LUD WIGIA DECURRENS WALT. - A NEW RICE-FIELD WEED IN SRI LANKA

: Ludwigia decuwens, a member of the family Onagraceae, is a new weed in the rice-fields of Sri Lanka. It has probably been introduced to the country relatively recently. This weed is presently found in great abundance in the rice-field habitats of Colombo and Kalutara districts, and to a lesser extent in Gampaha, Kegde, Ratnapura and Galle districts. The weed produces large numbers of seeds which are easily spread by irrigation water and rain water. Successful management of the spread of this weed will depend on applying control measures before the plant comes into bloom and sets seeds. Its botanical features are described.


Introduction
The genus Ludwigia belongs to the -family Onagraceae, a family that has many temperate and sub-tropical species which are mainly herbs, but includes some shrubs as well. The family Onagraceae also includes the genus Jussiaea will-known to botanists. However, due-to evidence of many close relationships and inter-connections between Jussiaea and Ludwi'ga, the two genera have now been merged and the name Ludwijga accepted as the generic name most tenable for the aggregate genus. 4 ~r i r n e n~ in the Handbook to the Flora of Ceylon (1893-1900) described two species of Ludwipa, namely Lzrdwigia pamiflora Roxb. and Ludwigia prostrata Roxb., as well as two species of Jussiaea, namely Jussiaea repens L. and Jussiaea suffruticosa L. as occurring in Sri Lanka He also noted that a variety of J. suffruticosa L. var. fl subglabra Thw., appeared so different from the type (J. suffruticosa) and stated that he was inclined to make a second species. Trimen also stated that L prostrata was very. rare and that he had only seen the specimen collected by Moon at Kalutara.
Alstonl in his Supplkment t o Trimen's Flora of-Ceylon, viz. Vol VI (1931) recorded four species of Jussiaea for Sri Lanka. He accepted the previously described J. repens L., but did several alterations t o the other species recorded by Trimen. The nanie of 1. suff;ruticosa sensu Trimen was altered to Jussiaea peruviana L., while the name J. suffrutieosa L: was retained for the variety subglabra noted earlier. Alston also described a new specie's, Jussicea tcnella Burm. f. Of the Ludwigia species, Alston altered the name of L. pamiflora sensu Trimen, to read as Ludwigia perennis L.
Chandrasena and ~m a r a s i n~h e~ in a preliminary communication, presented evidence of seven species of Ludwigia found growing naturally in Sri Lanka. They also favoured the view that the name Ludwi'gia be accepted as the valid name in describing these species, following Raven. The seven Walt. They stated that L. prostrata, which Trirnen himself had not collected, was never found by them despite extensive searching and preferred to leave this species out. Their list included Ludwigia uruguayertsis and L. decurrens, two species which had not been previously recorded from Sri Lanka. Whilst L. uruguayensis was of rare occurrence, L. decurrens was reported as a new rice-fieId weed of considerable importance, already well established in the Wet zone.
The objective o' f the present paper is to record the occurrence, botanical nature, present distribution and significance of L. decurrens, which is a new species for Sri Lanka.

Distribution of L. decurrens in Sri Lanka
Up to tl;e end of the Yala season (April-August) of 1986, the author has found L. decurrens in four districts of the low-country wet zone growing in rice-fields and nearby wet marshy lands. Heaviest infestations were seen in the rice-fields on the eastern side of the Colombo district, (viz. Maharagama, Homagama, Avissawella) and in the Kdutara district around Bombuwela, Nagoda, Matugama and Agalawatta. Isolated patches of the weed were found in the Gampaha district, north of Colombo, in several fields close to the district boundary. In the G d e district, only one population of L, decurrens was found in 1985/1986, and this was in a village called Ampegama. This population was one of the heaviest infestations of the weed seen by the author anywhere, and covere'd an area of 2-4 ha of rice-fields as the most dominant weed. In 1986 and 1987, the species was collected from several isolated locations in the Kegale and Ratnapura districts. These occurrences were always close to the main roads (viz. Colombo-Kandy; Colombo-Ratnapura via Avissawella), suggesting that the weed was spreading slowly to other districts as well.
Whether the plant dccurs in the dry zonemd montane zone is not yet b o w i to the author, who has not done any extensive searching for the species in these areas, up to the time of writing this paper.

. . 4, Significance
The commonest and by far the most troublesome broad-leaved weed m ricefields in the Wet zone of Sri Lanka is L. hyssopifolia, a close relative of L. d e~u r r e n s .~ Along with L. hyssopifolia, L. decurrens has also become a rice-field weed of considerable importance. The majority of the local farmers interviewed by the author did not distinguish between the two species and referred to the plants by the local names 'Bim-Uru' or 'Wel-Karabu'. The origin of the former word could be due to a popular belief that these plants arise from the dead bodies of a common soil insect called 'Bim-Ura'. The origin of the latter name is appalrently the resemblence of the mature capsules of both Ludwigia spp. in rice-fields t o 'cloves' (Eugenia  ~a r~a p h y l l a t a ) , the well known spice, called 'Karabu' in Sinhala. Some of the local farmers expressed the view that the larger-flowered L. decurrens was not known until a few years ago, possibly suggesting a relatively recent introduction and rapid spread.
L. decurrens is known to be native t o the New World from South Eastern United States to Northern ~r~e n t i n a .~ ~a v e n~ is of the opinion that it has been introduced recently to Japan and Africa. This may be true for parts of Asia too, including Sri Lanka. There is little doubt +at the seeds are carried to nearby fields and lands by irrigation water, raiq water or by agricultural implements. Once the seeds are set, rapid spread is inevitable and appears to be one of the main causes of its success as a weed.
Farmers and agriculturd-extension officers interviewed by the author stated that both L. 'hyssopifolia and L. decurrens are easily controlled by MCPA (4-chloro-2-methylphenoxy acetic acid) and paraquat (1,1 '-dimethyl-4,4'-bipyridylium), two of the commonest foliage-applied herbicides used in rice-culture in Sri Lanka, However, these species when mature are not easily uprooted by hand-weeding, due to their deeply penetrating root systems. The control of L. decurrens therefore depends upon the judicious use of clean seed (rice) for sowing in areas where the weed is not present, and by avoiding the use of irrigation water from infested fields to supply water for non-infested fields.
this work, and Mrs. P. Arulgnanam for the illustrations. The author is also grateful to Drs: Peter Raven, Peter Hoch and particularly, Dr.Elsa Zardini, all of the Missouri Botanic Gardens for assistance in the verification of the identity of the species. Thanks are also due to the colleagues in the Department of Botany for their encouragement.