Potamogeton distinctus

Crop: Rice

Why is it a problem? Potamogeton distinctus, locally called shochum, is one of the most serious weeds in transplanted rice in Bhutan. It is an aquatic plant so grows together with the rice. Yield reductions of up to 37% have been reported, even when farmers hand weed one to two times a season. Manual weeding is difficult owing to the weeds ability to propagate from any vegetative part, and effective weedicides are not yet available in Bhutan. Once it is established on a farm it becomes very difficult to eradicate, in part owing to the dormant root stage (turions).

Where and when is it a problem? It was first reported in 1992 and is now widespread in mid-altitude areas (1200-2500 m asl). It has not been reported in lowland rice cultivation.

IDENTIFICATION

Shochum is a perennial, aquatic plant with floating leaves. The leaves are elliptical in shape and form a thick cover on the surface of the water. Roots (rhizomes) grow to 20 cm deep in the soil. The plant produces peduncles that support the cylindrical spike on which flowers are borne. The cylindrical spikes bend down into the water when in fruit. Late in the season the roots form dormant turions which are characteristic banana-shaped organs.

SYMPTOMS

During the early stages of crop growth, rice plants do not show any observable symptoms of weed infestations. However, as the weed progresses, the rice plants show yellowing of leaves and stems becomes weak and slender. Tillering is reduced and the plants remain stunted. If left uncontrolled the rice plants may ultimately die.

Confusion with other pests: This is the only aquatic weed with floating leaves in transplanted rice.

BIOLOGY

Life cycle: Shochum is an aquatic weed with excellent adaptations to dry seasons and to cultivation. It mainly propagates through turions that form late in the season and which can remain dormant until the right conditions are encountered. Turions can sprout and develop within 7-10 days of flooding. Sprouting is favoured by anaerobic conditions. High levels of sprouting can occur at temperatures as low as 10^oC. If the right conditions do not occur, for example if the rice paddy is temporarily converted to dryland agriculture, the turions are expected to be able to remain dormant for many years. Flowering occurs between June and October. Viable seeds are produced which remain dormant for 120 days after seeding. However, the importance of seeds for reproduction is uncertain. Shochum can also reshoot from plant fragments during the season, such as those left behind after weeding

Dispersal: Seeds and turions are dispersed through irrigation water, and by machines and tools used during field preparation. They can also be moved around in contaminated transplanting stock. Seeds, being so small, are likely to be more easily dispersed, but their subsequent germination rate has not yet been established.

When can damage be expected? Shochum can already be visible three weeks after rice transplantation. Damage to rice can be expected to be noticed within 30 to 50 days. Once established shochum populations can build up very quickly. For example, it covered most of the open water in affected rice terraces in Bumthang Valley within about five years of introduction as a contaminant in rice seedling stock.

Habitats: Shochum is a perennial aquatic weed. In Bhutan, it is found in transplanted rice fields, where fields are generally flooded with 3-5 cm water throughout the growing season. Shochum can also grow in drainage ditches where water remains stagnant or in areas where water pools. It appears to be a temperate weed, not having been found in the low-land sub-tropical and tropical rice-growing areas.

MANAGEMENT PRACTICES

The weed can be controlled by hand pulling, although it is laborious and needs to be done carefully as the plant can regenerate from plant fragments left behind after weeding. Herbicides to control shochum are currently being evaluated by the NPPC. Preliminary results are promising. Implementing strict on-farm quarantine practices is the best way to prevent the entry and spread of shochum.

Monitoring

Newly emerged, floating leaves can normally be seen within three weeks of rice transplantation and flooding. If densities are low, or the plant has a restricted distribution on the farm, then monitor its distribution and spread through time. This will help to focus management effort, and may help identify how it is being spread onto and through the farm. While monitoring for shochum inspect all possible habitats, including irrigation channels and stagnant pools.

Quarantine

If the farm or terrace is free of shochum then:

Ensure all planting material comes from a source that is free of shochum.
If possible alter irrigation channels to prevent seeds and turions in infested fields from moving to clean rice terraces.
When working in the fields, start in the clean fields first to prevent infesting them through the movement of contaminated soil and plant material.
Inspect regularly through the season for shochum and remove thoroughly if found, returning to the same location regularly to ensure all has been removed.
Minimise standing water outside of the rice terraces in which shochum might grow.
Keep irrigation channels free of weeds to aid inspection and control activities.

Non-chemical management

Hand weed 2-3 times per rice-growing season. When weeding, dig deep into the soil to uproot and remove all stems and roots. Place weeds away from water to dry out.
- First weeding should be done within three weeks of transplanting
- Second weeding needs to be done four weeks later.
- The final weeding needs to occur before turions are formed, in September-October. Once formed they will break off and be left in the soil.
Keep irrigation channels free of weeds to aid inspection and control activities.
Autumn ploughing to the depth of 20 cm may be useful in killing the dormant turions.
Intermittent drainage of terraces can result in the drying of all shochum vegetative parts, thereby reducing competition with rice.
Mulching with barley straw has been found effective in reducing the growth of the weed.
Temporarily switching to dryland agricultural has been suggested. It is not known exactly how long turions can remain dormant in the absence of flooding, but high densities were still present after at least a year in fallow fields in Bumthang.
In severe cases, resting infested paddy fields for a season, but flooding them periodically to allow turions to break dormancy, may be an effective way of removing them from the soil. However, this has not yet been tested.

Chemical management

Butachlor is the only herbicide available for use in rice in Bhutan. However, it is not effective against shochum.
In countries like Korea, Japan and China, the weed is fully controlled by the use of herbicides like pometryn and propanil.
In Bhutan, ethoxysulfuron is being tested, showing promising results. It is not yet available for use by farmers.
During the 1990s herbicides like SANBIRD and NC311 were tested in Bhutan and found to be very effective in controlling shochum as well as other grassy weeds and sedges. However, at Nu. 8,000 per acre they were considered uneconomical and therefore not made available

Image acknowledgements: NPPC & Kiyoshi Kojima, Biology of Hirumushiro in Japan and its control (life cycle of shochum).