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

Based on non-host status to the root-knot nematode (Meloidogyne javanica), high biomass, and lack of major pest or disease problems in both greenhouse trials and field trials, the most promising green manure crop is sorghum x sudangrass (Sorghum × drummondii ‘Sordan 79’ or ‘Graze-all MST’). Another promising green manure crop that fixes nitrogen, has a low-host status for reniform nematodes (Rotylenchulus reniformis), and good biomass accumulation is sunn hemp (Crotalaria juncea). Overall, initial populations of root-knot nematodes were low and barely at the level of detection in several field trials, particularly those on the islands of Molokai and Maui. When root-knot nematodes were present in the soil at the start of the field trial (e.g. on the island of Hawaii), growth of green manures for 2 or 3 months had a beneficial effect on the individual fresh corm weight of the subsequent crop of taro. This beneficial effect could be due to: a) lower initial numbers of root-knot nematodes; b) lower numbers of reniform nematodes; and/or c) to greater exchangeable potassium (K) in both soil and taro leaves, perhaps caused by slow release of nutrients during decomposition of green manure crops. Field days were held to demonstrate the growth of various green manure crops and management techniques such as flail mowing of green manures and treatment of vegetative propagating materials of taro to minimize spread of nematodes.

Objectives/Performance Targets

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

Performance Target 1: Determine resistance/tolerance of newly introduced taro germplasm against root-knot nematodes.
This target was completed and previously reported on. To summarize, no resistance to the root-knot nematode was found in the taro germplasm screened. Potential candidates for further tolerance evaluation were Thailand cultivar 035 and hybrid 40 from a Hawaiian x Palauan cross, because these varieties displayed a consistently low nematode reproductive factor (Rf) and high growth.

Performance Target 2: Evaluate biomass potential of green manure crops, resistance to root-knot nematodes and Pythium.

This performance target was completed and reported on in the last report. In the greenhouse, sorghum x sudangrass Sorghum × drummondii ‘Graze All MST’ was the best candidate with a low reproduction factor (Rf = 0.19). In addition, it grew well when inoculated with M. javanica and P. aphanidermatum combined or individually.

Performance Target 3: Determine the best management practices for green manure crops.
3a. Green Manure x Growing Period Duration Trial on Island of Hawaii: One crop of nematode-susceptible buckwheat (Fagopyrum esculentum) was grown for 3 months, then plowed under and allowed to decompose for 1 month. Then, two promising green manure crops, sorghum x sudangrass hybrid ‘Sordan 79’ and sunn hemp, were grown for 1, 2.5, and 4 months in Pepeekeo, Hawaii (Figure 1). A weed mat treatment and a weedy treatment were included for a total of eight treatments and four blocks. Root-knot nematode populations differed significantly in green manure treatments after decomposition with the highest mean number in 1-month sunn hemp and the lowest mean numbers in 4-month sunn hemp and 2.5-month ‘Sordan 79.’ Reproduction of reniform nematodes differed significantly in green manure treatments with 4-month ‘Sordan 79’ supporting the highest reniform nematode reproduction (Figure 2). In contrast, 2.5- and 4-month sunn hemp was a poor host to the reniform nematode with lower reniform nematode reproduction compared to 4-month ‘Sordan 79.’

Glyphosate was applied to kill green manures and then taro planted 1 month later. Soil samples were taken prior to planting of taro. Exchangeable potassium (K) differed significantly among treatments with the greatest level in 4-month ‘Sordan 79’ followed by 2.5-month ‘Sordan 79,’ and the lowest level in the weed mat treatment. After 1 month of growth, tissue analyses of taro were conducted. Significant differences in leaf K were found in taro grown for 1 month, with the highest K concentration in taro grown after 2.5-month ‘Sordan 79’ and the lowest K concentration in taro grown after the weed mat treatment.

Visible treatment differences in growth of taro were evident at 3 months after planting (Figure 3). A representative taro plant was harvested in each plot at 5 months after planting and nematode populations associated with roots measured. Fresh weight of taro corms was negatively correlated to reniform nematode populations, with smaller corms having a higher nematode population (Figure 4). These results suggested that the reniform nematodes possibly contribute to yield reduction in taro. Reniform nematode populations increased up to 6-fold during the growth of green manure crops (Figure 5).

