Survival of Taro: Agronomic and Pathological Research For Sustainable Production
Single hyphal-tip cultures of the new undescribed Phytophthora have been prepared. These cultures will be used to characterize and identify this new pathogen. Two new experiments using cover crops and organic or standard chemical fertilizers are in progress. Growers are using new research knowledge to employ and create disease control methods, strong evidence of project success.
Objective 1: Efforts to characterize the new homothallic Phytophthora species isolated from pocket rots of taro (Colocasia esculenta) are continuing. Great difficulty has been encountered with this pathogen and simple procedures, such as the establishment of single hyphal tip cultures, have been problematic. Single hyphal tip cultures are needed to insure the genetic purity of each isolate before characterization can begin. Single tips of other Phytophthora species, which are less than 1 mm, grow well. However, for this new Phytophthora species, tips that were l mm long did not grow. Several types of agar media were also used to encourage tips to grow. Finally, a combination of several methods was employed to establish single hyphal tip cultures. These included growing the parent culture on thin agar that restricted mycelial growth to a single layer, transferring long tips/branches (2 mm+) to water agar with sterilized rye seeds and placing tips adjacent to the seed. The Phytophthora tips grew slowly but were reestablished by using these methods. Single tips of the three populations of the new Phytophthora were prepared and characterizations were begun. Preliminary measurements taken of the parental cultures demonstrated no difference among the isolates for length or diameter for sporangia produced on 10% vegetable juice agar (V-8).
Data for rate of colony expansion, hyphal diameter, growth in a temperature range, sporangial dimensions, sporangial cauducity (detachment) and other sporangial characteristics, sexual spore morphology (oogonial, antheridial and oospore diameter and length), presence and size of survival spores (chlamydospores), and other characteristics will be determined. Procedures for zoospore production and other morphological characteristics will be described. At this point, our inclinations are that this is an undescribed species of Phytophthora and these data are required to establish that this is a distinct species.
Objective 2: Kaala Program Site: The plan for the second test at the Kaala Educational Center was placed on hold as the Center did not receive funds to continue beyond 2003. Most of the staff was released and only one person and one part-time person are at the Center to maintain it. Tours for youngsters are continuing. Previous: In a previous cropping cycle, a plot at the Kaala Center was fallowed and cover cropped with Sunn Hemp, a legume that increases the nitrogen level in soil. The crop was cut and allowed to decompose. Blood meal was added to increase the amount of nitrogen needed for the crop. A control plot, that was not fallowed, had no cover crop grown in the paddy, but received blood meal to equalize the nitrogen level. The field with the legume cover crop yielded very high survival rates (80%) of planted taro huli (young plants) and total biomass produced was very high (370 kg), but overall yield was poor after a year, possibly because of the frequent addition of blood meal to the paddy. Only about 32% of plants survived in the control plot and biomass was very low (80 kg). Progress: At the Kaala Educational Center, the plot of land where the cover crop was grown and tilled into the soil still contained a lot of nutrients after the original taro crop was harvested. The remaining nutrients are from the residual organic matter and the unused organic fertilizers that had been added for the first taro crop. This plot was replanted and after six months we visited the site. The taro crop was growing very well. After 12 months, the crop was harvested and a high yield was reported by the grower. Corms were large and corm rots were very few. Blood meal was only added at planting. The growers at Kaala are very pleased although we were not able to quantify this harvest. This planting was not part of our experiment and the second experiment was designed for an area adjacent to the paddy used in the first experiment.
After receiving news that the Kaala project did not receive funds for 2004, Drs. Janice Uchida and James Silva met with the leaders of the Kaala Educational Center, the field managers, growers, and other personnel including volunteers. We reviewed the entire project and what we had learned. They were anxious for us to return in the future to build on what we had learned in the first test. Beyond the scientific information that we gathered at this site, it was even more important for this group that we made a tremendous impact demonstrating that faculty from the University was willing to travel on numerous occasions to Waianae and make the effort to share our knowledge with them. We built this project as a partnership, that each of us had things to test and learn. An important bond has been built with the members of the Waianae community, an area with high crime, low educational levels, high numbers of food-insecure residents, and a needy social population with native Hawaiians in the ethnic mix.
Haleiwa site: A taro paddy was drained, tilled, and planted with Sunn Hemp as a cover crop, and another adjacent paddy was planted with sections of Sunn Hemp, clover, and alfalfa. Overall, Sunn Hemp grew quite well but was not as tall as in other sites, averaging about 4-5 ft at flowering. A few Sunn Hemp plants wilted and isolations revealed a Pythium tentatively identified as P. carolinianum. This Pythium species has not been pathogenic to taro. Sections of the cover crop in each paddy grew poorly as the Haleiwa grower struggled without an effective method to water these crops. When the fields are constructed, drainage is deliberately made poor allowing the taro crop to be grown partially submerged in water. Normally, the taro paddy is flooded and the water slowly drains through the bottom of the paddy. This is different from other taro farms that drain the water from the paddy. For the cover crops, the water was left on overnight at a low flow rate but this was excessive and plants grew poorly. Eventually the water was reduced to only a few hours late in the day but growth was not greatly improved. Soil analysis did not reveal high salt levels, excessive nutrients, or nutrient deficiencies. Seeds that accidentally landed on the banks and walkways grew very well and were more than 3 times the size of plants in the paddy. The level of nodulation was good with Sunn Hemp but poor for alfalfa and clover growing in the paddy and on the banks. After 6 weeks, the cover crops were cut and tilled into the soil. At least 4 weeks were allowed for decomposition of the organic matter. The paddies were then formed, flooded, and planted with taro cuttings.
