effective control measures. On the other hand, there are only a few reports of synergistic pathogen-pathogen interactions in plant diseases and the mechanisms of interactions are currently unknown. Here we review some of these reports of synergism between different plant pathogens and their possible implications in crop health. Finally, we briefly highlight the recent technological advances in diagnostics as these are beginning to provide important insights into the microbial communities associated with complex plant diseases. These examples of synergistic interactions of plant pathogens that lead to disease complexes might prove to be more common than expected and understanding the underlying mechanisms might have important implications in plant disease epidemiology and management.

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Synergisms between microbial pathogens in plant disease complexes: a growing trend.

Notoginseng (Panax notoginseng), a valuable herbal medicine, has high death rates in continuous cropping systems. Variation in the soil microbial community is considered the primary cause of notoginseng mortality, although the taxa responsible for crop failure remains unidentified. This study used high-throughput sequencing methods to characterize changes in the microbial community and screen microbial taxa related to the death rate. Fungal diversity significantly decreased in soils cropped with notoginseng for three years. The death rate and the fungal diversity were significantly negatively correlated, suggesting that fungal diversity might be a potential bioindicator of soil health. Positive correlation coefficients revealed that Burkholderiales, Syntrophobacteraceae, Myrmecridium, Phaeosphaeria, Fusarium, and Phoma were better adapted to colonization of diseased plants. The relative abundance of Fusarium oxysporum (R = 0.841, P < 0.05) and Phaeosphaeria rousseliana (R = 0.830, P < 0.05) were positively associated with the death rate. F. oxysporum was a pathogen of notoginseng root-rot that caused seedling death. Negative correlation coefficients indicated that Thermogemmatisporaceae, Actinosynnemataceae, Hydnodontaceae, Herpotrichiellaceae, and Coniosporium might be antagonists of pathogens, and the relative abundance of Coniosporium perforans was negatively correlated with the death rate. Our findings provide a dynamic overview of the microbial community and present a clear scope for screening beneficial microbes and pathogens of notoginseng.

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Soil bacterial and fungal community dynamics in relation to Panax notoginseng death rate in a continuous cropping system

Rhizospheric soil and root endogenous fungal diversity and composition in response to continuous Panax notoginseng cropping practices

Endophytic Trichoderma gamsii YIM PH30019: a promising biocontrol agent with hyperosmolar, mycoparasitism, and antagonistic activities of induced volatile organic compounds on root-rot pathogenic fungi of Panax notoginseng

Background and Aims 
Sanqi ginseng (Panax notoginseng) growth is often hampered by replant failure. In this study, we aimed to examine the role of autotoxicity in Sanqi replant failures and assess the role of ginsenosides in autotoxicity.


Methods 
The autotoxicities were measured using seedling emergence bioassays and root cell vigor staining. The ginsenosides in the roots, soils, and root exudates were identified with HPLC-MS.


Results 
The seedling emergence and survival rate decreased significantly with the continuous number of planting years from one to three years. The root exudates, root extracts, and extracts from consecutively cultivated soils also showed significant autotoxicity against seedling emergence and growth. Ginsenosides, including R1, Rg1, Re, Rb1, Rb3, Rg2, and Rd, were identified in the roots and consecutively cultivated soil. The ginsenosides, Rg1, Re, Rg2, and Rd, were identified in the root exudates. Furthermore, the ginsenosides, R1, Rg1, Re, Rg2, and Rd, caused autotoxicity against seedling emergence and growth and root cell vigor at a concentration of 1.0 µg/mL.


Conclusion 
Our results demonstrated that autotoxicity results in replant failure of Sanqi ginseng. While Sanqi ginseng consecutively cultivated, some ginsenosides can accumulate in rhizosphere soils through root exudates or root decomposition, which impedes seedling emergence and growth.

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Autotoxic ginsenosides in the rhizosphere contribute to the replant failure of Panax notoginseng.

【Objective】Panax notoginseng, cultivated in Yunnan, China, is an important medicinal herb. Rot root disease is the most serious problem limiting the production of the herb. 【Method】 Panax notoginseng root rot disease was surveyed and sampled in the field. The pathogens of the root rot was isolated and identificated. Pathogenecity tests in vitro and in field were carried out. 【Result】A survey on the disease indicated that the symptoms of the disease are expressed as dry chap rot, medullary tissue rot, slowly yellow rot, stem base dry rot, suddenly wilt rot and wet rot. Among them, slowly yellow rot and suddenly wilt wet rot were the most popular ones. Dry chap rot, medullary tissue rot and stem base dry rot are new records for symptoms of the disease. Cylindrocarpon destructans, C. didynum, Fusarium solani, Phytophthora cactorum, Phoma herbarum, Monilia sp., Verticillium sp., Trichoderma sp., Rhizoctonia solani were involved in the disease. In vitro pathogenecity tests demonstrated that C. destructans, C. didynum, P. cactorum, P. herbarum and R. solani could result in the disease at different levels. Field inoculation tests showed that P. cactorum and P. herbarum were the most pathogenic fungi, leading to disease incidences at 48.4% and 50.0%, respectively. 【Conclusion】However, C. destructans and C. didynum were considered as two of the most important fungi causeing Panax notoginseng root-rot diseases because of their wide distribution, high frequency of isolation, and close relationship withdominant symptom.

