Survival of Taro: Agronomic and Pathological Research For Sustainable Production

Survival of Taro: Agronomic and Pathological Research For Sustainable Production

Summary

Three populations of the new homothallic Phytophthora, the cause of pocket rots, have been collected and are being used to characterize this pathogen. The best method to isolate this Phytophthora is use of internal tissue from young lesions and plating on water agar. Disinfestation with chlorine inhibits growth and prevents isolation. A fallow period with a legume cover crop increases plant survival and yields. However, disease levels were also high and were caused, for the first time by Phytophthora colocasiae collar rots. Addition of nitrogen fertilizer after 6 months increased corm rots.

Objectives/Performance Targets

Objective l: The new homothallic Phytophthora is being cultured on 10% vegetable juice agar. Growth has remained slow but transferring colony sectors that produce oospores has helped to maintain this nearly obligate pathogen. This pathogen causes the mysterious pocket rots and is only isolated in the initial stages of infection. This phase, referred to as the active rot stage, is distinct, as there is no wound periderm around the corm lesion. These lesions always occur at the base of the petiole or sometimes expand into the apical tip of the taro corm. Once the wound periderm forms, the Phytophthora does not grow. This is an intriguing phenomenon and one that needs further study, as it has disease control implications. Three populations of this new Phytophthora have been collected from: Hanalei Kauai, Haleiwa, and Waianae Oahu, all important taro production areas. Over 70% of the taro grown for the traditional Hawaiian poi dish, is grown in Hanalei valley. The Haleiwa site is a large wetland farm of a major commercial grower and taro processor. The Waianae site is the Ka?ala Community Educational Center that is using taro to revive the local community and to rehabilitate young offenders. With these three populations we can begin to characterize this new Phytophthora and determine if it is a new species. Tests to reisolate the homothallic Phytophthora were not successful and efforts will continue on this objective (inoculation followed by searches for active rots and reisolation).

Presently, the best method developed to isolate this new Phytophthora is to: carefully clean the specimen by washing or wiping with alcohol, trimming all excess tissue from the lesion, wiping with alcohol, and exposing the internal section of the lesion. A sterile blade is used to remove the interface tissue sections from the lesion, which are placed on 1.7% water agar. Microscopic examination of this tissue reveals wide, coenocytic, irregular mycelium, characteristic of the Oomycota. The pathogen is intolerant of low concentrations of disinfesting solutions (chlorine or alcohol). Tests show that growth will not occur if the same disease material is surface disinfested with 0.52, 0.25 or 0.05% sodium hypochlorite. Disinfestation with 50 or 70% ethyl alcohol also inhibited pathogen growth. Prolonged washing in running water was also unsuccessful. The most common contaminants are bacteria, which rapidly develop large populations on the starch-rich corm tissue. The present method works but is far from ideal. Over twenty plates are needed per corm, allowing a lucky piece to produce the Phytophthora without a bacterial contaminant. Substantial hyphal growths must be transferred in order to establish a pure culture. Small sections of single hyphal tips or single sporangia do not grow. Improvements in isolation procedures will continue. A direct relationship between the occurrence of Phytophthora colocasiae leaf blights and the susceptibility of the taro corm to pocket rot has been repeatedly observed. Phytophthora colocasiae causes severe leaf blight during wet weather and frequently reduces the number of leaves per plant from 4 to 7, to half of the youngest leaf. Thus, beyond the reduction in starch for the corm, clearly the ability of the plant to manufacture defensive compounds is also compromised during Phytophthora foliar epidemics

