Cropping Systems to Control Tropical Soil-Borne Pests in Dryland-Grown Taro

Cropping Systems to Control Tropical Soil-Borne Pests in Dryland-Grown Taro

Summary

The goal of this research is to provide information necessary to develop sustainable control of M. javanica in dryland taro cropping systems. Sustainable dryland taro systems for the control of soil-borne pathogens can be obtained using resistant varieties in conjunction with effective green manures.

Objectives/Performance Targets

1. Determine resistance and tolerance of newly introduced taro germplasm to Meloidogyne javanica, a root-knot nematode.

   Evaluate biomass potential of green manure crops, resistance to root-knot nematodes and Pytium, and nematicidal or fungicidal activities of decomposition products.

   Determine the best management practices for green manure crops.

   Assess the impact of green manure crops on soil bacterial, fungal, and nematode communities in the field.

   Conduct effective educational outreach to disseminate project information to taro growers.

Accomplishments/Milestones

Performance Target 1: Determine resistance and tolerance of taro to root-knot nematode

The initial screening of the taro germplasm for tolerance, susceptibility, and resistance toward infection by Meloidogyne javanica is nearly complete. The Thaipalm and Cho2000 collections have been screened twice. The Thaipalm collection contains 43 different accessions—32 from Thailand, 6 from Nepal, and 5 originating from Vietnam. The Cho2000 collection contains 11 hybrids from parents originating from Indonesia, Papua New Guinea, Guam, Hawaii, Palau, Samoa, and Thailand. The results from these screenings were presented last year. In general, all accessions and hybrids supported nematode reproduction. Some taro cultivars supported much greater numbers of nematodes than other cultivars, but no cultivar was entirely resistant to M. javanica.

The third collection, the Tansao germplasm, contains 56 cultivars gathered from Papua New Guinea, Indonesia, Thailand, Malaysia, and the Philippines. Of the 56 cultivars in the Tansao collection, only 50 accessions have produced sufficient propagules to complete the screening. These 50 cultivars have been inoculated with nematodes and are growing in the greenhouse. The plants will be harvested, weighed, and nematode reproduction determined by September 2005. The remaining six cultivars are being grown to produce sufficient material for screening. If the plants reach sufficient size within the next 3 months, they will be inoculated and screened for resistance also.

Performance Target 2: Evaluate biomass potential of green manure crops, resistance to root-knot nematodes and Pytium, and nematicidal or fungicidal activities of decomposition products.

Goal 2.1: Determine the host status of green manure crop species to M. javanica. Green manure screenings for nematode host status were completed and reported on last year. Ten green manure crops—four brassicas, two marigolds, two sorghum×sudangrass hybrids, a legume, and a black hollyhock were evaluated. Marigolds and sorghum×sudangrass hybrids were extremely poor hosts to the nematode. Nematode reproduction factors ranged from 0-0.61. The sorghum×sudangrass hybrids produced the greatest amounts of biomass of the 10 green manures evaluated, averaging 5.2 g dry weight in the greenhouse.

Goal 2.2: Determine interactive effects of M. javanica and Pythium sp. on green manure crops. Upon completion of the green manure host-status evaluation, assay of interactive effects between M. javanica and a Pythium sp. on 22 green manure crop species was initiated. Green manure crop species used for this test provide inhibitory and/or toxic byproducts upon decomposition and includes seven cultivars of Tagetes, four Brassica sp., seven sorghum×sudangrass hybrids, and four other species of interest. A 2×2 factorial experiment was established in the greenhouse. Factors consisted of M. javanica and Pythium inoculation. The levels of each factor were +/-. All Brassica sp. were killed by the combined infection of M. javanica and Pythium (Fig. 1). Four of the marigold cultivars were killed in the combination treatment. Sunn hemp and all of the sorghum×sudangrass hybrids were not affected by either pathogen alone nor combined (Fig. 2). Of all species tested, the sorghum×sudangrass hybrids yielded the greatest amounts of biomass and seem to have the most promise for field use. A repeat of the experiment with M. javanica and P. aphanidermatum is currently underway. The experiment has been modified eliminating susceptible host cultivars. Consequently, seven Tagetes and seven sorghum×sudangrass hybrids are being investigated. Completion of this experiment is scheduled for mid June 2005.

Goal 2.3: Evaluate six different green manure crop species to determine species best suited to Hamakua Coast growing conditions and to determine management practices for the green manure crops. Plots of yellow mustard ‘Ida Gold,’ marigold ‘Nema-gone’ and ‘Golden Guardian,’ Sorghum x Sudangrass ‘Sordan 79’ and ‘Tastemaker,’ and sunn hemp were compared to a weedy unplanted and weed-free weed cloth planted. The field was prepared by a boadcast application of lime at 2000 lbs/Ac of CaCO3 equivalents. Marigolds seedlings were transplanted into the field whereas the other green manure crops were direct seeded into the plots. Fertilizer was broadcast to each treatment at 250 lbs 16-16-16 fertilizer/ac (40 lbs N/ac) at planting and 1 month later. The plants were irrigated if rainfall was less than 0.7 inches per week. Four criteria were used to evaluate the two best green manure crops: biomass production; absence of serious diseases and pests; ease of management; and host status for root-knot nematode.

