Survival of Taro: Agronomic and Pathological Research For Sustainable Production

Survival of Taro: Agronomic and Pathological Research For Sustainable Production

Summary

Taro (Colocasia esculenta), suffers from a mysterious and destructive pocket rot disease. A new, very difficult to culture homothallic Phytophthora species was isolated from diseased taro corms. Slow growth and unknown nutritional requirements make this new Phytophthora extremely difficult to isolate, but it has reproduced pocket rots on inoculated disease-free taro plants. Plants are more vigorous and larger in a test plot using fallow plus legume-incorporation compared to a plot without fallow. Thus a major causal organism of taro corm rot has been identified and a sustainable method to control this and other pathogens is being tested.

Objectives/Performance Targets

1)To deterrmine the cause or causes of taro pocket rot, the influence of leaf blight on corm rots, and to develop control measures for these disease complexes.
2)To determine the effect of fallow type and period, on disease severity of the following crop.
3)To develop nitrogen fertilizer recommendations that maximize growers economic resources and protect the environment from movement of excess nitrogen.
4)To educate growers, discuss results and implement use of research findings.

Specific results:
Objective 1: Repeated transfers show that growth rates of the new homothallic (self fertile) Phytophthora are adequate on 10% vegetable juice agar. Culture blocks of Phytophthora were applied to disease-free taro plants, planted in vermiculite and maintained in water in the greenhouse. In 3 tests, taro plants that were inoculated with the new Phytophthora, developed pocket rots. Our hypothesis is that this Phytophthora attacks the corm near the base of the petiole and forms a small rot. The rot expands but secondary fungi (Rhizoctonia, Fusarium, Acremonium, etc) invade the pocket and apparently overgrows the Phytophthora. Furthermore, in the hundreds of taro corms assayed, small active rots, characteristic of rots caused by Phytophthora are never present in the lower two-thirds of the corm. Inoculations with these secondary fungi have not reproduced the pocket rot symptom. Phytophthora has not been present in the pocket rots, although only plants inoculated with Phytophthora developed pocket rots. Frequency of pocket rot formation by this new Phytophthora has been high while an occasional corm rot was caused by P. colocasiae. Reflecting the absence of the new Phytophthora in pocket rots, it was not reisolated at the end of the test from the pockets. Reisolation of Phytophthora must be attempted in the earlier stage of pocket rot formation. This test is in progress, although the goal will be difficult to achieve. Isolation of this new Phytophthora has been extremely problematical, probably accounting for its years of elusiveness. Efforts to develop a selective medium are in progress but the Phytopthora has been extremely sensitive to nearly all inhibitors used to eliminate bacteria and other fungi. Our best efforts will continue on this objective and will be focused on novel methods to surface disinfest sections of diseased tissue.

Objective 2. A new cooperator was located on the island of Oahu. This group, the Kaala Educational Program, is growing taro in Waianae valley and reviving taro culture as a method to save the youth at risk in the community. Disease levels and declining yield were major problems. Two experimental plots are being compared at this site. The taro corms from plot #1 were harvested, the soil was dried, and the plot was planted with the legume, Sunn hemp. Sunn hemp was selected because it fixes nitrogen and improves the nutrition of the soil. After about 6 weeks, the Sunn hemp was cut, added to the soil, and allowed to decompose. In plot #2, the traditional practice of taro harvest followed by replanting was used. Before planting, a soil analysis was conducted for levels of nitrogen, potassium, phosphorus, calcium and soil acidity for both plots. The plots were amended with organic fertilizers to make the nutritional components as equal as possible at planting. All plants planted in both plots were disease-free taro plants prepared by our program. Three weeks after planting, the plants in plot #1 were larger and more vigorous. Thus preliminary results are encouraging. Plant height measurements and tissue analysis will be conducted at 3, and 6 months.

These disease prevention treatments are the most difficult for growers, as they are required to change traditional methods and routine cropping practices. It has not been easy to find a commercial farmer who would conduct this experiment. Fortunately the Kaala Program, has an educational mission, was very interested in decreasing pathogens in their paddies, and agreed to a cooperative study with us. As non-commercial fields, their paddies are smaller but well cared for and will give good preliminary results. Similar experiments on Kauai and Maui are also being planned in commercial fields. These new cultural practices tested on the farms are one of the best communication tools.