Objective 2: The taro crop planted at the Ka?ala Program was harvested. Two treatments were compared: one plot had a cover crop of Sunn hemp (Crotalaria) and one did not (the control). The Sunn hemp was grown for about 6 weeks, cut, plowed into the plot, and allowed to decompose. The control plot used the traditional method of harvest, followed by replanting. The nutritional status of the soil was determined before planting and organic fertilizer was added to equalize the nutritional components between plots as best as possible. The plots were flooded and planted with 135 plants per plot. During the experiment, leaf tissue analysis indicated low nitrogen content for both plots. Thus, blood meal was added to the soil. Blood meal was added a total of 7 times (once before the test began and once per month for 6 months) to maintain high levels of nitrogen in the early months of the crop. This procedure was successfully employed in Hanalei Valley and growers using inorganic fertilizers also maintain high fertility early in the crop cycle. However, the soil at Waianae became dark, odoriferous, and plants were stunted. The growth at 3 months was better in the fallowed plot but in the next 3 months growth remained poor for both plots. The suspected cause was the blood meal applications. At 6 months, the application of fertilizer is usually stopped for this crop and this was done at Ka?ala. There was a gradual improvement in the growth of the plants. However, during the first 6 months and especially in the 4th and 5th month, many plants in the control plot died. Stunted plants had poorly developed roots, and shoots were missing. There was little root or corm rots. Some of the leaves were blighted with Phytophthora colocasiae.

At harvest, 36% of plants survived in the control plot while 80% survived in the fallowed plot. The total biomass for the control plot was 80 kg verses 370 kg for the fallow plot (Figs 1 and 2).

Unexpectedly, disease levels were high in both plots. Within the dense canopy of the fallowed plot, 86% of the plants had collar rots (Fig. 3). In the control plot, with fewer plants surviving and better air movement, 40% of the plants were diseased. When corms were separated into those with high disease (more than 10% rot) and little disease (less than 10% rot) there was little difference between categories 47% and 39% for the fallow plot and 22% and 29% for the control plot. The plants harvested from the Waianae site were extremely different from all other sites harvested. Normally, the original plant or mother corm is harvested after 12 months and weighs about 1 kilogram. It produces several smaller cormels that are about 200 to 500 g each. At Waianae, many of the mother corms were tiny (<150 g) without apical tips and had produced 2-4 smaller cormels without apical tips. These cormels had produced 1-2 cormels, and these then produced the huge cormels that were harvested. For the fallowed plot, the total corm plus cormel weight was 162 kg and total top or foliage weight was 199 kg. This high foliage weight indicates that the harvest was early since maturing taro converts the foliage biomass into starch in the corm. For the control plot, total corm plus cormel weight was 52 kg and foliage weight was 28 kg. Diseased collars were assayed at the lab. The rots were large (30 to 80 mm) and not typical of pocket rots (<10 mm). All of the rots assayed produced Phytophthora colocasiae. This is the first time severe corm rot caused by this pathogen, at such high frequencies, has been observed. Over 20 isolates of P. colocasiae were collected and purified. Morphological studies show that they belong to P. colocasiae, are of the A2 mating type and are not different from other P. colocasiae populations in Hawaii. Drs. Janice Uchida and James Silva met with the leaders of the Ka?ala project along with the field managers and other personnel at Ka?ala. We reviewed the problem with the use of blood meal and the first ever finding of P. colocasiae causing a collar rot epidemic. They were eager to conduct a second test. Modifications for the second test are as follows: a different taro cultivar (a Palauan hybrid that grows well there) will be used; the entire amount of blood meal needed, will be added at the beginning of the test; the blood meal will be allowed to decompose for a month or more in the plots; and a better water flow system will be installed. This test will be started in the next 4-5 months when the site is ready. In the interim, Ka?ala managers will increase the number of cuttings that can be obtained from the preferred Palauan hybrid. In general, at least 300 plants will be needed for two plots. The Ka?ala program was the first group to agree to test the fallow with a legume cover crop, as a disease management option. While the organic matter helped the taro growth, addition of blood meal was a problem. Compared to Hanalei, the temperature at Waianae is much higher and the amount of water is very low, perhaps only 10% of what is available at Hanalei. This apparently made a huge difference in the soils ability to assimilate the blood meal and convert it rapidly to nitrates and ammonia. The bulk of the water that originally flowed to Waianae is now diverted for military and residential uses elsewhere. The Ka?ala project is seeking to increase the allocation of water to the Waianae community and the need for taro culture is an important component. Fortunately, a commercial grower in Haleiwa has agreed to conduct the fallow with legume experiment. This is very significant, as a large commercial operation can better test our hypotheses. The Haleiwa experiment will also serve as an example for many others in the industry. At this time, the plot has been dried, amended with compost, plowed, and planted with Sunn Hemp. In addition, alfalfa and clover have been planted to determine if they can serve as good cover crops. Sunn hemp seeds are very expensive in Hawaii and local growers without proper equipment are having difficulty separating the seeds from the plant. Thus cover crop options are also being considered. Objective 3: We have endeavored to obtain a new site to test nitrogen levels and the timing of the N application. The test requires 12 separate plots and this condition is difficult for many growers. We will continue to search for a site. In a test at Haleiwa, research initially funded by the College attempted to determine whether slow-release fertilizers, alternative N sources and N rates would sustain taro growth with minimal leaching into the environment. At this farm, addition of compost to each paddy is a routine practice and the fields are unique since water does not move from the paddy back into a river or stream. Instead, all of the water drains into the ground. To determine if excess nitrogen was moving into the ground water, long plastic probes were inserted at 1 and 3 feet depths in the paddies for every treatment. A ceramic cap allowed water to seep into the tip of the probe and this water was used for analysis. All treatments were repeated. After 6 months, leaf analysis showed that there was no difference in leaf nitrogen content, between the ?No nitrogen? plot and the highest level of nitrogen (400 lbs/acre). Continued application of treatments were halted in one replicate, but continued in the other replicate. At harvest, growth was excellent for some of the treatments in the second replicate (that had received a second application of nitrogen). However, the first replicate, which had no added nitrogen applied after 6 months, had almost no corm rots. The second replicate, with higher corm weights, had high disease levels. This indicates that application of nitrogen late in the crop cycle will stimulate disease in the corm. Water analysis of the ecosystem showed that the water entering the farm contained 3 ppm nitrogen. This high background nitrogen level prevents the testing of any nitrogen treatments at that site. Objective 4: A Statewide Taro Conference was held in Hanalei in October. Both Drs. Uchida and Silva made presentations at the Taro Conference and the results of our work were shared with growers. Benefits of sustainable agricultural practices were covered (fallow with legume, green mature, crop rotation, etc). Dr. Uchida also conducted a field workshop for growers to observe disease symptoms on freshly harvested taro. Concepts of disease development and the normal microbial processes in the field that reduce pathogen spores during the fallow period were discussed. Good discussions occurred at these field sites as growers appear to be more relaxed and willing to share their experiences. A few growers were anxious to help with our next tests and two volunteered to conduct the fallow plus legume test at Hanalei. On Oahu, discussions were also held with the Ka?ala Waianae group several times during the year. These meetings are in a large traditional hut and full-page colored photographs were successfully used. During the summer Drs. Silva and Uchida mentored 5 high school students in the College?s ?Fast Track? program. The taro project was used as a focal point and students were very interested in the heritage and growth of taro. They participated in water analysis of streams, wetland field trips, and conducted a greenhouse test of the effect of nitrogen on taro plants. We visited Ka?ala and shared our goals and research results from the SARE project. The college faculty mentored a total of 20 students and 5 are now attending the University at CTAHR. The environmental goals and the practical application of our work intrigued the students. Two high school students with their own resources have joined our effort to control P. colocasiae on taro.

Accomplishments/Milestones

Excellent progress has been made on all objectives except the experiment to determine the timing of nitrogen application and the movement of nitrogen into the ecosystem. This will be our top priority for the next year, as we now have cooperators for the fallow tests. In greenhouse tests, efforts will be made to reproduce active rots and reisolate the new homothallic Phytophthora.

Impacts and Contributions/Outcomes

Our project goals remains to improve yield by controlling disease, to improve the soil ecosystem by increasing biodiversity through the use of crop rotation, composting, cover cropping, and fallow, to reduce environmental damage from excess loss of nitrogen fertilizers, and to help the community by increasing farm profits and rural development through sustainable agricultural practices. A major cause of destructive pocket rots has been identified and control measures are being developed. Growers are increasingly interested in using fallow to reduce disease levels. However, as we visit taro growers and make presentation in different communities we continually emphasize how different the taro patches are from island to island. With the diversification of agriculture in Hawaii, farmers are attempting to grow taro in non-traditional ecosystem (i.e. Waianae) and much needs to be learned. Thus, although not economically quantifiable, there has been a gradual change among growers that experimentation can lead to information that will help their crops and livelihood. They have become active partners in our efforts to achieve our goals.
Photographs will be sent with hardcopy.

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