After 3 months growth the green manures were evaluated. The Sorghum x Sudangrass hybrids and sunn hemp grew well and had few disease problems (Fig. 3). The marigolds grew sufficiently well but the seed was very costly. The yellow mustards were severely infected by Rhizoctonia solanii. We also found Pythium and root-knot nematodes associated the yellow mustards. Root-knot nematode population densities were low (2-11 nematodes/50 cm3 soil) in all plots at green manure incorporation. Nematode We concluded that the sorghum x sudangrass and sunn hemp would be best suited for use as a green manure (Table 1).

Goal 2.4: Determine optimal biomass for maximum effect on nematodes. An on farm test was established to determine biomass yield for Sordan 79 and sunn hemp. A weed mat control was included. The Sordan 79 and sunn hemp were planted at staggered intervals such that the green manures grew for 1, 2, or 3 months. The test began by increasing the nematode population densities with a planting of buckwheat for 3 months. The green manure crops were planted starting in November. All were treated with herbicide to allow decomposition. The taro was planted in April 2005 (Fig. 4). No tillage was used on the green manures and the stumps of the plants provided a wind break and seemed to improve taro stand establishment compared to weed mat. Soil samples were taken before and at the termination of the buckwheat crop to determine nematode population densities and for soil microbial analysis. A tissue analysis was also conducted on the buckwheat. Similar samples were taken for the green manures.

Performance Target 3: Determine the best management practices for green manure crops.

The field activities addressing this project objective are being conducted on Molokai (Goal 3.1) and Hawaii (Goal 3.2).

Goal 3.1: In November 2004, five green manure crops the with greatest resistance to M. javanica, as determined in Performance Target 1, were selected for the Molokai field experiment. The five green manure crops selected were the sorghum×sudangrass hybrid “Graze All MS,” Crotolaria juncea (sunn hemp), Tagetes erecta “Scarletade,” T. erecta “Orangeade,” and T. erecta “Marigold Scarletade Single #25.” The best management practice for the cropping system experiment calls for three phases. Phase 1 is the growing of green manure crop for soil borne pathogen suppression and encompassed six steps. Phase 2 is for the incorporation and decomposition of green manure into the soil. Phase 3 is to grow the subsequent taro crop and consists of six steps.

Phase 1, Growing the Green Manure Crop

1) Soil Management. A soil analysis was conducted. The pre-plant soil pH and nutrient levels were adjusted and the amendments incorporated into the soil. The soil analysis called for nutrient adjustment. Consequently, 6,640 lbs gypsum/ac and 1,036 lbs MgSO4/ac was applied. The soil analysis did not call for any soil pH adjustment or other pre-plant soil nutrient. The soil analysis did recommend application of post-plant nitrogen.
2) Select and Install Irrigation. The cropping system project required field planting of the green manure; mowing, tilling, and decomposing the green manure into the soil; and subsequent planting of taro. With these requirements, a sprinkler irrigation was selected and installed and will be used throughout phases 1 and 2. The sprinkler irrigation will also be used to implement weed management procedures. For the taro crop, drip irrigation will be used.
3) Weed Management. Sterile seed procedures will be used. Surface weed seeds will be germinated by applying sprinkler irrigation for 30 days. A herbicide will be applied to the germinated weeds serving to reduce the seed bank.
4) Plant Green Manure Crop. The green manure seeds will be planted using a seed spreader or planting seed drill. For the field experiment, plots were seeded by hand sowing. Seeding rates varied with each green manure crop (Table 2).
5) Post-Plant Nutrition. Per soil and plant nutrition analysis recommendations, 200 lbs N will be applied to green manure crops, except sun hemp because it is a legume crop.
6) Mowing and Tilling Green Manure Crop. When a green manure crop reaches 50% flower, it will be mowed using a flail mower and tilled into the soil with a rotor tiller.

Phase 2, Green Manure Decomposition

The mowed and tilled green manure crop will be left in the soil to decompose. The decomposition process is expected to require 45 days to complete. Weekly irrigation will be applied to hasten to the decomposition process.

