Jassid Resistance and Hairiness of the Cotton Plant

Abstract

A wide range of material has been studied in an investigation of the relationship between “resistance” to attack by Empoasca facialis and “hairiness” of the cotton plant.A continuous range has been found between full susceptibility and very high resistance.In general relative susceptibility, as gauged by visual assessment of symptoms, has been found to correspond closely with relative infestation, determined by counts of nymphs.With attention confined to grown plants, the relative susceptibility of varieties has been found substantially constant under varying levels of exposure to infestation.Hairiness of the cotton plant has been found to show an extremely wide range in degree, style and distribution. A method has been devised for expressing hairiness numerically, taking into account both length and density of hairs.A very close and consistent relationship has been found between degree of hairiness of the under surface of the leaf, and degree of resistance to Jassid. Without exception, all thoroughly hairy types have been found highly resistant, and all non-hairy types fully susceptible. Intermediate degrees of hairiness ate associated with intermediate degrees of resistance.The relationship has been found to hold good between varieties and between plants within varieties, of the species G. hirsutum; also between plants of G. barbadense, and in segregating progenies of hybrids between these two species.The hairiness of resistant strains of cotton has been found to develop gradually with the growth of the plant; the first few leaves on the seedlings being virtually non-hairy. This lack of hairiness in the early stages of growth is associated with a lack of resistance. Hairiness and resistance to Jassid develop concurrently.The conclusion is reached that hairiness of the leaf confers resistance to Jassid and that degree of leaf hairiness, measured in an appropriate manner, is a thoroughly reliable guide to degree of resistance.Length of hairs is shown to be of prime importance, and high densities without adequate length are ineffective.The relative importance of hairs on lamina and midrib has not been conclusively determined. Both have an influence on resistance but a high degree of midrib hairiness is not essential if the lamina is hairy.Hairs on stem and petiole are shown to be of very little direct importance.