Progress 09/01/14 to 08/31/18
Outputs
Target Audience:The target audiences of this project are the California strawberry and lettuce producers in both conventional and organic production systems and shippers, pesticide/fumigant application companies, pest control advisors and Verticillium researchers. Other critical stakeholders are the California Strawberry Commission, California Leafy Greens Research Board, and Driscoll's. Because the status of the disease on strawberry affects the crops that follow, the California Leafy Greens Research Program also has an intense interest in the outcome of this research. The results obtained from this project will be broadly applicable to crops heavily dependent on soil fumigation in the western US as well as other states in the US and beyond. Changes/Problems:Field implementation of the successful treatments from the greenhouse experiment encountered initial setbacks as growers hosting the studies backed out as their priorities changed. Despite implementing the studies as outlined in the proposal during the first two years of the project, the studies had to be abandoned prematurely for this reason. A third grower then committed to hosting the studies over a longer term. Hence, the cropping sequences outlined in the proposal had to be altered in the field study. What opportunities for training and professional development has the project provided?In addition to accomplishing the project objectives, one of main goals of the project was to offer training opportunities to postdoctoral researchers and developing them into independent scientists. During the reporting period, the project has had one project scientist, one staff research associate, and a postdoctoral researcher involved in generating the information described in the previous section. A graduate student at UC Davis from the Agricultural and Resource Economics Department was also supported by the project and conducted the cost-benefit analyses of the systems approach relative to chemical fumigants. How have the results been disseminated to communities of interest?The results from the project have been disseminated through talks at the mid-year and annual meeting of the California Leafy Greens Board and the global assembly of Driscoll's clientele including researchers and managers, and at professional society meetings in the US and overseas. Additional talks at grower meetings have been delivered and the study has generated much interest throughout the Salinas and Pajaro Valleys and even on a 5000-acre Navajo-managed farm in New Mexico, where Verticillium wilt has been a major problem on several crops that they grow. Based on the results presented in international meetings, researchers in several countries (China, Spain, Belgium) are also testing these techniques at this time. The first manuscript from the study was published in 2018. Several other manuscripts are currently being planned including the one dealing with the economic analyses. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Two naturally infested Verticillium wilt conducive soils from the Salinas Valley of coastal California were amended with disease-suppressive broccoli residue and/or crab meal amendments, & changes to the soil prokaryote community were monitored using Illumina sequencing of a 16S rRNA gene library generated from 160 bulk soil samples.The experiment was run in a greenhouse 4 times with eggplant as the Verticillium wilt susceptible host.Disease suppression, plant height, soil microsclerotia density & soil chitinase activity were assessed at the conclusion of each experiment.In high microsclerotia density soil, all amendments significantly reduced Verticillium wilt severity and microsclerotia density, & increased soil chitinase activity.Plant height was increased only in the broccoli-containing treatments.The treatments had a significant impact on the soil microbiome community structure, but measures of alpha diversity didn't vary between treatments.Community structure correlated with disease score, plant height, microsclerotia density and soil chitinase activity, suggesting that the prokaryote community may impact the disease-related response variables or vice versa.Similarly, the abundance of 107 genera correlated with disease-related response variables, which included variants from genera with known antagonists of filamentous fungal plant pathogens, such as Pseudomonas and Streptomyces.Overall, genera with anti-fungal antagonists were more abundant in amended soils than in unamended soils, & constituted up to 8.9% of all genera in broccoli+crabmeal amended soil.This study demonstrated that substrate-mediated shifts in soil prokaryote communities are associated with the transition of Verticillium wilt conducive soils to Verticillium wilt suppressive soils, & suggests that soils likely harbor numerous additional antagonists of fungal plant pathogens that contribute to the biological suppression of plant disease. Based on the success of the above study, we completed an additional 4 experiments in the greenhouse involving the same treatments but on soils from an agricultural field, a non-agricultural field & a field that is in transition from pasture to organic agriculture under similar Verticillium dahliae inoculum densities. Treatments that were best in the above experiment were also significantly better across the three soil types in this study.The microbiome data including both prokaryote & fungal communities influenced by the different treatments in the three soil types are being analyzed currently.The specific prokaryote & fungal communities responsible for disease suppression will be identified identified from the