Phase 3, Planting of Taro Crop

1) Land Preparation. Following the mowing and tilling of green manure crop, no addition soil tillage or pre-plant soil amendments will be required.
2) Weed Management. Herbicides will be applied to weeds that germinated during Phase 2.
3) Irrigation. A drip irrigation system consisting of lines 5 feet apart will be installed in the field.
4) Plant Taro. Taro cuttings will be planted 2 feet apart within a row. Rows are 5 feet apart on center.
5) Pre-plant Fertilizer. Each month from the 2nd to the 7th month after planting, 60 lbs N/ac will be applied to the taro. The sources of nitrogen will be rotated among urea, potassium nitrate, and calcium nitrate.
6) Harvesting: The taro crop will be harvested 9 to 12 months after planting.

Performance Target 4: Assess the impact of green manure crops on soil bacterial, fungal, and nematode communities in the field.

Goal 4.1: Two field experiments have been designed and installed to address this objective. One test is on Molokai and the other on Hawaii. The tests are generally similar but differ slightly in specifics, so each will be presented separately. Soil samples have been collected and assayed in the laboratory. On Molokai, the field was conditioned to establish high nematode pressure. In 2003, soil samples showed a wide variation in nematode population densities in the field. Consequently from fall 2003 through fall 2004, nematode-susceptible buckwheat, Fagopyrum esculentum, was grown in the field to increase nematode populations in the green manure test field (Fig. 5). Subsequent soil samplings indicate increased nematode population densities throughout the field.

The field experiment is a randomized complete block design, with 6 blocks (replications). In addition to the five selected green manure crop treatments, the nematode-susceptible buckwheat, and a no green manure or bare fallow are included. The subsequent planting of taro will occur in each block and treatment plot, resulting in six replications. Soil samples will be collected at pre-plant, mid-maturity, and post-plant for phases 1 and 3. Data on soil-borne diseases on the taro and taro yields will be collected at harvest in phase 3. The experimental design and data collection will determine the impact of the green manure crops on soil pathogen communities in the field.

In addition to these fields the trial from Goal 3.2 was also subjected to molecular and microbiological analysis. Soil samples for the trials were collected prior to green manure planting, at the end of the green manure growing period (~16wks), and at the end of the green manure decomposition period (~8wks). A total of 176 soil samples have been collected. Total soil DNA has been isolated from each sample and, due to the low microbial density in some of the soil samples, 5g soil was used for each 9217256DNA isolation.

Methods for the analysis of the soil bacterial and fungal communities have been altered from the original proposal. Microbiological analysis has been shifted to serial dilution plating. The fresh equivalent to 1g dry soil is suspended in 100ml sterile distilled water. Serial dilutions of this suspension are plated on PYEA + mycostatin and 10% V8A + streptomycin media. Colony forming units (CFU) are counted 24-120 hours after inoculation and used to calculate CFU per gram soil.

Amplified Fragment Length Polymorphism (AFLP) was proposed for the molecular analysis of the total soil microbial community. A preliminary study with various samples from Oahu, Molokai, and Hawaii were processed in duplicate along with a sugarcane DNA control (Fig. 6). Total soil DNA was isolated, then 10ng DNA was digested with EcoRI and MseI and selectively amplified with primers based on the enzyme restriction sites. The soil microbial community fingerprints generated with this technique could not be replicated, however the sugarcane control duplicated well. Duplication of the control demonstrates the reproducibility of the technique and infers a potential problem with the microbial community DNA. From AFLP analyses of pooled bacterial artificial chromosomes (BACs), we have learned that if the number of DNA molecules with variable concentrations exceeded 5,000, the AFLP banding patterns were not reproducible. The difficulty of reproducing AFLP banding patterns from these soil DNA samples indicates that thousands of microbial genomes with variable population sizes may have been represented in the soil samples. Alternative techniques, such as Terminal Restriction Fragment Length Polymorphism (T-RFLP) and Serial Analysis of Ribosomal Sequence Tags (SARST), for analysis of the soil bacterial and fungal communities are currently being studied and will be assessed for reproducibility.

Performance Target 5:Conduct effective educational outreach to disseminate project information to taro growers.

Goal 5.1: Field Day. In conjunction with the Molokai Annual Taro Field Day in February 2005, a workshop and field day was held on the project (Figs. 7 and 8). The project objectives and desired outcomes were explained and participants were able to view the buckwheat planting in the experimental field. A second field day on the green manure crop production was scheduled for June 2005. Participants were to have the opportunity to see the selected green manure crop at mid-maturity. The third field day will be scheduled at taro harvest and data collection. Participants will be able to view the results and impact of the green manure cropping system. On-farm demonstrations on the cropping system will be conducted using the most effective green manure crop identified in the experiment.

Impacts and Contributions/Outcomes

• Identified potential green manure crops for control of M. javanica in dryland taro

  Developed a protocol for green manure cultivation in dryland taro

  Showed the diversity of soil bacteria in green manure planted taro fields

  Showed and excited taro farmers about the potential of green manure cover crops

Collaborators: