Control of Apple Replant Disease Using Mixed Cropping with Brassica juncea or Allium fistulosum

Evidence indicates that Allium and Brassica species which release bioactive compounds are widely used in bio-fumigation to suppress soil-borne diseases. However, the active molecules of such plant residues are easily volatilized. In this study, we conducted mixed cropping of the apple tree with Allium fistulosum or Brassica juncea; the results demonstrated that such mixed cropping significantly improved the growth of the grafted apple seedlings and alleviated apple replant disease (ARD) for two years. The terminal-restriction fragment length polymorphism profile results showed that the soil fungal community demonstrated distinct variation and diversity in terms of composition. A. fistulosum and B. juncea significantly improved the Margalef, Pielou, and Shannon indices. In addition, the analyses of clone libraries showed that A. fistulosum and B. juncea promoted the proliferation of antagonistic fungi such as Mortierella, Trichoderma, and Penicillium, and inhibited the proliferation of pathogens such as Fusarium. Fusarium. Proliferatum (F. proliferatum) was abundant in replanted soil and proved to be an aggressive pathogen of apple seedlings. Our findings thus indicate that apple tree mixed cropping with A. fistulosum and B. juncea was an effective long-term method for modifying the resident fungal community and alleviating ARD.

Detection of Root Physiological Parameters and Potassium and Calcium Currents in the Rhizoplane of the Apple Rootstock Superior Line 12-2 With Improved Apple Replant Disease Resistance

The cultivation of resistant rootstocks is one of the more effective ways to mitigate apple replant disease (ARD). We performed an ion current test, a pot experiment, and a pathogen infection test on the apple rootstocks 12-2 (self-named), T337, and M26. The ion current test showed that exposure to ARD soil extract for 30 min had a significant effect on K+ ion currents at the meristem, elongation, and mature zones of the M26 rhizoplane and on Ca2+ currents in the meristem and elongation zones. ARD also had a significant effect on Ca2+ currents in the meristem, elongation, and mature zones of the T337 rhizoplane. Exposure to ARD soil extract for 5 min had a significant effect on K+ currents in the meristem, elongation, and mature zones of 12-2 and on the Ca2+ currents in the elongation and mature zones. Compared to a 5-min exposure, a 30-min exposure to ARD extract had a less pronounced effect on K+ and Ca2+ currents in the 12-2 rhizoplane. The pot experiment showed that ARD soil had no significant effect on any root architectural or physiological parameters of 12-2. By contrast, ARD soil significantly reduced some root growth indices and the dry and fresh weights of T337 and M26 compared with controls on sterilized soil. ARD also had a significant effect on root metabolic activity, root antioxidant enzyme activity (except superoxide dismutase for T337), and malondialdehyde content of T337 and M26. Pathogen infection tests showed that Fusarium proliferatum MR5 significantly affected the root structure and reduced the root metabolic activity of T337 and M26. It also reduced their root antioxidant enzyme activities (except catalase for T337) and significantly increased the root malondialdehyde content, reactive oxygen levels, and proline and soluble sugar contents. By contrast, MR5 had no such effects on 12-2. Based on these results, 12-2 has the potential to serve as an important ARD-resistant rootstock.

Characterization of soils conducive and non-conducive to Prunus replant disease

Successive orchard plantings of almond and other Prunus species exhibit reduced growth and yield in many California soils. This phenomenon, known as Prunus replant disease (PRD), can be prevented by preplant soil fumigation or anaerobic soil disinfestation, but its etiology is poorly understood and its incidence and severity are hard to predict. We report here on relationships among physicochemical variables, microbial community structure, and PRD induction in 25 diverse replant soils from California. In a greenhouse bioassay, soil was considered to be “PRD-inducing” when growth of peach seedlings in it was significantly increased by preplant fumigation and pasteurization, compared to an untreated control. PRD was induced in 18 of the 25 soils, and PRD severity correlated positively with soil exchangeable-K, pH, %clay, total %N, and electrical conductivity. The structure of bacterial, fungal, and oomycete communities differed significantly between the PRD-inducing and non-inducing soils, based on PERMANOVA of Bray Curtis dissimilarities. Bacterial class MB-A2-108 of phylum Actinobacteria had high relative abundances among PRD-inducing soils, while Bacteroidia were relatively abundant among non-inducing soils. Among fungi, many ASVs classified only to kingdom level were relatively abundant among PRD-inducing soils whereas ASVs of Trichoderma were relatively abundant among non-inducing soils. Random forest classification effectively discriminated between PRD-inducing and non-inducing soils, revealing many bacterial ASVs with high explanatory values. Random forest regression effectively accounted for PRD severity, with soil exchangeable-K and pH having high predictive value. Our work revealed several biotic and abiotic variables worthy of further examination in PRD etiology.

The Endophytic Strain Trichoderma asperellum 6S-2: An Efficient Biocontrol Agent against Apple Replant Disease in China and a Potential Plant-Growth-Promoting Fungus

A study was conducted for endophytic antagonistic fungi obtained from the roots of healthy apple trees growing in nine replanted orchards in Shandong Province, China. The fungi were assessed for their ability to inhibit Fusarium proliferatum f. sp. malus domestica MR5, a fungal strain associated with apple replant disease (ARD). An effective endophyte, designated as strain 6S-2, was isolated and identified as Trichoderma asperellum. Strain 6S-2 demonstrated protease, amylase, cellulase, and laccase activities, which are important for the parasitic and antagonistic functions of pathogenic fungi. The inhibition rate of 6S-2 against Fusarium proliferatum f. sp. malus domestica MR5 was 52.41%. Strain 6S-2 also secreted iron carriers, auxin, ammonia and was able to solubilize phosphorus. Its fermentation extract and volatile substances inhibited the growth of MR5, causing its hyphae to twist, shrink, swell, and rupture. The antifungal activity of the 6S-2 fermentation extract increased with increasing concentrations. It promoted the production and elongation of Arabidopsis thaliana lateral roots, and the strongest effects were seen at a concentration of 50 mg/mL. A GC-MS analysis of the 6S-2 fermentation extract and volatile substances showed that they comprised mainly alkanes, alcohols, and furanones, as well as the specific volatile substance 6-PP. The application of 6S-2 spore suspension to replanted apple orchard soils reduced plant oxidative damage and promoted plant growth in a pot experiment. Therefore, the endophytic strain T. asperellum 6S-2 has the potential to serve as an effective biocontrol fungus for the prevention of ARD in China, and appears to promote plant growth.