ongoing data analyses. In this study, the treatments were altered to test additional treatments.Prior to initiating a systems-based approach to the management of soilborne diseases in this field, the incidence of Verticillium wilt ranged from 70 to 100% in the standing lettuce crop.The additional treatment tested was to terminate infected lettuce crop by the application of allyl-isothiocyanate (AITC) at 155 kg ha-1through the drip irrigation system after lettuce harvest to prevent inoculum from being returned to soil from infected plants.AITC occurs naturally in broccoli & Brussels sprouts, two crops common to coastal California.However, the amounts returned to the soil are less than 10 kg ha-1, whereas the amount registered for application is well above 224.2 kg. ha-1.Since endemic soil microbial communities have been exposed to AITC for decades from decomposing crucifer crops, they have had ample time to adapt to the chemical as a carbon source. In a 2.67 ha field study, following the first lettuce crop and AITC application, the entire field was amended with chitin-rich crustacean shell at 1.1 t ha-1incorporated to a depth of 12 cm.Soil sampling was done 2 days after chitin amendment & used to monitor chitinase enzyme activity, Verticillium microsclerotia count, & DNA extraction.Broccoli was planted as the second crop in the sequence.Following broccoli harvest, the remaining broccoli biomass was incorporated into the soil through disking to facilitate tissue decomposition. One month after the incorporation of the residue, the soil was re-amended with chitin and prepared for the third crop (lettuce) in the rotation.Soil samples were taken before lettuce transplanting for chitinase enzyme analysis, soil microsclerotia count, & DNA extraction.The disease incidence & severity of its impact on lettuce was also recorded.The grower planted another broccoli crop that was harvested in 2018 August. The grower then planted lettuce that was monitored for Verticillium wilt & other soilborne diseases. We also sampled soil for V. dahliae microsclerotia & microbial & nutrient analyses before & after the crop. Considerably higher numbers of MS were found in the plot that was selected for the AITC application relative to untreated control plots.It does not appear that the crop termination treatment was successful in reducing MS populations.Due to the ineffectiveness of AITC treatments, we recommend not using this chemical & instead to adjust the system plans by increasing chitin applications and having more frequent broccoli cultivations.The number of soil microsclerotia increased significantly when crisphead lettuce was planted & later incorporated in the soil.However, the numbers decreased significantly when broccoli was incorporated into the soil.The disease incidence on lettuce was significantly lower.The effectiveness of treatment combinations was demonstrated in the fields, pointing to success with broccoli rotation & chitin application treatments as proposed. The data on changes in the soil microbial communities is still being analyzed. The economic analyses of using broccoli rotations to evaluate the suppression of Verticillium wilt were assessed in terms of a partial budgeting analysis that evaluates the net effect of treatment-related differences in costs & yields on net returns (from here we will refer to partial budgeting net returns as just net returns).MS levels & cost data were obtained from the trial above trial & potential yield losses were computed. Price data was obtained from the USDA Agricultural Marketing Service (AMS) & averaged over the 2016-17 period.Prices from outside the trial period were used to reflect values closer to growers' current price expectations.Annual yield data for strawberries from Monterey County for 2011-2014. In order to capture fully the flexibility of this system-based approach, even if the grower always chooses to plant strawberry after three rotational plantings.It is necessary to evaluate the net returns for every possibility of broccoli & lettuce use over the three possible planting periods.This leads to eight possible combinations (L=Lettuce, B=Broccoli):LLL, LLB, LBL, BLL, LBB, BLB, BBL, BBB. Although this systems approach allows for the grower to change their broccoli/lettuce decision based on the relative price of broccoli & lettuce, levels in the soil were only available for one combination: BBL.We then evaluate a counterfactual, in which the grower plants a 2nd strawberry crop after 1 broccoli planting. Strawberries are followed by a 2nd broccoli planting & a lettuce planting. The economic value of the flexibility associated with a systems approach is highly dependent upon expected prices.Future research should include more experimental data which captures the flexibility of the systems-based approach & an assessment of the sensitivity of outcomes to relative prices, as well as using a discount factor in line with growers' expectations about the future.The examples presented utilize interpolated values of MS, which may vary with rotations in ways this approach does not capture
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2016
Citation:
Triky-Dotan, S., B. Westerdahl, F. N. Martin, K. V. Subbarao, S. T. Koike and H. A. Ajwa. 2016. Fumigant dosages below maximum label rate control some soilborne pathogens. California Agriculture 70:130-136.
- Type:
Journal Articles
Status:
Published
Year Published:
2016
Citation:
Daugovish, O., A. Howell, S. A. Fennimore, S. T. Koike, T. R. Gordon, and K. V. Subbarao. 2016. Non-fumigant treatments and their combinations affect soil pathogens and strawberry performance in Southern California. Int. J. Fruit Sci. 16:37-46.
