Kühn is a persistent pathogen of potato (Solanum tuberosum
L.) that causes stem and stolon canker and is also responsible for black scurf, a tuber disease caused by the formation of sclerotia, the long-term survival structures of the pathogen, on newly formed tubers [1
]. These symptoms result in reduced tuber quality and yield. Rhizoctonia solani
is classified into numerous discrete Anastomosis Groups (AGs), and AG-3 is the predominant AG responsible for Rhizoctonia disease of potato in the U.S. and throughout most of the world [1
]. Current cultural and chemical controls, including crop rotation, reduced time to emergence, and fungicide seed treatments, are not always practical or effective, and Rhizoctonia disease of potato continues to be a serious problem wherever potatoes are grown [1
Effective, alternative disease control options that are consistent with sustainable production practices are needed. Improved crop rotations and biocontrol organisms represent an integrated approach that may effectively reduce Rhizoctonia disease, improve yield, and contribute to agricultural sustainability and environmental quality. Crop rotations can suppress disease by allowing inoculum levels to decline in the absence of the host, directly inhibiting the pathogen by producing a toxic compound, increasing specific antagonists that inhibit the pathogen, or by increasing general microbial populations that compete with the pathogen [5
]. Crop plants are primary drivers of changes in soil microbial communities and may be responsible for changes that increase soil microbial activity, diversity, populations of plant-beneficial organisms, and antagonism towards pathogens, potentially resulting in disease suppression and improved yield [8
]. Previous research in potato systems has demonstrated that each type of rotation crop produces distinctive changes in soil microbial community characteristics, as well as that these characteristics may be related to disease characteristics [6
Presently, barley (Hordeum vulgare
L.) or oats (Avena sativa
L.) underseeded with a cover crop of red clover (Trifolium pratense
L.) in a 2-year rotation with potato is a standard rotation practiced on many commercial farms in Maine [13
]. Barley rotations can suppress Rhizoctonia diseases to some degree [6
]; however, more effective cover crops may be available for use in conjunction with barley [14
]. Legume rotations have been implicated in decreased yields of subsequent crops, mainly as a result of increases in disease by R. solani
]. Observations in the field have suggested that ryegrass (Lolium multiflorum
Lam.), and possibly a particular variety known as “Lemtal”, may be more effective as a cover crop with barley for suppressing stem canker and black scurf, but this has not been sufficiently documented and tested.
Another potentially sustainable method of disease suppression is the addition of microbial antagonists. Many biocontrol organisms have shown suppressive activity towards R. solani
in various pathosystems [16
], as well as on potato [19
], but disease control has not been consistent. Previously, Brewer and Larkin [22
] evaluated many of these organisms against Rhizoctonia disease on potato in greenhouse trials and observed strains of the rhizosphere bacteria Bacillus subtilis
and the fungi Trichoderma virens
and Laetisaria arvalis
to be among the most effective biocontrol organisms tested, reducing stem canker and black scurf by 30% to 60%. In subsequent trials, commercially available formulations of B. subtilis
and T. virens
provided some control of Rhizoctonia disease of potato in the field [23
]. One strategy to improve disease control is to combine the use of biocontrol organisms with more effective crop rotations [22
]. Different crop rotations and biological control treatments have been shown to have significant and distinctive effects on soil microbial communities [5
]. These changes may include overall increases in microbial populations, population changes within specific groups of organisms, or changes in the structural and functional characteristics of the soil microbial communities. In addition, there may be significant interactive effects of the rotations and treatments on each other, as well as on the soil microbial communities and disease relationships. Biological control may be enhanced or reduced depending on the specific rotation [28
Another important soilborne potato disease that occurs wherever potatoes are grown is common scab, caused by the actinomycete Streptomyces scabies
]. Common scab also results in tuber surface lesions that can reduce tuber quality and yield [4
]. Several biocontrol organisms have also shown activity against this pathogen [29
] Although the focus of this investigation is on Rhizoctonia disease, because common scab routinely occurs in Maine potato fields, potential treatment effects on common scab will also be evaluated.
