Phenetic resistance in sugarcane, tentative conclusions and new research directions

Sugarcane genotypes selected on the basis of dead hearts formation were studied for phenetic based characters of resistance. Shoot borer preferred to bore accessions/cultivars with light green and drooping foliage as well as droopiness of canopy seems to have been preferred over erectness. Low intensity of light reflected by leaves at 60 days after planting (DAP) in least susceptible group as compared to moderately susceptible and highly susceptible genotypes varying from 1.32x10 lux (Co H 70) to 1.46x10 lux (Co J 64) than the least susceptible. Apart from other leaves characteristics the leaf size has also been observed to be significantly related to borer attack, the short and broad leaves facilitating higher infestation than the long and narrow leaves. In present investigation, a gradual decline in sheath hardness with an increase in shoot borer susceptibility indicated a strong inverse correlation between sheath hardness and shoot borer incidence. The average spindle length of least susceptible (17.9 cm) and moderately susceptible (17.5 cm) entries at 60 DAP was higher than the highly susceptible genotypes (13.7cm). The susceptible variety Co1148 recorded the shortest spindle length (12.9 cm) indicating that genotypes with longer spindles were shown to offer greater resistance to Chilo infuscatellus damage than those with shorter spindles. In highly susceptible genotypes, the average chlorophyll content was recorded to be 27.7 mg/gm fresh weight as against 33.1 mg/gm observed in least susceptible. A consistent increase in total chlorophyll content was evident with in shoot borer resistance, being maximum in least susceptible entries and minimum in highly susceptible ones.