- Type:
Journal Articles
Status:
Awaiting Publication
Year Published:
2018
Citation:
Carroll, C. L., C. A. Carter, R. E. Goodhue, C.-Y. Cynthia Lin Lawell, and K. V. Subbarao. 2018. The economics of Verticillium wilt management: Control options and externalities. Phytopathology doi.org/10.1094/PHYTO-03-17-0083-RVW.
- Type:
Journal Articles
Status:
Awaiting Publication
Year Published:
2018
Citation:
Inderbitzin, P., Ward, J., Barbella, A., Solares, N., Izyumin, D., Burman, P., Chellemi, D., Subbarao, K. V. 2018. Soil microbiomes associated with Verticillium wilt suppressive broccoli and chitin amendments are enriched with potential biocontrol agents. Phytopathology doi:10.1094/PHYTO-07-17-0242-R.
- Type:
Journal Articles
Status:
Published
Year Published:
2016
Citation:
Chellemi, D. O., Gamliel, A., Katan, J., and Subbarao, K. V. 2016. Development and deployment of system-based approaches for the management of soilborne plant pathogens. Phytopathology 106:216-225.
- Type:
Journal Articles
Status:
Published
Year Published:
2017
Citation:
Puri, K. D., S. Gurung, D. P. G. Short, G. Sandoya, R. J. Hayes, and K. V. Subbarao. 2017. Stability of a lettuce disease resistance locus against a monoclonal population of Verticillium dahliae race 1. Phytopathology 107: 1417-1425.
- Type:
Conference Papers and Presentations
Status:
Published
Year Published:
2017
Citation:
Carroll, C. L., C. A. Carter, R. E. Goodhue, C.-Y. Cynthia Lin Lawell, and K. V. Subbarao. 2017. The economics of managing Verticillium wilt, an imported disease in California lettuce. California Agriculture 71:178-183.
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Progress 09/01/16 to 08/31/17
Outputs
Target Audience:The target audiences of this project are the California strawberry and lettuce producers in both conventional and organic production systems and shippers, seed/fertilizer/pesticide/fumigant/irrigation sales and application companies, pest control advisors and Verticillium researchers. Other critical stakeholders are the California Strawberry Commission (CSC), California Leafy Greens Research Board, and Driscoll's. Because the status of the disease on strawberry affects the crops that follow, the California Leafy Greens Research Program (CLGRP) also has an intense interest in the outcome of this research. The results obtained from this project will be broadly applicable to crops heavily dependent on soil fumigation in the western US as well as other states in the US. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?In addition to accomplishing the project objectives, one of main goals of the project is to offer training opportunities to postdoctoral researchers and developing them into independent scientists. During the reporting period, the project has had one project scientist, one staff research associate, and a postdoctoral researcher involved in generating the information described in the previous section. A graduate student at UC Davis from the Agricultural and Resource Economics Department is being supported by the project and is currently conducting the cost-benefit analyses of the systems approach relative to chemical fumigants. How have the results been disseminated to communities of interest?The results from the project have been disseminated through talks at the annual meeting of the California Leafy Greens Board and the global assembly of Driscoll's clientele including researchers and managers. Additional talks at grower meetings have been delivered and the study has generated much interest throughout the Salinas and Pajaro Valleys and even on a 5000-acre Navajo-managed farm in New Mexico, where Verticillium wilt has been a major problem on several crops that they grow. The first manuscript from the study is in press currently and will appear in print early next year. Several other manuscripts are currently being planned including the one dealing with the economic analyses. What do you plan to do during the next reporting period to accomplish the goals?The project will continue as proposed during the next reporting period.
Impacts
What was accomplished under these goals?
