A Study on the Parasitism of Striga SPP. on Different Host Plants in Bangladesh

Abstract

A brief survey of Striga, an angiospermic root parasite of sugarcane in the northern part of Bangladesh was carried out during three successive cropping seasons. Due to heavy Striga infestation in the field, most of the host plants (sugarcane) died and the whole field showed a burned appearance. Incidence of Striga infestation of sugarcane fields was highest in sandy loam soil in high lands. Growers generally abandon sugarcane cultivation in newly infested fields for a certain period due to its long period of seed viability. Striga spp. started to appear above the ground from the end of April after early rain, during summer the parasite continued to grow up to October. Even in the early June population of Striga consists of plants with mature fruits, dispersing seeds, plants beginning to flower and plants still in juvenile stage of growth. During the months of June and July heavy occurrences were noticed. In the cane areas where Striga appeared comparatively earlier in the season were more affected. The population density of Striga per sq. metre was more or less equal in number in all the visited areas due to more or less similar land topography, soil texture and environmental conditions. Striga spp. covered an area of 1300 sq. km and is more prevalent between the region of Varamara to Joypurhat. Comparatively higher degree of variation was observed for maximum quantitative characters of vegetative, floral and reproductive parts of Striga spp. Such variations reveal genetic differences. Three species of Striga are present in Bangladesh such as S. densiflora, S. lutea and S. euphrasioides. Among them S. densiflora is more common and destructive. It is suspected that the place of first appearance of Striga was nearest to the oldest sugar mill of the country where the improved cane varieties were introduced as a seed cane from India and distributed among the growers. The seeds of Striga spp. are very minute and tiny. Striga seeds were introduced here along with the seed cane from India, multiplied day by day and disseminated the adjoining region of the oldest sugar mill .of the country. It was observed that three species of Striga parasitized sugarcane, rice, cynodon and two unidentified grasses. S. densiflora attains one-third of its final height within juvenile stage and more than two-thirds of the final height within fruit setting stage. Final height was attained at seed release stage. The growth pattern, recorded as plant height (cm/day) was faster at juvenile stage, which increase slowly afterwards and coinciding with fruit setting stage. Striga spp. completed its life cycle quickly. The root exudates of suitable host plant stimulated Striga seeds to germinate when come in contact with it and a parasitic relationship becomes established. The ill-developed root system of this parasitic weed is less efficient to absorb water and mineral nutrient from the soil. Multicellular haustorium maintains connection between host and parasitic weed and possibly absorbs minerals and water from the host root. As a result, host plants are suffering from physiological disorder. Striga parasitized canes were stunted and attacked by different types of diseases. Finally the cane plants died when Striga released its seeds, resulting huge crop yield losses. The high significant value between control and Striga parasitized sugarcane in yield components of all the selected 5 improved cane varieties revealed that all the selected varieties were susceptible to Striga spp. The best way to manage Striga problem is to eradicate it before fruit set and more than 9 weeding is needed. It is very costly, which makes this method discouraging to the growers. It is a localized problem in Bangladesh. Long distance dissemination is controlled by plant quarantine. Urea solution (2.5%) killed Striga, and high doze of urea fertilizer inhibited seed germination of Striga. It is fairly available, less costly and also less toxic. Cultivation of legume crops especially Cicer arietinum as crop rotation and burning of stubble are the easiest solution for Striga problem.