Traditionally, soil dilution plating on artificial media was used to estimate populations of different components of soil microbial communities, but this approach has many limitations, such as only measuring the small proportion of microbes that are culturable on media, generally not distinguishing active organisms from dormant propagules, and not assessing soil microbial communities as a whole [31
]. Community level approaches that include the full range of soil microbes are needed for assessing the changes and effects on soil microbial communities. The use of whole soil fatty acid methyl ester (FAME) analysis, which is based on profiling the array of fatty acids produced by soil microorganisms, is one such approach that accounts for a much broader array of microbes, is very sensitive in detecting changes in soil microbial communities, and is a relatively quick and simple technique to compare and evaluate soil microbial communities [6
]. Although this technique cannot identify the specific taxa composition of the microbial communities, it can effectively show changes in community structure associated with different cropping systems and the addition of biological amendments [11
The purpose of this research was to assess whether the integrated use of a specific crop rotation, barley and ryegrass, and selected biocontrol organisms could effectively reduce Rhizoctonia disease of potato in the field, as well as to evaluate their interactions and effects on soil microbial communities. The biocontrol organisms studied were T. virens GL-21, L. arvalis ZH-1, and B. subtilis GBO3.
2. Materials and Methods
2.1. Inoculum Preparation
Inoculum of Rhizoctonia solani
RS31B, a virulent isolate of AG-3 recovered from a potato plant in Maine [22
], was used for all experiments. Inoculum was prepared by transferring four plugs of potato dextrose agar (PDA, Difco Laboratories, Inc., Sparks, MD, USA) containing 7-to 14-day-old cultures of R. solani
to petri dishes filled with 20 to 30 g of sterile organic cracked wheat. The grain was prepared by adding 3 mL deionized water per 5 g of organic cracked wheat and autoclaving for 60 min on two consecutive days [22
]. The inoculated wheat was incubated at room temperature (21 to 25 °C) in the dark for 8 days, then air dried in a sterile hood for 48 h, passed through a 2.0 mm sieve, and stored in a paper bag at 4 to 5 °C for no more than 1 month, until needed. Viability of inoculum was confirmed by plating on PDA.
2.2. Greenhouse Rotation Trials
Crop treatments tested in the greenhouse experiments included two varieties of annual ryegrass (“Lemtal” and a generic ryegrass just referred to as common), barley (“Robust”), red clover (“Cinnamon”), barley and Lemtal ryegrass together, barley and red clover together, and potato. Potato variety “Shepody” was used for all experiments, as it is susceptible to multiple soilborne diseases, such as Rhizoctonia stem canker and black scurf, and common scab. Field soil from Newport, ME, a Nokomis sandy loam (coarse-loamy, mixed, frigid Typic Haplorthod), was used for these experiments. The soil was sieved through a 6 × 6 mm screen and combined with sterile sand at a 3:1 w/w soil to sand ratio. Pathogen inoculum was incorporated with the soil mix at a rate of 4 g per kilogram and incubated for 24 h at room temperature.
Two kilograms of pathogen-infested soil mix was added to each 25 cm × 17 cm × 8 cm plastic potting tray and trays were seeded in the following amounts for each respective rotation treatment: annual ryegrass, 0.67 g; barley, 4 g; clover with Nitro-Fix clover/alfalfa inoculant (Trace Chemicals LLC, Pekin, IL, USA), 0.5 g; and potato, four cut seedpieces treated for 2 min in 2% formaldehyde [22
] and green-sprouted for two weeks [35
]. These seeding rates were chosen because they resulted in dense growth of each crop. Rates used in the greenhouse studies were several times higher than those used in commercial fields in order to maximize short-term cropping effects. The experimental design was a randomized complete block with four replications per treatment (each potting tray was a replicate), and the experiment was repeated following the same protocols.
Crops were grown in the greenhouse for approximately one month under normal greenhouse conditions (sunlight, regular watering, 15–27 °C), and the treatments containing barley, ryegrass, or clover were cut to soil level and replanted to keep crop growth active. All crops were grown for approximately one more month so that the length of active crop growth was comparable to that of field crops in Maine. Crop biomass from all treatments was cut and incorporated into the soil by hand and, approximately one month later, four potato seedpieces, treated as described above, were planted and grown for approximately 3 weeks.
Soils were sampled after infestation with R. solani
, after two months of crop growth, and one month after crop incorporation to determine the effects of crop growth and incorporation on populations of R. solani
. Six cores (8 cm deep × 2 cm diameter) were taken and combined into one composite sample for each potting tray (four potting trays per treatment). Samples were plated on five plates of semi-selective media [36
] amended with neomycin sulfate (700 mg/L), pyroxychlor (90 mg/L), and benomyl (2 mg/L) using the soil pellet sampler method [37
]. Plates were incubated at room temperature for 24 h, and Rhizoctonia-like mycelia emanating from the soil pellets were enumerated by observation at 10× to 20× magnification using a dissecting microscope. The total number of R. solani
propagules per gram of soil was calculated from the percent of pellets colonized. This number was subjected to the multiple colonization correction formula [38
After 3 weeks of growth, total plant emergence was noted and all potato plants were harvested, washed, and rated for stem canker on a scale of 0 to 5 (0 = no disease symptoms; 1 = brown discoloration of stems; 2 = cankers covering <25% of the stem circumference; 3 = 25 to 75% coverage by cankers; 4 = >75% coverage by stem cankers; and 5 = stem completely nipped off or death of the plant). Differences in potato shoot size were detected, so shoots were also rated for size on a scale of 0 to 3 (0 = no growth; 1 = shoot height <2 cm; 2 = shoot height of 2 to 5 cm; and 3 = shoot height >5 cm).
2.3. Field Set-Up and Design
The field experiment was conducted over two field seasons on research plots in Newport (location 1) and Presque Isle (location 2), Maine. The Newport site is located in central Maine (N 44°52′, W 69°17′) and the Presque Isle site is in northern Maine (N 46°38′, W 68°00′), approximately 300 km northeast of the Newport site. Soil at the Newport site is a Nokomis sandy loam (coarse-loamy), and soil at the Presque Isle site is a Caribou sandy loam (fine-loamy), and both are mixed, frigid Typic Haplorthods. The Newport site had previously been planted to millet, ryegrass, or was fallow the previous 5 years, with potatoes grown within the past 10 years. The Presque Isle site had been in sod the previous 3 to 9 years, but it also had potatoes grown within the past ten years. All procedures were conducted similarly at the two locations, except that planting, harvesting, and sampling was approximately one week later for location 2 than location 1. The experimental design was a randomized complete block split-plot with four replicate plots per treatment. Three rotation crop treatments, barley underseeded with “Lemtal” ryegrass, barley underseeded with red clover, and potato, were planted in early June of the first year and followed by potato planted in early June of the second year. The main plots (18 m × 3.6 m) contained the rotation crops, and the subplots (4.5 m × 3.6 m, four potato rows each) consisted of three different biocontrol treatments applied to the potato crop and a control plot with no biocontrol amendment. Prior to planting the rotation crops, R. solani inoculum was added to all field plots at a rate of 64 kg/ha and incorporated into the soil approximately 15 cm by disk or harrow.
Barley was planted at a rate of 135 kg/ha with 45 kg N/ha added using a commercial 10-10-10 fertilizer. The cover crops, clover and ryegrass, were planted with the barley at rates of 17 kg/ha and 23 kg/ha, respectively. Nitro-Fix clover/alfalfa inoculant (Trace Chemicals LLC) was added to the clover seed prior to planting. The potatoes were hand-planted with 35 cm spacing within rows and 0.9 m spacing between rows and fertilized at a rate of 169 kg N/ha. Imidacloprid (Bayer CropScience, Research Triangle Park, NC, USA) and chlorothalonil (Syngenta Crop Protection, Greensboro, NC, USA) were applied to potato plots at recommended rates as needed for control of Colorado potato beetle and late blight (Phytophthora infestans), respectively. Crops were managed and cultivated using recommended production practices.