The Causal Microorganisms of Panax notoginseng Root Rot Disease

Gao Y.-H., Miao W., Guo R.-J., Li S.-D. (2015): Real time PCR quantification of Sclerotium rolfsii in chilli tissue and soil. Plant Protect. Sci., 51: 61–66. Root rot disease caused by Sclerotium rolfsii is one of the important factors limiting chilli production in southern China. Rapid methods of detection and identification of S. rolfsii are required for disease prediction and management. Herein, we adapted a four-step real time quantitative PCR method to detect S. rolfsii using new designed primer pair S301S/S301A. By using this assay, S. rolfsii was detected in ten soil samples and eight plant samples at concentrations as low as 10 –7 g mycelium/g soil or plant, both with and without typical symptoms, while tissue-culturing method could only detect the fungus in two samples. This rapid and quantitative method is efficient for detection and determination of S. rolfsii population in soil and plants.

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Real time PCR quantification of Sclerotium rolfsii in chilli tissue and soil

Two Bacillus strains B006 and B010 were screened on SWB medium plates for their rapid spreading and inhibitive ability against growth of Fusarium oxysporum and F.solani,the pathogens of soybean root rot disease.These two strains were identified as Bacillus subtilis based on 16S rDNA sequence analysis,morphology and physiological and biochemical characteristics.In field,B006 and B010 were applied as seed and furrow treatment,and showed 74.6% and 63.5% efficiency to control soybean root rot disease at seedling stage respectively,were more efficient than the treatment with 35% Duofu(P=0.05).And they increased the soybean output by 13.8% and 24.0% respectively in comparison with that gained without any treatment.The gene fragments of bmyB,fenD,bioA,srfAA and srfAB were amplified from genomic DNA of strains B006 and B010 with specific primers,while only gene ituC was amplified from strain B010.The analysis of functional genes of isolates B006 and B010 might benefit the quick selection of potentially useful biocontrol strains and understanding of the antagonistic mechanism.

Characterization of Bacillus strains screened via nutritional competition for biocontrol of soybean root rot disease

The control efficacies of such organic sulfide fumigants as allylIsothiocyante(AITC),diallyldisulfide (DADS),dimethlyldisulfide(DMDS),methylene-(bis)thiocyanate(MBTC),sodium tetra-thiocyanate (STTC),dazomet(DZM)and K-Vapam on soil plant pathogens were tested under laboratory and greenhouse.The results showed that the order of the inhibiting effects of tested fumigants on soil fungi was DZM=MBTC≥DADS=AITC≥STTC=DMDS,and the order of their inhibiting effects on soil bacteria was AITC≥MBTC=DZM=DADS≥DMDS=STTC.Among the tested fumigants only AITC and DZM demonstrated a strong inhibiting effect on weeds,decreasing by 73.6%～94.7%in weed density.All the tested fumigants had a quite high toxicity to plant nematodes and free-living nematodes.AITC had the greatest toxicity to the 2nd-stadium larvae of root-knot nematodes, with LC_(90)0.5μg/mL.The soil treatment with fumigants before seeding significantly decreased root-rot (Fusarium solani) of kidney bean and root knot index,and the order of fumigants in the control efficacy was AITC=MBTC=DZM≥DADS=DMDS≥STTC.The order of fumigants in safety to vegetables was STTCMBTCDADSTITCK-Vapam=DZM.Cabbage was more tolerant to AITC than other crops.Compared with CK,treating planting holes with fumigants(except 2%DADS dust and 2%AITC dust) and then transplanting two-leaf-stage seedlings of tomato together with medium on the same day could significantly reduce soil nematode population and root-knot index,and there was not significant difference in seedling establishment between CK and the treatment.The treatment with 2%DADS dust and 2%AITC dust produced chemical injury for the tomato seedlings.

Guo RongJun

Papers

The Causal Microorganisms of Panax notoginseng Root Rot Disease

Real time PCR quantification of Sclerotium rolfsii in chilli tissue and soil

Characterization of Bacillus strains screened via nutritional competition for biocontrol of soybean root rot disease

Efficacy evaluation of applying organic sulfide fumigants to plant pathogens-polluted soil in vegetable greenhouse.