Objective 3: An experiment to test the amount of nitrogen needed and the timing of the nitrogen applications was conducted on the island of Maui. This area is very different from any of the other experimental site. In many parts of the state, fresh water supplies are decreasing, while water is relatively abundant there. Testing of this new environment was viewed as a benefit at the onset of the test. Historically, large fields of taro had been grown there by the Hawaiians. Today, it is abandoned rangeland covered with thick shrubs that the farmer removed, and recreated the paddies using clean water from springs at the base of the mountains. The site is cool, but low temperatures in the 50?s and 40?s during the winter months (Jan, Feb) were unexpected. The taro planted in the experiment did not grow during that period. Root development was extremely slow and growth did not resume until March or early April. The long period of poor growth, killed many of the cuttings in the test. At planting, the taro cuttings have two petioles, no leaves but a new leaf within the apical tip. Without roots, the new leaf failed to develop and the cutting was lost. The grower tried to salvage the test by replacing the dead plants, but this increased variation within the test (by May, there were assorted plant sizes and the plots lacked uniformity). Fertilizer is commonly applied by, first draining the water level to about 2 inches, then broadcasting the granular fertilizer in the paddy. The fertilizer dissolves and the water is left to move into the soil. After a few days, a little water is re-added to the paddy and allowed to move into the soil again. This process is repeated three times. By turning off the water entering the paddy, the water was expected to drain at the Maui site. But the entire ecosystem was saturated and water did not move into the soil. After 2 weeks, water levels remained the high and the scheduled fertilizer treatments were added to the paddy. Frequent (daily) heavy rains kept the paddies filled with water, the soil saturated, and wet weather prevented significant evaporation. This was a problem throughout the experimental period. Thus, variation in this test was high and a clear growth response to each treatment was not observed at harvest. Leaf blight was occasionally severe but at harvest, pocket rots and soft rots were extremely low. This test will be repeated in another location, several miles closer to the ocean. The pathology of the area will be continued however. We need to identify the factors that contributed to such a low level of soft rot and pocket rots. Another researcher?s plot about 15 yards away, harvested a month before this test, was nearly completely lost to soft rots.

Objective 4: Presentations of recent pathological findings and agronomic studies were made by Drs. Uchida and Silva at the (Statewide) Taro symposium held at the University of Hawaii campus in August. Presentations were well received by growers who were very impressed with the reproduction of pocket rots and the range of agronomic studies being conducted. Full-page colored illustrations of photomicrographs of pathogens, microscopic biological control organisms that feed on pathogens, and survival structures of pathogens were used in discussions with the Waianae group. An open discussion using the 8?X11? colored photographs greatly aided the exchange of ideas with this group of traditional Hawaiian growers. In October, Drs. Silva and Uchida presented seminars and discussed taro growing and disease problems with high school teachers who are using taro culture in their curriculum. A field day was also held with these teachers from throughout the State. The country agents have been in close contact with the farms on each island. Farmers commonly consult with the agents monthly if not more often. This has built a valuable link to the taro growing community.

Accomplishments/Milestones

Excellent progress has been made on all objectives except for the experiments to determine the timing of nitrogen applications and the possible movement of nitrogen into the ecosystem. A major cause of pocket rot has been identified and efforts to control this pathogen and others are being tested with a legume fallow experiment. In the next year, we will repeat the legume/fallow and possibly an azolla/fallow experiment as a sustainable method to increase nitrogen and reduce pathogen populations. A cooperator has been located in Hanalei Valley for this test. The nitrogen test will be repeated at another location also. Testing of multiple fungi to reproduce pocket rots will be discontinued since the new Phytophthora has reproduced pocket rots. Efforts to develop a selective medium will continue but greater efforts will be made to test disease control compounds in greenhouse tests.

Impacts and Contributions/Outcomes

Our project goals are to improve yield by controlling disease, to improve the soil ecosystem by increasing biodiversity through 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 preliminary growth of taro in a legume/fallow treated plot is very encouraging. Significant reduction in the amount of nitrogen fertilizer is occurring throughout the state for taro growers. Our tests determined that nitrogen application of 800lbs/acre could be changed to 350 lbs without loss in productivity. This saves the grower at least $250 per acre from this practice alone. More growers are using less nitrogen and at this time we have not found any that are still using 800 to 1000 lbs per acre (a standard 2 years ago). Beyond the cost of the fertilizer are the reduced labor cost, the improved health of the plant (high nitrogen levels encourages succulent growth and more disease), and reduction in the amount of nitrogen added to the wetland ecosystems.

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