The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health.

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The rhizosphere: a playground and battlefield for soilborne pathogens and beneficial microorganisms

Mechanisms of nematode suppression by organic soil amendments—A review

Management of soilborne pests and diseases in cropping systems is often highly challenging—in implementation of acceptable methodologies and in dealing with secondary problems. The phase-out of methyl bromide brings this into particularly sharp focus. There is a need for diversified options and alternatives to fill different roles across the soilborne pest and disease management spectrum, but flexibility is limited, as practicalities demand that they fit into a prophylactic methodology. It is against such a backdrop that expectations and promotion of alternatives must be set. There is also a need to recognize potentially serious problems that may have been masked under historical management regimes, but for which the nature of the system offers little scope to avoid or manage. Biofumigation is the beneficial use of Brassica green manures that release isothiocyanates chemically similar to methyl isothiocyanate, the active agent from the synthetic fumigant metam sodium, which is used as a substitute for met...

Biofumigation and Enhanced Biodegradation: Opportunity and Challenge in Soilborne Pest and Disease Management

Application of organic amendments has been proposed as a strategy for the management of diseases caused by soilborne pathogens. However, inconsistent results seriously hinder their practical use. In this work we use an extensive data set of 2423 studies derived from 252 papers to explore this strategy. First, we assess the capability of a specific organic amendment to control different diseases; second, we investigate the influence of organic matter (OM) decomposition on disease suppressiveness; and third, we search for physical, chemical and biological parameters able to identify suppressive OM. OM was found to be consistently suppressive to different pathogens in only a few studies where a limited number of pathogens were tested. In the majority of studies a material suppressive to a pathogen was ineffective or even conducive to other pathogens, suggesting that OM suppressiveness is often pathogen-specific. OM decomposition in many studies (73%, n ¼ 426) emerged as a crucial process affecting suppressiveness. During decomposition, disease suppression either increased, decreased, was unchanged or showed more complex responses, such as ‘hump-shaped’ dynamics. Peat suppressiveness generally decreased during decomposition, while responses of composts and crop residues were more complex. However, due to the many interactions of contributing factors (OM quality, microbial community composition, pathosystem tested and decomposition time), it was difficult to identify specific predictors of disease suppression. Among the 81 parameters analysed, only some of the 643 correlations showed a consistent relationship with disease suppression. The response of pathogen populations to OM amendments was a reliable feature only for some organic matter types (e.g. crop residues and organic wastes with C-to-N ratio lower than w15) and for pathogens with a limited saprophytic ability (e.g., Thielaviopsis basicola and Verticillium dahliae). Instead, population responses of the pathogenic fungi Phytophthora spp., Rhizoctonia solani and Pythium spp. appeared unrelated to disease suppression. Overall, enzymatic and microbiological parameters, rather than chemical ones, were much more informative for predicting suppressiveness. The most useful features were FDA activity, substrate respiration, microbial biomass, total culturable bacteria, fluorescent pseudomonads and Trichoderma populations. We conclude that the integration of different parameters (e.g. FDA hydrolysis and chemical composition by 13 C NMR) may be a promising approach for identification of suppressive amendments.

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Identifying the characteristics of organic soil amendments that suppress soilborne plant diseases.

SUMMARY The use of organic matter (OM) has been proposed, for both conventional and biological agriculture systems, to decrease the incidence of plant diseases caused by soilborne pathogens. In this work we review reports on the application of OM amendments, focusing on the suppressive capacity of different OM materials and the response of different soilborne pathogens. A total of 250 articles were analysed, with 2423 experimental case studies. The effect of OM amendments was found to be suppressive in 45% and non-significant in 35% of the cases. In 20% of the cases, a significant increase of disease incidence was observed. Compost was the most suppressive material, with more than 50% of cases showing effective disease control. The effect of crop residues was more variable: it was suppressive in 45% of the cases, but enhanced disease in 28%. Finally, significant disease suppression with peat was recorded only in 4% of the experiments. The ability of OM to suppress disease varied largely with different pathogens: it was observed in more than 50% of the cases for Verticillium, Thielaviopsis, Fusarium and Phytophthora. In contrast, effective control of Rhizoctonia solani was achieved only in 26% of the cases. From this review it emerged that OM amendments have great potential but, at the same time, present some inconsistencies in their application. More investigation on the mechanisms by which OM acts on disease suppression is needed to make the use of these materials more predictable.

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Suppression of soilborne fungal diseases with organic amendments.