Two naturally infested Verticillium wilt conducive soils from the Salinas Valley of coastal California were amended with disease-suppressive broccoli residue and/or crab meal amendments, and changes to the soil prokaryote community were monitored using Illumina sequencing of a 16S rRNA gene library generated from 160 bulk soil samples. The experiment was run in a greenhouse twice with eggplant as the Verticillium wilt susceptible host. Disease suppression, plant height, soil microsclerotia density and soil chitinase activity were assessed at the conclusion of each experiment. In high microsclerotia density soil, all amendments significantly reduced Verticillium wilt severity and microsclerotia density, and increased soil chitinase activity. Plant height was increased only in the broccoli-containing treatments. A total of 8790 error-corrected sequence variants were included in the analyses. The treatments had a significant impact on the soil microbiome community structure, but measures of alpha diversity did not vary between treatments. Community structure correlated with disease score, plant height, microsclerotia density and soil chitinase activity, suggesting that the prokaryote community may impact the disease-related response variables or vice versa. Similarly, the abundance of 107 sequence variants correlated with disease-related response variables, which included variants from genera with known antagonists of filamentous fungal plant pathogens, such as Pseudomonas and Streptomyces. Overall, genera with anti-fungal antagonists were more abundant in amended soils than unamended soils, and constituted up to 8.9% of all sequences in broccoli+crabmeal amended soil. This study demonstrates that substrate-mediated shifts in soil prokaryote communities are associated with the transition of Verticillium wilt conducive soils to Verticillium wilt suppressive soils, and suggests that soils likely harbor numerous additional antagonists of fungal plant pathogens that contribute to the biological suppression of plant disease. Based on the success of the above study, we completed an additional three experiments in the greenhouse involving the same treatments but on soils from an agricultural field, a non-agricultural field and a field that is in transition from pasture to organic agriculture under similar Verticillium dahliae inoculum density. Regardless of the soil type, the treatments performed just as above. The microbiomes influenced by the different treatments in the three soils are being analyzed and work completed has provided results consistent with the above results. Field experiment on a grower's farm that had a significant Verticillium wilt incidence on a previous lettuce crops is ongoing. The first broccoli crop followed by a Romaine lettuce crop was planted. We monitored the soil inoculum densities in the different treatments, chitinase activity, disease incidence, and soil microbiomes as affected by the different treatments over the course of the past year. The grower will be planing the second broccoli crop during the 2018 winter and we will continue to follow all the soil parameters measured during the previous seasons.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2016
Citation:
Triky-Dotan, S., B. Westerdahl, F. N. Martin, K. V. Subbarao, S. T. Koike and H. A. Ajwa. 2016. Fumigant dosages below maximum label rate control some soilborne pathogens. California Agriculture 70:130-136.
- Type:
Journal Articles
Status:
Published
Year Published:
2016
Citation:
Daugovish, O., A. Howell, S. A. Fennimore, S. T. Koike, T. R. Gordon, and K. V. Subbarao. 2016. Non-fumigant treatments and their combinations affect soil pathogens and strawberry performance in Southern California. Int. J. Fruit Sci. 16:37-46.
- Type:
Journal Articles
Status:
Awaiting Publication
Year Published:
2018
Citation:
Carroll, C. L., C. A. Carter, R. E. Goodhue, C.-Y. Cynthia Lin Lawell, and K. V. Subbarao. 2018. The economics of Verticillium wilt management: Control options and externalities. Phytopathology doi.org/10.1094/PHYTO-03-17-0083-RVW.
- Type:
Journal Articles
Status:
Awaiting Publication
Year Published:
2018
Citation:
Inderbitzin, P., Ward, J., Barbella, A., Solares, N., Izyumin, D., Burman, P., Chellemi, D., Subbarao, K. V. 2018. Soil microbiomes associated with Verticillium wilt suppressive broccoli and chitin amendments are enriched with potential biocontrol agents. Phytopathology doi:10.1094/PHYTO-07-17-0242-R.
|
Progress 09/01/15 to 08/31/16
Outputs
Target Audience:The target audiences of this project are the California strawberry and lettuce producers in both conventional and organic production systems and shippers, seed/fertilizer/pesticide/fumigant/irrigation sales and application companies, pest control advisors and Verticillium researchers. Other critical stakeholders are the California Strawberry Commission (CSC), California Leafy Greens Research Board, and Driscoll's. Because the status of the disease on strawberry affects the crops that follow, the California Leafy Greens Research Program (CLGRP) also has an intense interest in the outcome of this research. The results obtained from this project will be broadly applicable to crops heavily dependent on soil fumigation in the western US as well as other states in the US. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?In addition to accomplishing the project objectives, one of main goals of the project is to offer training opportunities to postdoctoral researchers and developing them into independent scientists. During the reporting period, the project has had one project scientist, one staff research associate, and a postdoctoral researcher involved in generating the information described in the previous section. A graduate student at UC Davis from the Agricultural and Resource Economics Department will be supported by the project during years 2 and 3 of the project to conduct the cost-benefit analyses of the systems approach relative to chemical fumigants. How have the results been disseminated to communities of interest?The results from the project have been disseminated through talks at the annual meeting of the California Leafy Greens Board and the global assembly of Driscoll's clientele including researchers and managers. Additional talks at grower meetings have been delivered and the study has generated much interest throughout the Salinas and Pajaro Valleys and even on a 5000-acre Navajo-managed farm in New Mexico, where Verticillium wilt has been a major problem on several crops that they grow. The first manuscript that was an invited review was also published during this reporting period and a second manuscript is currently in review. What do you plan to do during the next reporting period to accomplish the goals?The project will continue as proposed during the next reporting period.
