Quantification of common root rot symptoms in resistant and susceptible barley by image analysis

A series of tests were carried out to determine the effect of common root r ot, caused by Cochliobolus sativus, on root growth and discolouration in th e moderately resistant cv. Bonanza and the susceptible cv. Gateway of barle y. An image analyser was used to quantify the growth and discolouration (me sa intensity) of the roots and the subcrown internode (SCI) of individual p lants. The plants were also visually rated for root rot severity and their root dry weight was determined. In the absence of disease, no differences i n root discolouration or growth were observed between the two barley cultiv ars. In plants grown in naturally infested soil, visual root rot ratings an d mean intensity values indicated that the SCIs of Bonanza were less discol oured than those of cv. Gateway. Tn cu, Gateway, higher levels of disease w ere usually associated with a smaller root system than in cv. Bonanza, bur the effect was significant (P < 0.01) only in the first experiment. The roo t system of cv. Gateway had lower (P > 0.05) mean intensity values (i.e., d arker) than those of cv. Bonanza in the first experiment. Seedling emergenc e of cv. Gateway was lower in naturally infested soil than in fumigated soi l, but this was not evident in cv. Bonanza. There was a strong inverse rela tionship between the mean intensity and visual rating of the SCI and a posi tive association between coral root area as measured using image analysis a nd root weight. Image analysis also determined that in the severe root rot category, plants of cv. Gateway had SCIs that were more discoloured than th ose of cv. Bonanza. Discolouration of the SCI of root rot susceptible barle y appeared to adversely influence the growth and discolouration of the root system, but this effect was not always significant.