The barley was harvested in early fall, with the cover crops allowed to overwinter and then incorporated by chisel plow the following spring one month prior to potato planting. Biocontrol treatments, including Bacillus subtilis GBO3 (Kodiak, Bayer CropScience, Research Triangle Park, NC, USA), Trichoderma virens GL-21 (SoilGard, Certis USA, Columbia, MD, USA), or Laetisaria arvalis ZH-1 (ATCC #62715), were added by hand as in-furrow amendments to the potato crop at planting. No organisms were added to the control plot. Potato plots were cultivated and maintained as previously described. In early October, six linear meters of tubers were harvested by hand from the middle two rows of each plot.
2.4. Biocontrol Treatment Preparation
B. subtilis inoculum was prepared by adding deionized water to plates of the isolate grown on 0.1% tryptic soy agar (Difco Laboratories, Franklin Lakes, NJ, USA). The slurry was transferred to 4 L of trypticase soy broth (Difco Laboratories) and incubated on a platform shaker for three days at 28 °C. The broth was added to 6 L of deionized water, and the preparation was added in-furrow at a rate of approximately 104 CFU/cm3 soil (109 CFU/linear meter). T. virens was added in-furrow as the SoilGard formulation (Certis USA), as directed by the manufacturer at a rate of 0.1% (g/cm3). Four plugs of L. arvalis cultures actively growing on PDA were added to petri dishes of sterile cracked wheat, prepared as mentioned earlier, and grown at 26 °C for 5 days. The inoculum was dried at room temperature (21 to 25 °C) for 24 h and added at a 0.1% (g/cm3) incorporation rate. All treatments were used in the field immediately after preparation.
2.5. Disease and Yield Assessments
Shoot emergence was determined by counting the number of plants visible in the middle two rows of each four-row plot over several weeks in late June and early July. Stem canker was assessed in mid-August. Two plants were harvested from each of the first and fourth rows of every plot, and the stems of the plants were rinsed with water to remove soil so that cankers would be visible. Stems were rated on a scale of 0 to 5, as described for the greenhouse trials. When two or more shoots emerged from one seed piece, the average rating of all shoots was recorded.
After harvest, tubers were washed and graded into three categories based on size, and total and marketable yield were determined. The categories consisted of small (<4.7 cm), medium (4.7 to 5.6 cm), and large (>5.6 cm) tuber sizes. Marketable yield included the medium and large size classes. In addition, the percentage of misshapen tubers (by weight) was determined. Thirty tubers from the marketable yield category in each plot were arbitrarily chosen and rated for black scurf and common scab, if present, based on the percent surface area of each tuber covered by sclerotia or lesions.
2.6. Soil Rhizoctonia Populations
Initial soil samples were taken prior to and after the addition of R. solani
inoculum in May, in late-June after emergence of the crops, and in mid-August in year 1. In the following year (year 2), soil samples were taken in May prior to incorporation of the cover crops, in June after incorporation and immediately prior to potato planting, and in mid-August during the potato growing season. Samples consisted of 10 cores (15 cm deep by 2 cm diameter) taken from each plot and combined into one composite sample per plot. Samples were sieved (3.35 mm mesh) and kept at 5 °C until processed. Inoculum levels of R. solani
were determined for all soil samples taken in year 1 and the two samples taken prior to planting in year 2 by the soil pellet sampler method [37
], as described previously.
