Bøger af Shyama Weerakoon

Weeds along with insect pests and plant diseases are sources of biotic stress in crop systems. Weeds are responsible for serious problems in rice worldwide affecting growth and causing a considerable reduction in quality and quantity in yield. Herbicide-resistant rice varieties in combination with pre-emergent-broad-spectrum herbicides and post-emergent broad-spectrum herbicides improve the effectiveness of weed management in rice fields. Herbicide resistance can be acquired naturally or induced by classical or modern breeding techniques. High concentration of Glyphosate, a pre-emergent-broad-spectrum systemic herbicide, is prevalently applied to control rice weeds in Sri Lanka, which intern causes severe damages to cultivated rice varieties which are susceptible to Glyphosate. However, there are rice varieties with natural Herbicide Resistance (HR). Six traditional and eighteen inbred, cultivated rice varieties (Bg, Bw, At and Ld series developed by Rice Research Development Institute, Sri Lanka) were used to screen their natural HR. As observations, time taken-to seed germination, time taking to flowering; plant height and number of leaves at 12-weeks after sawing, leaf-length, breadth, panicle-length, number of seeds/panicles of resistant plants and controls were recorded. Plants with >= 40% resistance were considered as resistant to Glyphosate. Ten inbred rice varieties (Bg250, Bg94-1, Bg304, Bg359, Bg406, Bg379-2, Bg366, Bg300, Bw364, At362) and three traditional rice varieties ("Kalu Heenati", "Sudu Heenati", "Pachchaperumal") were naturally resistant to 0.25 g L-1 Glyphosate concentration and when increased the concentration (0.5 gL-1) resistance were reduced. The post-emergent broad-spectrum herbicide, Glufosinate is also applied to control rice weeds in Sri Lanka. Two traditional varieties ("Pachchaperumal", "Ma Wee") and fifteen inbred varieties (At362, Bg250, Bg300, Bg352, Bg357, Bg358, Bg359, Bg360, Bg366, Bg369, Bg379/2, Bg403, Bg450, Bg454, and Bg94-1) were screened for their natural HR resistant to Glufosinate.

Weedy rice (Oryza sativa f. spontanea) is one of the most nuisance weeds, referred as ¿red rice¿ possess higher morphological variability and varying degrees of similarities to wild and cultivated rice. Close similarity between weedy and cultivated rice prevents use of selective herbicides in controlling weedy rice. Weedy rice affects both quantity and quality of national rice production. Efforts were made to understand morphological and physiological properties of weedy rice and its epidemiology to design controlling methods and to disseminate information to farmers. Weedy rice is strongly characterized by seed shattering and dormancy characters. Long term sympatric distribution has led to similarities between weedy and cultivated rice through natural hybridization and introgression. Major wild rice species (O. nivara and O. rufipogon) are also distributed all over the country. The content of the book discusses levels of genetic diversity of weedy rice populations in different climatic zones in Sri Lanka through agro-morphological and molecular studies and explore the possible origin of weedy rice by comparing the genetic relationships with cultivated rice and wild rice varieties.