The history, etiology, and management of ginseng replant disease: a Canadian perspective in review

Ginseng replant disease (GRD) has had a major impact on the American ginseng (Panax quinquefolius L.) industry in Canada and is a threat to its survival. With only 150 yr of domestic ginseng cultivation, GRD and its effect on the industry in North America is well documented compared with replant diseases in other crops. However, minimal research has been published on the etiology and management of GRD. Research and observations of replant diseases of the major commercial Panax species worldwide are presented to propose mechanisms of GRD and potential management options. The available evidence suggests the specific involvement of the soil-borne fungus Ilyonectria mors-panacis (A.A. Hildebr.) A. Cabral & Crous combined with an unknown host-related factor as the ultimate cause of GRD, since other proposed mechanisms seem unlikely to have occurred in the diversity of regions where GRD is reported. Other abiotic and biotic factors influencing the severity of the disease are also likely to be involved. Given the lack of clarity in the scientific literature between problems associated with continuous cultivation of ginseng and true replant disease, a definition of GRD is proposed. The development of an effective and economical management regime will require a better understanding of the mechanisms of GRD. Potential management options include reducing ginseng debris after harvest of the first crop, preplant testing to identify low-risk sites for replanting, fumigation, alternative disinfestation techniques, and fungicides (including biofungicides) targeting I. mors-panacis.

Biocontrol Potential of The Phloridin-Degrading Bacillus Licheniformis XNRB-3 Against Apple Replant Disease

Background: Apple replant disease (ARD) is a common occurrence in many major apple-growing areas worldwide, seriously hindering the development of the apple industry. To avoid the shortcomings of chemical fungicides currently used to control ARD, it is necessary to find sustainable and effective control methods. Here, an endophytic phloridin-degrading Bacillus licheniformis XNRB-3 was isolated from the root tissue of healthy apple trees, and its control effect on apple replant disease (ARD) and its how to alleviates the pathogen pressure via changes in soil microbiomes were studied.Results: The addition of strain XNRB-3 in Fusarium infested soils significantly reduced the number of pathogens in the soil, thus resulting in a lower disease incidence, and the relative control effect reached more than 60%. The fermentation broth can also protect the roots of the plants from Fusarium infection. These antagonistic effects were further validated using an in vitro assay in which the pathogen control was related to growth and spore germination inhibition via directly secreted antimicrobial substances and and indirect interspecific competition for nutrients. The antifungal organic compounds in the fermentation metabolites were identified using GC-MS technology. Among them, alpha-bisabolol and 2,4-di-tert-butylphenol had significant inhibitory effects on many planted pathogenic fungi. Butanedioic acid, monomethyl ester, and dibutyl phthalate can promote the root elongation and lateral root development of Arabidopsis plants. The potential of strain XNRB-3 to control ARD was later validated using microbial fertilizer inoculation in pot and field experiment. The addition of strain XNRB-3 significantly promoted the growth of plants, and the activity of enzymes related to disease resistance (SOD, POD, and CAT) was also significantly enhanced. It also reduced the abundance of Fusarium and the content of phenolic acids in the rhizosphere soil, improved soil microbial community structure and nutritional conditions, and increased soil microbial diversity and activity, as well as soil enzyme activity. Conclusions: The incorporation of strain XNRB-3 in the soil alleviated the damage of soil-borne pathogens to plants by reducing the relative abundance of pathogenic fungi and the content of phenolic acids, and inducing disease resistance of plants. Taken together, B. licheniformis XNRB-3 could be developed into a promising biocontrol and plant-growth-promoting agent. This provides a new management strategy to control ARD.

Alleviation of Nematode-Mediated Apple Replant Disease by Pre-Cultivation of Tagetes

Apple replant disease (ARD) is a severe problem in orchards and tree nurseries caused by yet unknown soil biota that accumulate over replanting cycles. This study tested the contribution of nematodes to ARD, and cultivation of Tagetes as a control option. In a pot experiment, Tagetes patula or Tagetes tenuifolia were grown in ARD soil, incorporated or removed. Nematodes extracted from untreated ARD soil and washed on 20-µm sieves induced ARD symptoms when inoculated to apple plantlets growing in a sterile substrate. In contrast, nematodes from Tagetes treated ARD soil did not reduce root growth compared to uninoculated plants, irrespective of Tagetes species and incorporation. In plots of five apple tree nurseries or orchards, either Tagetes or grass was grown on ARD soil. Nematodes extracted from the grass plots and inoculated to apple plantlets significantly reduced plant growth compared to nematodes from Tagetes plots for all five farms. Apple rootstocks showed overall a significantly higher increase in shoot base diameter when grown on Tagetes-treated plots compared to grass plots, while this effect differed among farms. Plant-parasitic nematodes were too low in abundance to explain plant damage. In conclusion, Tagetes alleviated ARD by changing the nematode community in soil.