2.7. Analysis of Microbial Communities Using FAME Profiles
Whole soil fatty acid methyl ester (FAME) analysis was used to evaluate changes and differences in soil microbial communities. This analysis was performed on the initial soil samples of year 1 to obtain base level fatty acid data. The two sets of samples taken prior to planting in year 2 were analyzed to observe changes in the microbial communities prior to and after incorporation of crop residues. The mid-August soil samples from year 2 were also assayed to determine effects of both previous rotation crops and biocontrol organisms on soil microflora. Fatty acids were extracted from three soil subsamples per plot (4 g per subsample, four plots per treatment) according to a modification of the Microbial Identification System (MIS, MIDI, Inc, Newark, DE, USA), as previously described by Larkin [23
]. Fatty acid composition was determined by gas chromatography using an automated procedure by MIDI on an HP 6890 gas chromatograph (Hewlett-Packard, Wilmington, DE, USA). Fatty acids were identified by software developed for the MIS. Only fatty acids that accounted for at least 0.25% of the total fatty acid content were used for analysis [27
]. In addition, dicarboxylic acids and those with a chain length greater than 20 carbons were not included in the analyses. With these criteria, analyses consisted of 41 to 43 unique fatty acids. Fatty acids were also categorized by structural classes, including saturated straight chain, monounsaturated, polyunsaturated, branched, and hydroxy fatty acid classes. These classes and select individual fatty acids were used as indicators (biomarkers) for particular microorganism groups [6
2.8. Statistical Analysis
All significant treatment effects were detected using analysis of variance and means were separated using Fisher’s least significant difference at p
= 0.05. Fatty acid data was analyzed by principal components analysis (PCA) and canonical variates analysis (CVA), which maximizes group differences [39
]. Principal components and canonical variates were subjected to multivariate analysis of variance and one-way ANOVA was run on individual components and key fatty acid groups. SAS (ver. 9.4, SAS Institute, Cary, NC, USA) general linear models procedures were used to carry out statistical analyses.
This research demonstrated the potential benefits of replacing clover as a cover crop in barley rotations with ryegrass, for better control of Rhizoctonia disease and improved potato yield. This research also documented the complex interactions among crop rotations, biocontrol treatments, and soil microbial communities and the potential for improved integrated disease management with effective combinations of crop rotations and biocontrol organisms.
Ryegrass rotations reduced severity of Rhizoctonia stem canker alone in the greenhouse and at one of two field locations when combined with barley. An increase in tuber yield with the barley/ryegrass rotation also occurred at the same location where disease reductions were detected. Stem canker can result in severe losses in yield [41
], and the reductions in stem canker by the barley/ryegrass rotation may have led to the increase in yield. Reductions in severity were not detected at one of the two locations. It is hypothesized that this might be because both the ryegrass and clover cover crops were not fully established the following spring due to a hard winter and severe weed pressure at that location that substantially reduced cover crop stands from the previous fall. It has been well-established that cover crop biomass production is integral to effects on soil properties [42
]. As for whether the “Lemtal” ryegrass variety performed better than common ryegrass, based on the greenhouse tests, both types resulted in comparable suppression of Rhizoctonia disease, with no indication of any significant difference in their ability to reduce disease or their suitability as a cover crop. Previous indications of the efficacy of Lemtal ryegrass were based primarily on anecdotal observations of successful plantings, with no direct comparisons among different ryegrass varieties. Thus, there is no direct evidence that Lemtal ryegrass provides any better disease control than other ryegrass varieties.
