Abstract
Resistance of twenty one Aus and eleven Aman varieties of rice to yellow stem borer (YSB) infestation in relation to their physical and chemical factors was determined. These physical and chemical factors include plant morphological characters influencing YSB infestation, oviposition preference of YSB on these rice varieties, effect of antibiosis on YSB infestation, behaviour of YSB in relation to anatomical characters of rice plants, effect of time of transplanting and age of the plant on YSB infestation, major rice stem constituents contributing to the resistance to YSB infestation and effect of six chemical fertilizers on YSB infestation in different rice varieties.
It was observed that YSB infestation varied significantly among the rice varieties and also with the age of the rice plants. Maximum YSB infestations were observed in the variety TN1 in both seasons and lowest infestations were recorded in the varieties BR8 in Aus and Nizersail in Aman seasons. Percentages of YSB infestation in the Aus rice varieties were found in the following order of intensity: TN1 > BR14 > BR9 > IR8 > BR3 > BR7 > BR2 > BR16 > BR6 > BR21 > Gomvir> IR29 and BR15 > Dular > TKM6> BR12 > Purbachi > BR1 > BR20 > Hashikalmi > BR8. In Aman rice varieties the percentages of YSB infestations were found in the following order of intensity: TN1 > BR22 > TKM6 > BR4 > BR11 > Pajam > BR23 > Kalizira > BR10 > Tulsimala > Nizersail.
Different plant morphological characters varied significantly among test varieties. In Aus season stem diameter, leaf number, flag leaf length and width, 2nd leaf length and width and life duration of rice plant induced YSB infestation but greater plant height, increased tiller number and leaf hairs reduced the infestation rate. However, YSB infestation had an insignificant correlation with plant characters. In Aman season the correlation values indicated that wider stem increased tiller number, greater flag leaf length and width and wider 2nd leaf induced higher infestation. On the other hand, longer plant, more leaf number, greater 2nd leaf length, more leaf hairs and increased life duration of plant decreased YSB infestation rate.
There were no significant differences in the number of egg masses deposited on the rice varieties studied. The dorsal side of leaf was preferred to ventral side for egg deposition irrespective of the rice varieties. Again, the middle portion of leaf was preferred for egg laying to basal and apical portions in all varieties. Length and width of egg masses deposited on different rice varieties did not differ significantly except the length of the egg mass in aus season. The number of egg masses deposited by YSB was positively correlated with leaf blade colour but negatively correlated with plant height, 2nd leaf length and width, number of leaf, number of tiller and leaf hair in both Aus and Aman rice. So, the leaf blade colour induces egg deposition, while other plant characters reduce it.
Resistant rice varieties exhibited antibiosis mechanism of resistance against YSB infestation as expressed from lower percentage of larval survival, shorter length, lesser width and weight of larvae and pupae of YSB in less infested rice varieties as compared to those in highly infested ones.
Rice varieties having broader and thicker sclerenchymatous hypodermis, compact parenchyma cells of ground tissue, small air spaces in the ground tissue, more vascular bundles with narrower spaces between vascular bundles, ridged stem surface containing vascular bundles, ground tissue with prominent protoxylem lacuna and narrower pith are considered to be resistant.
Both plant age and time of transplanting had significant effect on the incidence of YSB. Highest percentage (2.19) of YSB infestation was observed in 40 days old rice plants and lowest percentage (0.22) was found in 100 days old rice plants. The rate of infestation gradually decreased with the increase of plant age. Lowest percentage (0.87) of YSB infestation was observed in late September transplanting while highest infestation (1.35) was recorded in late August transplanting of rice plants.
The twelve plant chemical constituents observed during the experiment varied significantly among the rice varieties. It was found that presence of plant nitrogen, phosphorus, sulphur, sodium and moisture in Aus varieties induced higher YSB infestation, while presence of higher amounts of potash, zinc, crude silica, crude fat, crude fibre and dry matter reduced the YSB infestation rate. But in Aman varieties presence of plant nitrogen, phosphorus, potash, zinc, sulphur, sodium, pH, crude fat and dry matter and absence of crude silica, crude fibre and moisture increased YSB infestation.
Experiments on the effects of chemical fertilizers containing nutrient elements of nitrogen, phosphorus, potash, zinc, sulphur and silicon on the YSB infestation rate in BR2 and BR11 Aus and Aman rice revealed that YSB infestation increased with the increase of nitrogen and sulphur but decreased with the increase of zinc and silicon. Phosphorus and potassium fertilizers had no significant effect on YSB incidence on these rice plants.