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

Fifty-five taro (Colocasia esculenta) cultivars were evaluated for resistance to the root-knot nematode, Meloidogyne javanica. Although all cultivars were hosts, significant differences in the reproductive success of M. javanica were found within the taro germplasm. Ten green manure species were evaluated in the greenhouse for biomass growth and resistance to M. javanica, and four promising green manure species were identified. Two experimental sites are being pre-conditioned to increase levels of M. javanica using buckwheat or taro. Preliminary research demonstrated that soil DNA can be extracted, amplified with bacterial 16s rDNA primers, and used to tentatively identify soil bacterial populations.

Objectives/Performance Targets

1. 1. Determine resistance/ tolerance of newly introduced taro germplasm against root-knot nematodes.
   2. Evaluate biomass potential of green manure crops, resistance to root-knot nematodes and Pythium, and nematicidal or fungicidal activities of decomposition products.
   3. Determine the best management practices for green manure crops.
   4. Assess the impact of green manure crops on soil bacterial, fungal, and nematode communities in the field.
   5. Conduct effective educational outreach to disseminate project information to taro growers.

Accomplishments/Milestones

1. Fifty-five taro (Colocasia esculenta) accessions were screened for resistance/tolerance of Meloidogyne javanica in a greenhouse pot study. All taro cultivars were found to be hosts to M. javanica. Significant differences within the taro germplasm were found; some cultivars were very good hosts (reproductive factor > 47), whereas others were only good hosts (reproductive factor > 1.5). A reproductive factor of 1.5 indicates that 1.5 times the number of eggs of M. javanica were found at the end of the experiment relative to that added at the beginning. A second, repeat screening is underway in the greenhouse and will be completed in September 2004. In addition, 57 new taro accessions have been planted and will be inoculated after sufficient growth has occurred.
2. Biomass potential and M. javanica host status of 10 green manure species was completed in the greenhouse. Four species or cultivars within species were identified to be promising, based on biomass growth and resistance to M. javanica: marigold (Tagetes patula cv. French Brocade); yellow mustard (Sinapis alba cv. Ida Gold); sorghum-sudangrass hybrid (Sorghum x drummondii cv. Sordan 79); and sunn hemp (Crotalaria juncea). Sorghum-sudangrass cv. Piper had the highest biomass growth of the green manure species. Marigold cv. French Brocade had the lowest number of eggs of M. javanica. In a second trial that has been initiated in the greenhouse, four Brassica sp., seven Tagetes sp., seven grasses, and four other green manure species of interest are being evaluated. At the cooperator’s farm on Hawaii island, plans are underway to evaluate six promising green manure crop species or cultivars based on results of the pot studies. These six green manure crops will include two cultivars of marigolds, two cultivars of sorghum-sudangrass hybrids, yellow mustard, and sunn hemp. Biomass and resistance to M. javanica will be evaluated under field conditions.
3. At the Applied Research Farm on Molokai, field pre-conditioning activities have been conducted to ensure exposure of nematodes to green manure treatments. Since January 30, 2004, three generations of buckwheat (Fagopyrum esculentum), a root-knot nematode susceptible crop, have been grown at the experimental site. Eggs of M. javanica per gram of buckwheat root have increased from 0 to 350 on average. Future research will investigate the effects of irrigation levels on glucosinolate levels in a selected Brassicaceae sp. At a cooperator’s farm on Hawaii island, nematodes were monitored during a taro cropping cycle. Incidence of M. javanica increased dramatically throughout the cropping cycle from 0 to an average of 1,832 per liter of soil at nine months after planting. Plans are underway to determine the two green manure crop species that are best adapted to this high rainfall area.
4. To measure the basal presence of soil microbial populations, soil samples were collected from an organic farm that grows taro and from the experimental field site on Molokai prior to planting of buckwheat. Total DNA was isolated from each of the soil samples using UltraClean Soil DNA kit. Interestingly, the DNA concentration of soil samples from the organic farm was approximately 20 times higher than that from the experimental field. DNA from soil samples were analyzed using bacterial 16s rDNA primers. The PCR products from the soil sample #3 (collected from an organic farm) and #7 (collected from the experimental field) were cloned into a Topo vector, and two clones each were selected for DNA sequencing from both ends. A GenBank search revealed that these four clones belonged to four different bacterial strains or species. The closest matches for these four clones are: 3-1: agricultural soil bacterium clone SC-I-19; 3-2: uncultured bacterium SJA-149; 7-1: uncultured bacterium SJA-22; and 7-2: uncultured gamma proteobacterium. Our preliminary results showed diverse microbial populations in the soil samples from both the organic farm and the experimental field. Soil samples were collected again in late May from the same experimental field on Molokai after pre-conditioning with buckwheat. Soil samples were also collected from the experimental field in Hilo 12 months after planting of taro. A new Graduate Research Assistant will start research during the fall of 2004, and large scale DNA analysis of soil samples will be carried out when the field trials are initiated on best management practices of green manure crops.
5. Meetings were held with farmer-cooperators to keep them informed about the progress of the research. Extension programs will be conducted after completion of the two field trials to determine best management practices.

Impacts and Contributions/Outcomes

Undergraduate students who were hired and trained in field research at the Molokai Applied Research Farm have found new positions working in agriculture. A graduate research assistant (GRA), Mr. Anthony Ortiz, is learning research techniques for evaluation of resistance/ tolerance of taro plants to root-knot nematodes. A new GRA, Ms. Andrea Blas, will start in the fall of 2004. Farmer-cooperators are very interested in the evaluation of green manure crops in Hawaii, not just for nematode control, but for improving soil fertility and plant health.

Collaborators: