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

Phytophthora capsici was first described by Leon H. Leonian at the New Mexico Agricultural Research station in Las Cruces in 1922 (65). In his report, he described a novel species of Phytophthora that caused considerable damage to chili pepper plants in the fall of 1918. A year later, the disease reappeared at the same site and also affected surrounding farms. During the late 1930s and early 1940s, recurrent problems with P. capsici in the Arkansas River Valley of Colorado were described on several vegetable hosts (51– 55,103). The first reported occurrence of P. capsici on a cucurbit crop occurred in 1937, when a 3.2-ha field of cucumbers became diseased resulting in 100% of the fruit rotting (51). By 1940, P. capsici had also been described on eggplant, honeydew melon fruit, summer squash, and tomato fruit (52,103). The disease on tomatoes was reportedly so severe that the viability of the processing tomato industry in the region was threatened. These early reports mirror the situation with P. capsici today on many modern vegetable production farms, especially those in the eastern United States (4,72,84,94). Our research was initiated in 1997, when crop losses caused by P. capsici threatened to bankrupt a number of vegetable producers in Michigan. Growers wanted to know why crop rotation and the use of fungicides in well-drained fields had not provided adequate protection against full-scale epidemics. At that time, there were fundamental gaps in our understanding of P. capsici’s epidemiology in Michigan, and it was difficult to answer these questions with any degree of certainty. We did not recognize the extent to which sexual recombination and genetic diversity could influence management options and success. In particular, the fungicide mefenoxam was being applied by some growers, and the sensitivity of natural populations of P. capsici in Michigan to mefenoxam was unknown at that time. Here we review recent advances in our understanding of P. capsici’s biology, in particular the role of sexual reproduction, and provide an overview of some of the management challenges presented by this information.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS.2004.88.12.1292

Phytophthora capsici on Vegetable Crops: Research Progress and Management Challenges.

Phytophthora blight, caused by the oomycete pathogen, Phytophthora capsici, is a devastating disease on bell pepper and cucurbit crops in the United States and worldwide (29,40). P. capsici causes a root and crown rot, as well as an aerial blight of leaves, fruit, and stems, on bell pepper (Capsicum annuum), tomatoes, cucumber, watermelon, squash, and pumpkin (29,35, 40,57,73). The disease was first described on bell pepper in New Mexico in 1922 (40). In recent years, epidemics have been severe in areas of North Carolina, Florida, Georgia, Michigan, and New Jersey. Oospores are believed to provide the initial source of inoculum in the field, and the disease is polycyclic within seasons (1,7,59,60,67). In this article, we discuss the biology and epidemiology of Phytophthora blight on bell pepper and also describe management strategies that can be implemented based on existing knowledge of the ecology of this devastating pathogen. The objectives of ecologically based pest management (EBPM) are the safe, profitable, and durable management of pests that includes a total systems approach (25). EBPM relies primarily on biological input of knowledge concerning a pathogen life cycle, and secondarily, when necessary, on physical, chemical, and biological supplements for disease management. An understanding of the ecological processes that are suppressive to plant diseases is emphasized rather than secondary inputs (25). Fortunately, we have a considerable amount of information available on the biology and ecology of P. capsici, which can now be integrated to improve our ability to manage the disease using ecologically based approaches. Strategies recommended for management of Phytophthora blight involve integrated approaches that focus first on cultural practices that reduce high soil moisture conditions, but also include monitoring and reduction of propagules of P. capsici that persist in the soil, utilization of cultivars with resistance to the disease, and when necessary, judicious fungicide applications. Symptoms and Life Cycle P. capsici can infect virtually every part of the pepper plant. The pathogen causes a root and crown rot on pepper (Fig. 1) and also forms distinctive black lesions on the stem (Fig. 2). P. capsici can also infect the leaves and causes lesions that are circular, grayish brown, and water-soaked (Fig. 3). Leaf lesions and stem lesions are common when inoculum is splash dispersed from the soil to lower portions of the plant. The pathogen can also infect fruit and causes lesions that are typically covered with white sporangia, a sign of the pathogen (Fig. 4). P. capsici typically causes a fruit rot or stem rot on cucumbers and squash (Fig. 5). P. capsici reproduces by both sexual and asexual means (Fig. 6). The pathogen produces two mating types, known as the A1 and A2. These are actually compatibility types and do not correspond to dimorphic forms. Each mating type produces hormones that are responsible for gametangia differentiation in the opposite mating type. Both A1 and A2 mating types are common in fields in North Carolina and have also been identified within the same plant (59). P. capsici produces a male gametangium, called the antheridium, and a female gametangium, called the oogonium. The antheridium is amphigynous in this species. Meiosis occurs within the gametangia, and plasmogamy and karyogamy result

https://apsjournals.apsnet.org/doi/pdfplus/10.1094/PDIS.1999.83.12.1080

Ecologically Based Approaches to Management of Phytophthora Blight on Bell Pepper

Incidence of Phytophthora blight in bell pepper fields that were sprayed for the first time with Ridomil Gold (mefenoxam) according to labeled recommendations was higher in North Carolina in 1997 than in previous years. Mefenoxam is the more active enantiomer contained in the racemic fungicide metalaxyl. A total of 150 isolates were obtained from 17 fields at eight grower locations. Among isolates from all locations, 30% were classified as sensitive, 10% as intermediate, and 59% were resistant to mefenoxam. Mefenoxam-resistant isolates were found in 82% of the fields sampled (14 of 17 fields). The proportion of resistant isolates in individual (fields ranged from 28 to 100%. The mean effective concentration (EC50) values for mefenoxam-sensitive isolates was 0.568 μg ml-1 (ranging from 0.12 to 1.1 μg ml-1), whereas the mean EC50 value for mefenoxam-resistant isolates was 366.5 μg ml-1 (ranging from 3 to 863 μg ml-1). The mean EC50 value for metalaxyl-sensitive isolates was 0.27 μg ml-1 (ranging from 0.00002 to 1.3 μg ml-1) and for metalaxyl-resistant isolates was 470.34 μg ml-1 (ranging from 10 to 966 μg ml-1). The greatest proportion of resistant isolates came from fields where mefenoxam was used alone rather than in combination with other fungicides. Both mating types were found among resistant isolates, suggesting that these isolates may persist in soil in subsequent years. Field isolates of Phytophthora capsici resistant to mefenoxam on pepper have not been reported previously and now pose new challenges for management of this important disease.

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Resistance to Mefenoxam and Metalaxyl Among Field Isolates of Phytophthora capsici Causing Phytophthora Blight of Bell Pepper.

One stop mycology

Spatial patterns of disease, soil population levels of Phytophthora capsici, and soil water content were characterized during epidemics of Phytophthora root and crown rot using geostatistical techniques in three commercial bell pepper fields in two consecutive years. Disease incidence and severity increased with time in all fields, ranging from a maximum linear rate of 1.01% per day and 0.060 severity units per day (0-6 scale) in the lower portion of field three in 1992 to a minimum rate of 0.33% per day and 0.011 severity units per day in the upper portion of field three in 1993. Semivariograms constructed over four directions of orientation (0°, 45°, 90°, and 135°) were used to determine the spatial dependence of mean disease severity by quadrat (1 m 2 ) in each field over time. In most fields, disease severity displayed strong spatial dependence and a high degree of anisotropy over time, indicating strongly aggregated patterns of disease with distinct directional orientation. The within-row (0°) orientation generally was the direction of maximum spatial correlation and the predominant direction of disease spread. Diagonal directions also demonstrated considerable spatial dependence in most fields, whereas spatial dependence in the across-row direction varied with soil water conditions and was lower than other directions in three of four fields. The range of spatial dependence for final disease severity averaged 15 m or greater within rows and 3-15 m across rows for fields one and two in 1992 and the lower portion of field three in 1992 and 1993. Disease severity displayed little spatial dependence in field one in 1993 and the upper portion of field three in both years. Two-dimensional distance class analysis indicated nonrandom spatial patterns of disease incidence in all fields, as well as an increasing level of aggregation of disease over time and greater spread of disease within rows than across rows in most fields. Patterns of soil water content also were spatially dependent and correlated with patterns of disease severity in fields that had distinct moisture gradients. Early season population levels of P. capsici, as estimated by either a leaf disk bioassay or soil dilution plating, did not consistently demonstrate strong spatial dependence and were not closely associated with the occurrence or severity of disease throughout the season. These results emphasized the importance of soil water in the development and spread of disease caused by P. capsici in bell pepper

Geostatistical analysis of Phytophthora epidemic development in commercial bell pepper fields.

Epidemics in bell pepper caused by Phytophthora capsici were monitored in three commercial fields to characterize the spatial pattern of disease and to gain a preliminary indication of dispersal mechanisms that may influence spatial disease progress. Disease incidence increased from 3.8 to 35.8% and from 13.6 to 38.5%, and the final percentage of quadrats with plants with wilt, crown lesions, stem lesions, or dead plants was 15.8, 16.8, 15.5, and 15.3 in field one and 16.5, 23.5, 10.5, and 31 in field two, respectively. In field three, disease incidence increased from 15.9 to 22.6% in the upper portion of the field and from 53.1 to 67.3% in the lower portion of the field [...]

Spatial and temporal dynamics of Phytophthora epidemics in commercial bell pepper fields

Several assay methods were compared for their efficacy in the detection and quantification of specific propagule types of Phytophthora capsici in soil. Zoospores, oospores, or sporangia and mycelial fragments were added to microwave-treated soil and nontreated field soil at densities from 1 to 1 x 10 4 propagules per g (ppg) of soil. Assay methods included standard soil dilution plating on selective medium without prior sample incubation, as well as dilution plating of soil, saturation water, or a pepper leaf disk bioassay after sample saturation, drainage, incubation for 5 days, and a 24-h resaturation period. Zoospore inoculum was detected at 10 ppg of soil or higher with standard soil dilution plating and the leaf disk bioassay, compared to >100 ppg of soil with dilution plating of soil or saturation water after sample incubation. Sporangial inoculum was detected at 1 ppg of soil with all assays when soil water matric potential was controlled during sample incubation. Oospores were detected at 1 ppg of soil with soil dilution and leaf disk assays after sample incubation. Maximum recovery rates were 10 and 100% of added zoospore and sporangial inoculum, respectively, with standard soil dilution plating (no incubation), and 30% of oospore inoculum with soil dilution plating after sample incubation. A constant soil water matric potential of -10 J/kg during the sample incubation period improved inoculum recovery with all assays, compared to incubation without controlled soil water matric potential. The sample incubation and saturation periods stimulated oospore germination and allowed detection and recovery of oospores added to soil. For most assay methods, recovery of all propagule types was lower in field soil than in microwave-treated soil ; however, the lower limits of detection were comparable in both soils. Although no single assay method was suitable for the accurate detection and quantification of all propagule types of P. capsici, the leaf disk assay provided the best detection of all propagule types but was not sufficiently quantitative to estimate inoculum densities. Soil dilution plating without prior sample incubation was the best assay for the quantification of zoospores and sporangia, whereas soil dilution plating after sample saturation and incubation provided the best recovery of oospores.

Detection and quantification of Phytophthora capsici in soil.

Epidemics caused by Phytophthora capsici were monitored in three commercial fields of bell pepper (Capsicum annuum) in order to characterize the spatial progression of symptom expression over rime and to provide evidence for possible mechanisms of inoculum dispersal. Spatial dynamics were characterized using two dimensional distance class analysis. Symptom expression was nonrandom in each field, and quadrats containing dead plants or plants with wilt, crown lesions, or stem lesions were clearly aggregated. Wilting preceded crown symptoms, suggesting that root infection and subsequent colonization to crowns of plants occurred most frequently. Large clusters of dead plants or plants with wilt or crown lesions were observed, and these clusters changed in size over time

R. P. Larkin

Papers

Geostatistical analysis of Phytophthora epidemic development in commercial bell pepper fields.

Spatial and temporal dynamics of Phytophthora epidemics in commercial bell pepper fields

Detection and quantification of Phytophthora capsici in soil.

Spatial dynamics of disease symptom expression during Phytophthora epidemics in bell pepper