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

The harvested tobacco leaves of K326 were treated with bacterial suspension of strain K7 and K3(Arthrobacter spp.) and their nicotine contents were analyzed after flue-curing.The results indicated that the nicotine contents reduced in all treatments comparing with the control,the reduction rate in tobacco leaves grown in paddy field was slightly higher than that grown in hillside land,and the reducing effect of K7 was better than that of K3. For tobacco grown in paddy field,the reduction rates in lower,upper and middle leaves treated with K7 were 39.7%,29.5% and 28.4%,respectively,and those treated with K3 were 34.9%,29.2% and 26.1%,respectively.The sensory evaluation of cigarettes made of each single grade tobacco showed that bacterial suspension could increase the aromatic quality and volume of tobacco leaf(especially for middle leaves),reduce irritation and strength,and improve aftertaste.

Xia ZhenYuan

Papers

The Causal Microorganisms of Panax notoginseng Root Rot Disease

Reduction of Nicotine in Tobacco Leaf Treated with Arthrobacter spp

Studies on the interactions of Paecilomyces Lilacinus (Thom) samson IPC and tobacco rhizosphere

Analyzing on the ecology factor of the soil infected by tobacco root knot nematodes in yunnan