The impact of Brassica napus seed meal on the microbial complex that incites apple replant disease was evaluated in greenhouse trials. Regardless of glucosinolate content, seed meal amendment at a rate of 0.1% (vol/vol) significantly enhanced growth of apple and suppressed apple root infection by Rhizoctonia spp. and Pratylenchus penetrans. High glucosinolate B. napus cv. Dwarf Essex seed meal amendments did not consistently suppress soil populations of Pythium spp. or apple root infection by this pathogen. Application of a low glucosinolate containing B. napus seed meal at a rate of 1.0% (vol/vol) resulted in a significant increase in recovery of Pythium spp. from apple roots, and a corresponding reduction in apple seedling root biomass. When applied at lower rates, B. napus seed meal amendments enhanced populations of fluorescent Pseudomonas spp., but these bacteria were not recovered from soils amended with seed meal at a rate of 2% (vol/vol). Seed meal amendments resulted in increased soil populations of total bacteria and actinomycetes. B. napus cv. Dwarf Essex seed meal amendments were phytotoxic to apple when applied at a rate of 2% (vol/vol), and phytotoxicity was not diminished when planting was delayed for as long as 12 weeks after application. These findings suggest that B. napus seed meal amendments can be a useful tool in the management of apple replant disease and, in the case of Rhizoctonia spp., that disease control operates through mechanisms other than production of glucosinolate hydrolysis products.

Suppression of specific apple root pathogens by Brassica napus seed meal amendment regardless of glucosinolate content.

A sprayable formulation of 1-methylcyclopropene (1-MCP; AgroFresh, Spring House, PA) was applied to 'Scarletspur Delicious' and 'Cameo' apples in the orchard 1 to 3 weeks before harvest and compared in different postharvest studies with the commercial postharvest 1-MCP fruit treatment (SmartFresh; AgroFresh) and with amino-ethoxyvinylglycine (AVG; ReTain; Valent BioSciences, Walnut Creek, CA). Treated apples were held in air storage for 50 to 60 d or in controlled-atmosphere storage for 120 to 125 or 215 to 225 d. With increased concentration, sprayable 1-MCP treatments were effective at controlling flesh firmness loss and internal ethylene concentration (IEC) up to 225 d of storage as well as during a 7-d poststorage shelf life simulation at room temperature. Application closer to harvest improved the effect of sprayable 1-MCP on control of flesh firmness loss and IEC. Concentrations of sprayable 1-MCP above 90 mg a.i./L produced similar fruit effects to 1-MCP. Treatment with 1-MCP showed little effect on soluble solids concentration (SSC), titratable acidity (TA), or skin or flesh color in 'Delicious' but slightly increased SSC and TA in 'Cameo' apples. AVG applied 4 weeks before commercial harvest controlled IEC nearly as well as either sprayable 1-MCP or 1-MCP during storage, but AVG-treated fruit allowed to ripen for 7 d at room temperature after storage lost much more flesh firmness regardless of storage regime. Sprayable 1-MCP also reduced starch hydrolysis, IEC and fruit drop at harvest. Sprayable 1-MCP may offer new opportunities for effective preharvest management of apple fruit condition, storability, and poststorage fruit quality.

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Preharvest Applications of Sprayable 1-methylcyclopropene in the Orchard for Management of Apple Harvest and Postharvest Condition

This paper reports on the potential of gibberellic acid (GA 3 and GA 4+7 ) to reduce sweet cherry (Prunus avium L.) floral bud induction and balance fruit number and improve fruit quality in the season following application. In 2003, GA 3 was applied to 'Bing'/'Gisela 1' trees at 50 and 100 mg·L -1 at the end of stage I of fruit development, end of stage II, and on both dates. These treatments were compared to the industry standard application of 30 mg·L -1 applied at the end of stage II and an untreated control. Fruit quality was evaluated in the year of application (i.e., nontarget crop) and return bloom, fruit yield and quality were assessed in the subsequent season (2004). In 2003, GA 3 delayed fruit maturity proportional to rate. In 2004, bloom density and fruit yield were related negatively and linearly to GA 3 concentration. GA 3 reduced the number of reproductive buds per spur and did not affect the number of flowers per reproductive bud. Nonspur flowering at the base of 1-year-old shoots was more inhibited by GA 3 than flowering on spurs. Double applications significantly reduced bloom density and yield versus single applications. Trees treated with two applications of 50 and 100 mg·L -1 yielded fruit with 7% and 12% higher soluble solids, 15% and 20% higher firmness, and 7% and 14% greater weight, respectively. However, no treatment improved crop value per tree. In a separate isomer trial, GA 3 and GA 4+7 were applied to 'Bing'/'Gisela 1' trees at 100 and 200 mg·L -1 at both the end of stage I and II in 2004. GA 3 and GA 4+7 applied at 100 mg·L -1 reduced bloom density similarly by 65%. GA 3 was more inhibiting than GA 4+7 at 200 mg·L -1 , reducing bloom density by 92% versus 68%. We observed a 4- to 5-day delay in flowering from both GA formulations at 200 mg·L -1 . At both concentrations, GA 3 reduced yield by 71% and 95% versus 34% and 37% reduction by GA 4+7 . Fruit weight and soluble solids were unaffected but fruit firmness was increased by all treatments (6% to 17%). However, crop value per tree was highest from untreated control because improvements in fruit quality were insufficient to offset reductions in yield. GA 3 shows potential as a novel crop load management tool in productive 'Bing' sweet cherry orchard systems.

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Gibberellic Acid Inhibits Floral Bud Induction and Improves `Bing' Sweet Cherry Fruit Quality

Mazzola, M., Granatstein, D. M., Elfving, D. C., Mullinix, K., and Gu, Y.-H. 2002. Cultural management of microbial community structure to enhance growth of apple in replant soils. Phytopathology 92:1363-1366. Apple replant disease typically is managed through pre-plant application of broad-spectrum soil fumigants including methyl bromide. The impending loss or restricted use of soil fumigants and the needs of an expanding organic tree fruit industry necessitate the development of alternative control measures. The microbial community resident in a wheat field soil was shown to suppress components of the microbial complex that incites apple replant disease. Pseudomonas putida was the primary fluorescent pseudomonad recovered from suppressive soil, whereas Pseudomonas fluorescens bv. III was dominant in a conducive soil; the latter developed within 3 years of orchard establishment at the same site. In greenhouse studies, cultivation of wheat in replant orchard soils prior to planting apple suppressed disease development. Disease suppression was induced in a wheat cultivar-specific manner. Wheat cultivars that enhanced apple seedling growth altered the dominant fluorescent pseudomonad from Pseudomonas fluorescens bv. III to Pseudomonas putida. The microbial community resident in replant orchard soils after growing wheat also was suppressive to an introduced isolate of Rhizoctonia solani anastomosis group 5, which causes root rot of apple. Incorporation of high glucosinolate containing rapeseed (‘Dwarf Essex’) meal also enhanced growth of apple in replant soils through suppression of Rhizoctonia spp., Cylindrocarpon spp., and Pratylenchus penetrans. Integration of these methods will require knowledge of the impact of the biofumigant component on the wheat-induced disease-suppressive microbial community. Implementation of these control strategies for management of apple replant disease awaits confirmation from ongoing field validation trials.

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PHYTO.2002.92.12.1363

Cultural management of microbial community structure to enhance growth of apple in replant soils.

Three commercial ‘Lapins’ sweet cherry (Prunus avium) orchards were used for this study during three crop seasons. Orchards were selected based on the historical average date of commercial harvest. The difference in commercial harvest date among the three orchards was 5 to 7 days. Three harvests were carried out in each orchard each year: 1) beginning 4 to 5 days before commercial harvest, 2) at commercial harvest, and 3) 4 to 5 days after commercial harvest. Fruit quality was determined after 0, 7, 14, and 21 days of storage. Harvesting fruit up to 5 days later than normal commercial harvest resulted in increases in fruit weight and soluble solids content along with no loss of firmness or change in acidity. Pedicel color did not change as harvest was delayed. Changes in visual ratings of both fruit and pedicel appearance with delayed harvest were detectable in only 1 of 3 years. Neither pitting nor bruising was influenced by harvest date. The amount of pitting or bruising present was related more to the year of harvest than to harvest date. Delaying harvest a short time beyond the normal commercial harvest date could enhance consumer appeal and increase fruit value. Storage time after harvest resulted in reduced fruit and pedicel appearance, but only beyond 14 days of storage.

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D.C. Elfving

Papers

Suppression of specific apple root pathogens by Brassica napus seed meal amendment regardless of glucosinolate content.

Preharvest Applications of Sprayable 1-methylcyclopropene in the Orchard for Management of Apple Harvest and Postharvest Condition

Gibberellic Acid Inhibits Floral Bud Induction and Improves `Bing' Sweet Cherry Fruit Quality

Cultural management of microbial community structure to enhance growth of apple in replant soils.

Indicators of Maturity and Storage Quality of `Lapins' Sweet Cherry