Corm yields of taro were determined after 9 months of growth. Green manures grown for 2.5 or 4 months had a beneficial effect on the individual fresh corm weight of the subsequent crop of taro. This beneficial effect could be due to: a) lower initial numbers of root-knot nematodes; b) lower reproduction of reniform nematodes; and/or c) greater exchangeable K in soil and in taro leaves, perhaps caused by slow release of nutrients during decomposition of green manure crops.

3b. Green Manure Species Trial on Island of Molokai: Two crops of nematode-susceptible buckwheat were grown for approximately 6 months. Root-knot nematode populations in the soil were extremely low, although an average of 6 root-knot nematodes was found per gram of buckwheat roots. After approximately 2 months of decomposition, six green manure crops plus a weedy control were planted and grown for approximately 3 months, then cut back with a flail mower (Figure 6), tilled under, and allowed to decompose for 4 months. Then, taro ‘Bun long’ was planted. No significant differences were found in numbers of root-knot nematodes at the start of the taro crop, nor mid-way through the taro crop. No significant differences in yield of taro were found when taro was harvested at 9 months after planting. It is difficult to measure the effect of green manures on root-knot nematodes when nematode populations were low at the start of the green manure crop.

3c. Tillage of Green Manure trial on Island of Hawaii: Sorghum x sudangrass hybrid ‘Sordan 79’ was grown for approximately 4 months. In the Delayed-Tillage (DT) plots, ‘Sordan 79’ was killed with glyphosate and taro planted into the dead stubble after 45 days of decomposition. In the Conventional Tillage (CT) plots, the green manure crop was plowed, tilled (Figure 7), and after 45 days of decomposition three varieties of taro (‘Bun long,’ ‘Maui lehua,’ and ‘Eleele naioea’) were planted. After 3 months of growth in the DT plots, soil was tilled and hilled around the base of taro plants. After 9 months of growth, fresh weight yield of taro corms with rotten portions removed was significantly greater in the CT plots compared to the DT plots. This yield difference was due to the significantly greater percent of corm rots in the DT plots, probably as a result of root damage caused by the delayed tillage that lead to invasion of corms by pathogens.

Performance Target 4: Assess the impact of green manure crops on soil bacterial, fungal, and nematode communities in the field.
Three field trials were selected for molecular analysis of the soil microbial community as affected by green manure treatment for control of root-knot nematodes in taro. The trials included a green manure species trial replicated on the islands of Molokai and Oahu and a green manure x growing period duration trial established on the Island of Hawaii. The green manure trial on Molokai has been completed (see results above in #3b) and is in progress on Oahu with a scheduled completion date of February 2007. The first green manure x growing period duration trial on the island of Hawaii has been completed (see results above in #3a). Soil samples collected at the end of the green manure growing period and the end of the green manure decomposition period were chosen for molecular analysis. Soil samples collected at all other time points have been archived and stored at -80°C.

Amplified Fragment Length Polymorphism (AFLP) was originally proposed for the molecular analysis of the total soil microbial community; however, due to problems with reproducibility an alternative method Terminal Restriction Fragment Length Polymorphism (T-RFLP) was used. A pilot study using several Molokai and Hilo soil samples verified the reproducibility of the T-RFLP fingerprints. Briefly, the target gene (16S rRNA) was amplified from the total community DNA with the universal bacterial primers P8F and P1492R. The amplified product, which contained one fluorescently labeled end, was then digested with one of three restriction enzymes (HaeIII, MspI or RsaI). The fluorescently labeled fragments (T-RFs) were separated by size on a Li-Cor IR2 DNA Sequencer to create the community fingerprint. The fingerprint data from the three restriction digests were submitted to phylogenetic assignment (PAT, http://trflp.limnology.wisc.edu) to classify individual fragments based on known sequence data available in the Ribosomal Database ProjectII (RDPII). Additionally, community diversity indices were calculated based on the number of T-RFs generated and the Dice similarity coefficient (S) was used to measure similarity between two communities and infer differences between treatments and times. T-RFLP fingerprint data have been generated for the Molokai trial, and the Oahu and Hawaii trials are currently being processed.