To obtain better growth of clover and alfalfa, in the future we want to grow these on the top of furrows to improve the drainage. The banks were only about 12 inches or less high and growth on these raised sections were good.
The original experimental design had the control paddy planted with taro until it was time to plant the new crop. Growers generally allow only a month between crops. The paddy adjacent to the cover crop paddies was selected as the control paddy and planted with taro. The taro crop was 4 months old when the Sunn hemp in the fallow paddy was cut and worked into the soil. Taro growth was extremely good (and is frequently good for that time of year and that stage of growth) and these plants were to serve as the source of cuttings for plants needed to plant the fallowed paddy and to replant the control paddy. The entire design and the schedule of treatments had been discussed twice with the owner. Unfortunately, when the owner saw the great growth of the taro in the control plot, he did not want to sacrifice this crop that was growing and instead provided another field in a different area to serve as a “control.” The field manager and county agent tried to reason with him but the concept of the control and its value could not be impressed upon him. He is a successful entrepreneur and well known for his obstinacy. Thus, although the best location for the control would have been in the nearby paddy, this could not be obtained. We were informed of this development after the fields had been planted.
The crop is now almost 7 months old and will be harvested in 5 to 6 months. The weather in the past 6 weeks has been extremely wet and high levels of the leaf blight pathogen, Phytophthora colocasiae, are present. The dew that persists on the leaves until well past 11 a.m. maintains moisture on the leaves and the humidity in the canopy is high. Sporangial (spore) production on every lesion is extremely high and is easily distributed by air movement. All fields were severely blighted and this is not unusual for this time of year.
Four samples of plants were obtained randomly from the fallowed paddy, the control paddy #1, that was assigned to us, and a second control paddy #2, adjacent to the fallowed field. The second control paddy # 2 was 4 months older but was needed since control paddy # 1 assigned to us had a different soil type and different microenvironment. Each sample consisted of a large corm and several new cormels. Disease levels and biomass produced were as follows:
Treatment Disease levels in cormels
Control 1 (assigned paddy away from fallow paddy) 34%*
Control 2 (paddy adjacent to fallow paddy but older) 31%
*about 32-35 cormels per treatment. The corms also had different levels of pocket rot but the data are not significant because only 4 corms were harvested per treatment. Root development in all treatments was good.
Treatment Average biomass
Control 1 1,435
Control 2 (4 months older) 1,353
Thus disease levels were lower in the fallowed paddy but yield was higher in Control paddy # 1.
According to the field manager, the paddy in which the Control # 1 is growing consistently has very high yields and is one of the best fields on the farm. He believes it is a poor comparison. Control # 2 is in the same soil series as the Fallow paddy but it is 4 months older, therefore biomass is larger. Thus, the strong growth in the paddy with Control # 1 is evident as the biomass is already the same or exceeding the biomass in a paddy that is four months older (Control # 2).
We find this change in the experimental design to be highly unfortunate but that is the disadvantage of working with growers. This grower however, began as an individual who had an extremely low opinion of the University and has now become a strong supporter of the College of Tropical Agriculture and Human Resources.
Waihole Site #3:
Most recently we secured an agreement with a family in the Waihole area, a traditional ecosystem for taro growing. It is located in a cooler area of Oahu and has adequate water supply. The family has been growing taro for two generations and also sells their taro and taro products. It is a small but very successful business. At this farm, Sunn Hemp was planted as the cover crop and the treatments were 4 levels of blood meal (0, 50, 100 and 200 lbs per 1,000 sq ft of blood meal). The Sunn Hemp was grown, cut, incorporated into the soil, and allowed to decompose. The field was tilled and sectioned with plastic barriers to separate treatments. This method has worked well in several other experiments. The blood meal was added to each section and volunteers helped to work the material into the paddy. The water level was kept low to avoid losing the blood meal. After a month, the field was planted with young plants. Because the area is cool, there is little taro growth from November to March. At this time, almost all plants have survived but growth is very slow.
