Phenotyping

Field experiments were conducted during 2021DS and 2021WS of 23 and 50 ILs, respectively at Zeigler experimental station (ZES), International Rice Research Institute. Randomized Complete Block Design (RCBD) was followed to lay out the experiments. Standard agronomic practices and plant protection measures to raise a good crop were followed. Data on agronomic and yield traits were collected following the standard evaluation system (SES of IRRI)⁷⁵. Days to 50% flowering (DF) were calculated based on the sowing date to when 50% of the plants in a plot flowered. Plant Height (PH) was measured from the base of the plant to the tip of the tallest panicle from three selected plants; Panicle length (PL) was measured from the panicle base to the tip of the top most grain. After harvesting grains were well dried and moisture content was adjusted to 14%, grain weight per plot was measured in grams (gm) and converted to tons/ha.

For grain quality (GQ) assessment, 140 g of well dried grains from each of replications were submitted at the Analytical Service Laboratory (ASL), IRRI. The grain morphological, milling and cooking quality data were estimated using standard protocols⁷⁶. The GQ traits measured were grain length (GL), grain width (GW), brown rice (BR), milled rice (MR), head rice recovery (HR), amylose content (AC), gelatinization temperature (GT) and gel consistency (GC).

Parental lines and ILs for RT resistance were screened by artificial inoculation under controlled conditions. Materials were seeded in a metal tray following Randomized Complete Block Design (RCBD) with three replications under the controlled condition at disease screening facilities of IRRI. The Taichung Native 1 (TN1) and BRRI dhan71 were used as susceptible checks, while TW16 and Matatag 1 were used as resistant checks. Uniformity was maintained between entries in terms of the age of the seedlings and the number (with twenty seedlings) of seedlings per entry by thinning. Ten days old seedlings were subjected to RT virus infestation by releasing viruliferous GLH and allowed them to feed on seedlings. The insects were already given chances to feed on infected plants for 5 days. Seed box that contains the test seedlings were placed inside the water tray and covered with a screen. Viruliferous GLH was released in the cage at an average of 6–7 insects per seedling for 2–3 h of inoculation access. The insects were disturbed every 30 min during the inoculation period to ensure the even distribution among the seedlings. After 21 days of infestation, disease symptoms, incidence and severity were recorded (Fig. 4). Disease severity was evaluated based on a scoring system following SES^17,75. Disease index (DI) was estimated based on disease percentage and symptoms. The DI value is determined by the severity of disease symptoms; the higher the DI value, the more severe symptoms⁷⁵ (Table ^S5).

DI was estimated as follows

where; n: the number of plants showed the corresponding score; tn: the total number of plants.