An additional trial examining the effect of Delayed Tillage and Conventional Tillage practices on the soil microbial community was conducted (see results above in #3c). Soil samples for molecular analysis were collected at the end of the green manure growth period and at the end of the green manure decomposition period (45 days). Total soil microbial DNA was isolated and the full-length 16S rRNA gene amplified using the bacterial primers P8F and P1492R. The amplified fragments were cloned using the TOPO-TA system and sequenced on an ABI 3700 DNA Analyzer. Four sequence libraries were constructed: conventional-till and no-till at the two time points noted above. A community profile was constructed from each sequence library, and species richness and community diversity indices were calculated based on the clone sequences. Further analysis of the sequence library data is being pursued.

Performance Target 5: Conduct effective educational outreach to disseminate project information to taro growers.
A field day was conducted on Molokai on June 16, 2005, when the green manure crops matured. Also, a field demonstration was held on the use of a flail mower to chop green manure into smaller pieces to enhance decomposition, instead of using a rotary mower. On December 2, 2006, Mr. Alton Arakaki conducted a demonstration workshop on cleaning and treating taro ‘huli’ (i.e. vegetative propagating material consisting of the upper 0.5 cm of corm plus the lower 30 cm of petiole) to minimize transferal of nematodes into a new field.

Our original farmer-cooperator on the island of Hawaii, Mr. Tom Menezes, sold his farm and is no longer growing taro. We located another farmer-cooperator, Mr. Bill Beach, who is growing taro using organic farming practices. We distributed seeds of the most promising green manure crops to Mr. Beach: marigolds (cvs. Scarletade and Orangeade), sunn hemp, and sorghum x sudangrass (cv. Graze-all). Unfortunately, the magnitude 6.7 earthquake of October 15, 2006, blocked the Hamakua ditch, resulting in loss of irrigation water to this farm and poor growth of the green manure crops.

Impacts and Contributions/Outcomes

Mr. Anthony Ortiz earned his master’s degree in the department of Plant and Environmental Protection Sciences in December 2005. He was responsible for screening taro germplasm for resistance to root-knot nematodes and for screening green manure species in the greenhouse for host status to M. javanica. In addition, Anthony presented his research results during 2005 at the Annual Meeting of the Society of Nematologist in Fort Lauderdale, FL.

Ms. Andrea Blas is in the process of earning a Ph.D. degree in the department of Molecular Biosciences and BioEngineering. She presented her research results as part of the Minority Graduate Student poster contest at the American Society of Agronomy – Crop Science Society of America – Soil Science Society of America during November 12-16 in Indianapolis, Indiana. She also presented a poster at the Society of Nematologists’ annual meeting held in Kauai, HI from June 18-21, 2006.

Publications/ Presentations
Ortiz, A. 2005. Sustainable control of soil-borne pathogens in dryland taro cropping systems. M.Sc. thesis, University of Hawaii, Department of Plant and Environmental Protection. 78 pp.
Ortiz, A., B.S. Sipes, J. Cho, J.Y. Uchida, and S. Miyasaka. 2005. Sustainable control of soil-borne pathogens in dryland taro. Journal of Nematology 37: 386-387.
Blas, A.L., Q. Yu, B. Sipes, R. Ming, and S.C. Miyasaka. 2006. Green Manure Effects on Soil Microbial Community in a Root-Knot Nematode (Meloidogyne javanica) control System in Taro (Colocasia esculenta). Abstract 37-8, American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, 2006 International-Annual meetings, November 12-16, Indianapolis, Indiana, p. 45.
Blas, A.L., Q. Yu, B. Sipes, S.C. Miyasaka and R. Ming. 2006. Characterization of soil microbial communities using 16S rDNA ribosomal sequence tags. Journal of Nematology 38:263.
Miyasaka, S.C., J. DeFrank, B. Sipes, and A. Blas. 2006. Green Manure Effects on Root-knot Nematodes (Meloidogyne javanica) and Following Taro (Colocasia esculenta) Crop. Abstract 283-7, American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, 2006 International-Annual meetings, November 12-16, Indianapolis, Indiana, p. 148.

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