This study indicates that ryegrass is a more suitable cover crop than clover for control of Rhizoctonia disease of potato. In the greenhouse, a rotation with clover alone increased R. solani
populations as much as or more than potato rotations. These increases in populations and disease levels by clover and other legumes have been observed in other studies [6
]. R. solani
AG-3 can produce stem lesions on clover in controlled studies, and leguminous crops, including clover, have shown increases in lesions caused by R. solani
on subsequent susceptible crops [14
]. However, one previous study observed lower soilborne disease levels for black scurf and common scab with a red clover rotation relative to barley and ryegrass, even though red clover resulted in higher plant-parasitic nematode populations and lower tuber yield than ryegrass [45
]. That study involved somewhat different soil and environmental conditions than the present study. The beneficial effects observed with the barley/ryegrass rotation were not observed with barley/clover in the present research. The barley/ryegrass combination can potentially provide better disease control, as was also demonstrated in a previous direct comparison of these combinations [14
A possible mechanism of disease suppression by ryegrass is that it stimulates microbial activity [42
]. In addition, the ryegrass rotation could stimulate specific antagonists that reduce disease. In the present study, ryegrass rotations increased populations of R. zeae
in the greenhouse. Although R. zeae
has mostly been considered a pathogen of turfgrasses [46
], isolates recovered in Maine did not cause disease symptoms on either ryegrass or barley (Brewer, unpublished). R. zeae
can cause some lesions on potato, but to a much lesser extent than R. solani
]. R. zeae
has shown potential biocontrol activity against R. solani
and may compete with R. solani
for colonization or infection of roots [49
]. In previous greenhouse biocontrol trials, an isolate of R. zeae
from this field location was among the most effective biocontrol isolates tested for reducing both stem canker and black scurf [22
]. Additional tests need to be conducted to determine the role, if any, of R. zeae
and suppression of Rhizoctonia disease by ryegrass rotations.
The barley/ryegrass rotations did not reduce black scurf in the present study. Differences in black scurf among rotations may not have been observed in the present study because severity levels were relatively low. In addition, disease measurements were taken after only one complete rotation cycle, and it may take more rotation cycles before differences can be detected. Cover crop effects on soil properties often take several seasons to fully develop, with longer-term studies generally showing greater effects than short-term trials [45
]. In a separate field study, in Maine, where a barley/ryegrass rotation was directly compared to a barley/clover rotation, a significant reduction in black scurf, as well as common scab, was observed with the barley/ryegrass rotation over multiple field seasons [14
]. More recent ongoing trials have also indicated consistent reductions in black scurf with barley/ryegrass rotations relative to some other rotations [51
FAME profiles, as represented by canonical variates analyses and the proportions of fatty acid structural classes and biomarkers, revealed distinct differences for all rotations, indicating that specific changes in soil microbial community characteristics occurred based on the preceding crop. This supports the growing body of literature demonstrating the considerable influence of plants and plant residues on shaping the soil microbiome [8
]. Many fatty acids and ratios of fatty acids have been identified as particularly useful biomarkers [30
]. The proportion of monounsaturated fatty acids was higher in the barley/ryegrass rotation than the other rotations, particularly after residue incorporation, and the ratios of monounsaturated to saturated fatty acids were higher in both barley rotations than continuous potato. Monounsaturated fatty acids are a biomarker for gram negative bacteria, as well as some fungi (16:1 ω5c and 18:1 ω9c), and these fatty acids tend to increase in soil with aerobic conditions, organic inputs, and high substrate availability [6
]. The ratio of monounsaturated to saturated fatty acids has been used as an indicator to evaluate communities and environmental conditions, with observed ratios of greater than 1 for cultivated soils with good C content and organic inputs, and values less than 1 representing low substrate, low organic input soils [53
]. Our results showing lower values for the potato rotation than the barley rotations are consistent with other studies, which have also shown even greater differences (with continuous potato values decreasing, barley and other rotation values increasing over time) the longer the rotations are in place [6
The proportions of polyunsaturated fatty acids, which are associated with fungi, were also higher in the barley rotations than the potato rotation, and also tended to increase after crop residue incorporation. Hydroxyl group fatty acids are indicators of gram-negative bacteria, whereas branched fatty acids are indicators of gram-positive bacteria [27
]. Results of this study indicate that the proportion of gram-positive bacteria was higher in continuous potato than the barley rotations, and the ratio of fungi to bacteria was higher in the barley rotations, especially barley/ryegrass, than the continuous potato rotation. These results are similar to those observed between grass-grain rotations and continuous potato treatments in other crop rotation studies in Maine [6
FAME profiles also revealed differences in soil microbial communities associated with different biocontrol treatments at both locations. However, the biocontrol treatments affected FAME profiles differently within each rotation, resulting in interactions between the rotations and biocontrol treatments on their effects. Interestingly, although, for the most part, rotations appeared to exert a greater effect on the soil microbial communities than the biocontrol treatments, at location 2, the B. subtilis
treatment resulted in similar microbial characteristics regardless of the rotation, indicating that the addition of B. subtilis
had a greater effect on defining the microbial characteristics than rotations for that soil. Proportions of fatty acid structural classes, class ratios, and biomarkers were also affected by biocontrol treatments. At location 1, fatty acid classes and ratios associated with the biocontrol treatments varied considerably among rotations, demonstrating the complex effects among rotations and biocontrol treatments on soil microbial communities. Previous research has documented that biocontrol organisms can be greatly affected by the indigenous microbial communities present but can also have significant effects on those microbial communities [24
]. In one study [28
], a mixture of various biocontrol organisms was more effective within a barley/ryegrass rotation than either a barley/clover or potato rotation, indicating the importance of a rotation that adequately supports the biocontrol activities of the added microbial agent. Both B. subtilis
and T. virens
has been previously shown to engage multiple mechanisms of action for disease control, including the induction of host defense responses, competition, and antibiosis [57
], whereas L. arvalis
is believed to function primarily as a mycoparasite in reducing pathogen populations [20
Many management practices for sustainable disease control affect soil microbiology characteristics. Changes in microbial activity or community structure can be mechanisms by which effective rotation crops and biocontrol reduce disease [5
]. Although community level approaches, such as FAME analyses, do not specifically identify the microorganisms that are responsible for the changes observed, these approaches are very sensitive to detecting changes in soil microbial characteristics and do provide useful information regarding the types of changes and groups of organisms involved. Larkin and Honeycutt [6
] demonstrated that many associations exist between microbial populations, FAME analysis, tuber yield, and Rhizoctonia disease of potato. In that study, the incidence and severity of black scurf in potato cropping systems were negatively correlated with FAME principal components and the ratio of monounsaturated to saturated fatty acids. Total and marketable yield were also negatively correlated with scurf incidence and positively correlated with fungal populations. In the present study, the barley/ryegrass rotation resulted in a greater proportion of monounsaturated fatty acids, a greater ratio of monounsaturated to saturated fatty acids, and an increase in the ratio of fungi to bacteria over the potato rotation. The barley/ryegrass rotation also increased yield and reduced stem canker, which may be influenced by specific changes in soil microbial communities.
Numerous interactions between crop rotations and biocontrol organisms were detected. Biocontrol treatments did provide some control of black scurf within the barley rotations at one location, although control by specific treatments varied within the different rotations. L. arvalis and B. subtilis reduced black scurf within the barley/ryegrass rotation, whereas T. virens reduced black scurf severity within the barley/clover rotation. L. arvalis and T. virens treatments increased the percentage of marketable and large size class tubers. Biocontrol by rotation interactions were evident in black scurf results, microbial populations, FAME profiles, and fatty acid structural classes. L. arvalis appeared to be more effective in the barley/ryegrass rotation, whereas T. virens was more effective in the barley/clover rotation at location 1.
Overall, both effective crop rotations and the addition of biological antagonists are potentially sustainable disease management practices that may reduce losses in tuber yield and quality from R. solani and other soilborne diseases of potato. The use of biocontrol within beneficial rotations may substantially improve efficacy against R. solani. This integrated approach demonstrates that microbial antagonists and effective crop rotations can potentially increase yield and suppress Rhizoctonia disease of potato, but the complex interactions involving crop rotations, biological amendments, and soil microbial communities and their roles in disease suppression needs to be better understood before the full potential of this approach can be achieved.