Dr. Jonathan Deenik, who was recently hired in the Tropical Plant and Soil Science Dept, has joined our program. He will replace Dr. Silva and is gradually learning about the tests we are conducting. At the Waihole site, he is measuring and tracking the level of ammonia and nitrates in the taro paddy. This intensive type of tracking has not been done for taro culture and is timely as the fate of nitrogen is so critical. Our Maui site, selected to monitor nitrogen levels, is still on hold as the county agent was injured recently. However, the resources for the Maui test are being used at Waihole and before the end of this SARE project we hope to have some soil and water analyses completed to determine the movement of nitrogen in the taro paddy.
The nitrogen was added to the paddy as complex organic nitrogen (primarily proteins) and forms ammonia in the soil. In aerobic soil environment, microbial action converts this ammonia to nitrates. To prevent this conversion, the paddy is kept flooded and maintained in an anaerobic state. A soil analysis from each of the treatments was made 2 weeks after the blood meal was added. This analysis confirmed that the control or 0 treatments had very low levels of ammonia, the 50 and 100 lbs had moderate levels, and the 200 lbs/1,000 sq ft had the highest level. At 4 weeks after planting the plants in the control treatments were smaller and more chlorotic than the 200 lbs/1000 sq ft treatment, but given the slow rate of growth in this cool season, leaf samples were not taken. Leaf samples will be taken in January to compare the nutritional levels through tissue analysis. Plant health was good for all treatments and a few plants that looked weak were carefully removed and roots and stems were examined. All were found to be healthy and replanted in the same location.
At this time most growers are using the 24″ spacing between plants. Thus, over the past two years many more growers have increased the spacing in their fields. Yields are nearly the same as for the 18″ spacing between plants yet cost of planting (labor), cost of new plants, and time (labor) for harvest are all reduced by converting to a 24″ interplant distance. More growers are aware that planting young plants with any blemish (small rots) will decrease survival of these plants. At this time growers have reduced the amount of nitrogen used from nearly 700 labs per acre to about 350 lbs per acre. They also see the value of testing the soil before planting to determine nutrient status and how it can be corrected.
Information Dissemination: Drs. Uchida and Silva made several public presentations on progress and findings. Dr. Uchida participated in the Conference on “Sustainable Pest Control in the Tropics,” held in Hilo, Hawaii, and presented the finding on results obtained with cover cropping and use of blood meal in Hawaii (January 2003). The conference was attended by, statewide county agents, farmers interested in sustainable agricultural practices, the Hawaii Organic Growers Association, and the Natural Resource Conservation Service. In June, Dr. Uchida presented the results of the “Pathological Findings for Taro Pocket Rot” at the annual American Phytopathological Pacific Division Conference held in Kona, Hawaii. Over 100 researchers and plant scientist attend the APS meetings. In July 2003, Dr. Uchida participated in a telecom program with the South Pacific Islands and made presentations on the “Concepts of Biological control” and the “Use of Compost and Cover Cropping in the Tropics” to an audience of researchers and major growers from the Pacific area. She also presented a Graduate Seminar in the Department of Natural Resource Environmental Management on a similar topic in November 2003. Dr. Silva made a presentation and conducted a field demonstration of our test at Haleiwa in July of 2003 to a statewide conference on Integrated Pest Management. In November 2003, Drs. Uchida and Silva presented a poster and discussed taro pocket rot at a Food Industry Conference at UH.
Excellent progress has been made on all objectives except to repeat an experiment tracking the movement of nitrogen from the paddy into the ecosystem. We will rely on soil and water analyses from an experiment on Oahu to gather information on the fate of nitrogen in the taro paddy.
This project has been given a 3-month extension and data from two fallow plus cover crop experiments will be gathered.
Impacts and Contributions/Outcomes
This project has goals that included the improvement of yield by controlling disease; the improvement of the soil environment by increasing biodiversity through the use of fallow periods, cover cropping, and composting to determine the occurrence of environmental damage from excess loss of nitrogen fertilizers; and to help the community by increasing farm profits and rural development through sustainable agricultural practices. After 3 years we are witness to a unique transfer of knowledge from researchers to the growers. In general, as we have made presentations on the cause of pocket rots and related host responses, growers now understand the pathology and are using this knowledge to create ways to reduce pocket rots. One plan independently developed by two growers is to reduce water levels during periods of heavy blight. We have repeatedly stressed that during high leaf blight periods, when the number of leaves are reduced to one young leaf per plant, the photosynthetic ability of the plant is tremendously reduced. Growers know that this translates to less sugar and therefore less starch to increase the size of the corm. Thus, the accompanying concept that biochemicals needed to protect the plant are also severely decreased during this time is reasonable to them and well accepted. Our finding that pocket rots are initiated at the base of the petiole has also helped growers. Combining this information with the knowledge that the pathogen we have identified is a Phytophthora that thrives in the water, they have reduced the water level in the field, keeping the base of the petiole drier. Without free water surrounding the petiole, infections are reduced during periods when the plant is highly susceptible. This change in cultural practice is much more labor intensive, but having created the plan for themselves, they are willing to do it. This is the best example of shared learning and improvements in the agricultural community.
Photographs will be sent as hardcopies.
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