Impacts
What was accomplished under these goals?
Two naturally infested Verticillium wilt conducive soils from the Salinas Valley of coastal California were amended with disease-suppressive broccoli residue and/or crab meal amendments, and the soil prokaryote community was monitored using Illumina sequencing of a 16S rRNA gene library generated from 160 bulk soil samples. The experiment was run in a greenhouse over two experimental cycles with eggplant as the Verticillium wilt susceptible host, and disease suppression, microsclerotia density, plant height and soil chitinase activity were assessed. In the high microsclerotia density soil, all amendments significantly reduced Verticillium wilt severity and microsclerotia density, but plant height was increased only in the broccoli-containing treatments, and soil chitinase activity was increased only in the broccoli+crabmeal treatment. A total of 2496 operational taxonomic units (OTUs), which approximately correspond to species, were included in the analyses. Amendment had a significant impact on the soil microbiome community structure, but measures of alpha diversity did not vary between amendment treatments. Antagonists of filamentous fungal plant pathogens as a group were more abundant in amendment communities than in the control and constituted up to 25% of DNA sequence reads in the broccoli+crabmeal community as compared to 16% in the control. This suggested that the antagonists may play a role in disease reduction but other, less characterized groups are more abundant, constituted 89% of all OTUs and may thus be more important. The abundance of 585 OTUs correlated with the disease-related response variables, suggesting an involvement of those OTUs in disease reduction and promotion of overall plant vigor by enzymatic degradation of hyphal cell walls and microsclerotia, and by other mechanisms. Only 3% of all OTUs were identified to species, but more than 90% of all amendment-enriched OTUs belonged to taxonomic orders that have been cultured. Overall, our results show that very little is known about the diversity of bacterial species and their impact on plant health in our system, which likely harbors numerous new, culturable antagonists of fungal plant pathogens that may have uses as biocontrol agents. Based on the success of the above study, we completed an additional three experiments in the greenhouse involving the same treatments but on soils from an agricultural field, a non-agricultural field and a field that is in transition from pasture to organic agriculture under similar Verticillium dahliae inoculum density. Regardless of the soil type, the treatments performed just as above. The microbiomes influenced by the different treatments in the three soils are being determined at this time. We also set up an experiment on a grower's farm that had a significant Verticillium wilt incidence on a previous lettuce crop. The first broccoli crop has been planted. We will monitor the soil inoculum densities in the different treatments, chitinase activity, disease incidence, and soil microbiomes as affected by the different treatments over the course of next few years.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2016
Citation:
Chellemi, D. O., Gamliel, A., Katan, J., and Subbarao, K. V. Development and deployment of system-based approaches for the management of soilborne plant pathogens. Phytopathology 106:216-225.
|
Progress 09/01/14 to 08/31/15
Outputs
Target Audience:The target audiences of this project are the California strawberry and lettuce growers and shippers, seed/fertilizer/pesticide/fumigant/irrigation sales and application companies, pest control advisors and Verticillium researchers. Other critical stakeholders are the California Strawberry Commission (CSC), California Leafy Greens Research Board, and Driscoll's. Because the status of the disease on strawberry affects the crops that follow, the California Leafy Greens Research Program (CLGRP) also has an intense interest in the outcome of this research. The results obtained from this project will be broadly applicable to crops heavily dependent on soil fumigation in the western US as well as other states in the US. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?In addition to accomplishing the project objectives, one of main goals of the project is to offer training opportunities to postdoctoral researchers and developing them into independent scientists. During the reporting period, the project has had one project scientist and a postdoctoral researcher involved in generating the information described in the previous section. A graduate student at UC Davis from the Agricultural and Resource Economics Department will be supported by the project during years 2 and 3 of the project to conduct the cost-benefit analyses of the systems approach relative to chemical fumigants. How have the results been disseminated to communities of interest?The results from the project have been disseminated through talks at the annual meeting of the California Leafy Greens Board and the global assembly of Driscoll's clientele including researchers and managers. What do you plan to do during the next reporting period to accomplish the goals?The project will continue as proposed during the next reporting period.
Impacts
What was accomplished under these goals?
A study was established on-farm at a site in Salinas with a severe infestation of Verticillium dahliae and a known history of Verticillium wilt on strawberry. The location has been subjected to a lettuce-lettuce-lettuce-strawberry rotation for many repeated cycles with pre-plant soil fumigation conducted prior to planting strawberry. During the season immediately before establishing this study, strawberry beds had been fumigated with drip applied of Pic Clor 60 before planting strawberry. We evaluated the crop for Verticillium wilt. Soil was collected in a section where the incidence of Verticillium wilt on strawberry was 31%. Disease confirmation on symptomatic plants was conducted by collaborator Koike. Soil properties and V. dahliae microsclerotia were determined. The soil is a loam with 40% sand, 46% silt, 14% clay, and 3.3% organic matter and contained an average of 1.2 microsclerotia per gram of soil. This level of inoculum is sufficient to cause losses on strawberries but not on lettuce. Two cycles of root guard application and broccoli crop cycles in replicated plots were completed before planting crisphead lettuce in all plots. Following lettuce harvest, synthetic allyl isothiocyanate was applied through the drip irrigation system to prevent inoculum build up from potential Verticillium wilt infections on lettuce. The second treatment cycle is currently underway as also the data analyses from the work done thus far. The results from this study and the potential conclusions will be included in next year's report. A greenhouse study involving soils from two different fields were completed twice to assess the efficacy of combinations of crab/feather meal (RootGuard) and fresh broccoli residue in Verticillium-infested field soil. Soils collected from the field experimental site described above served as the low inoculum density soil and soil from another field with a history of severe Verticillium wilt on lettuce served as the high inoculum density soil. Four treatments were established for each soil type and included: a) unamended control; b) 10% broccoli residue (fresh weight:weight of soil); c) 0.2% (2 tons per acre) crab/feather meal (90:10 blend of crab:feather meal commercially sold as Rootguard weight:weight of soil); and a combination of broccoli and rootguard at the corresponding rates. For each soil type, 2.65 kg of soil mixed with 237 g of Perlite (to prevent soil compaction) was placed into 3.7 L pots and the corresponding rates of broccoli residue and crab/feather meal were added. The broccoli residue was harvested from mature broccoli plants from a commercial field and stored at 5oC prior to amending the pots. Either 8 or 10 replicate plots were prepared for each treatment to accommodate the differences in texture between the two soil types and the volume of space occupied by the broccoli residue. Eggplant (the most susceptible host for Verticillium wilt - if treatments suppress the disease on this host, they would work on most other hosts) seedlings were transplanted into pots (4 plants per pot) one week following the establishment of 4 treatments. Verticillium wilt severity was assessed on a scale of 0 to 5, in which 0 = no vascular discoloration; 1=1 to 25, 2 = 26 to 50; 3 = 51 to 75, and 4 = 76 to 100% of vascular tissues in the tap root exhibited discoloration in the absence of foliar symptoms; and 5 = 100% of vascular tissues in the tap root exhibited discoloration extending into the crown and the presence of foliar symptoms typical of Verticillium wilt. Tap root and crown tissues of plants from treated and control were sampled randomly and placed on NP-10 medium (semi-selective for V. dahliae) to confirm the presence or absence of the pathogen. Soil and data were collected from replicates 1 to 5 to assess microbial communities, chitinase activity, and terminal V. dahliae microsclerotia. Soil from the remaining replicates (6 to 9) were mixed together with soil from replicates 1 to 5 prior to the 2nd amendment cycle to ensure enough soil (2.65 kg) was available for 5 replicate pots. Regardless of the soil type, broccoli residue significantly reduced V. dahliae microsclerotia and disease severity on eggplant. In addition, significantly higher chitinase activity was recorded in this treatment relative to other treatments suggesting that this treatment increased soil microflora with significant chitinase activity. The microbial communities were unique from all other treatments and formed a separate cluster. We have identified the differential taxa correlating with disease suppression involving Bacillus spp. and Actinomycete genera. Further analyses are currently being conducted to determine the functional groups responsible for increased chitinase activity and disease suppression, and for the potential of using these functional groups to reduce soilborne diseases. Although externally applied chitin increased chitinase activity in soil, the treatment failed to reduce Verticillium wilt significantly over the untreated control. We have now determined that V. dahliae itself possesses the machinery to synthesize chitinase and quite likely uses the chitin applied as a food base for its survival. This may be the reason why chitin failed to reduce Verticillium wilt on eggplants.
Publications
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