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Abstract
A series of tests were carried out to determine the effect of common root rot, caused by Cochliobolus sativus, on root growth and discolouration in the moderately resistant cv. Bonanza and the susceptible cv. Gateway of barley. An image analyser was used to quantify the growth and discolouration (mean intensity) of the roots and the subcrown internode (SCI) of individual plants. The plants were also visually rated for root rot severity and their root dry weight was determined. In the absence of disease, no differences in root discolouration or growth were observed between the two barley cultivars. In plants grown in naturally infested soil, visual root rot ratings and mean intensity values indicated that the SCls of Bonanza were less discoloured than those of cv. Gateway. In cv. Gateway, higher levels of disease were usually associated with a smaller root system than in cv. Bonanza, but the effect was significant (P < 0.01) only in the first experiment. The root system of cv. Gateway had lower (P > 0.05) mean intensity values (i.e., darker) than those of cv. Bonanza in the first experiment. Seedling emergence of cv. Gateway was lower in naturally infested soil than in fumigated soil, but this was not evident in cv. Bonanza. There was a strong inverse relationship between the mean intensity and visual rating of the SCI and a positive association between total root area as measured using image analysis and root weight. Image analysis also determined that in the severe root rot category, plants of cv. Gateway had SCIs that were more discoloured than those of cv. Bonanza. Discolouration of the SCI of root rot susceptible barley appeared to adversely influence the growth and discolouration of the root system, but this effect was not always significant.