Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/30/2008
Publication Date: 6/27/2008
Citation: Copes, W.E., Stevenson, K. 2008. A pictorial disease severity key and the relationship between severity and incidence for black root rot of pansy caused by thielaviopsis basicola. Plant Disease 92:1394-1399. Interpretive Summary: Root disease caused by the fungus Thielaviopsis basicola is assessed solely by visual ratings of the root system, however this assessment method usually may not generate the type of critical quantitative data needed to test interactions between the pathogen, plant, and environment. The root-line intercept method was used to sample pansy roots that had been previously inoculated with T. basicola, and disease incidence and severity data were assessed for seven experiments. This paper presents a pictorial guide for assessing disease severity levels and provides guidelines to interpret severity levels from incidence data. The research will be beneficial to scientists who study the development of diseases caused by T. basicola.
Technical Abstract: A pictorial key was developed and the relationship between disease severity and incidence was examined to aid in the assessment of black root rot of pansy caused by Thielaviopsis basicola. Pansies that had received different fertility treatments, as part of seven separate studies, were inoculated with T. basicola and typical black root rot symptoms were well developed at four weeks when disease was assessed. Root samples were distributed on a grid system and disease severity and incidence ratings determined for more than 100 line-root intercepts per plant. The key consisted of photographs of root segments that represent disease severity levels ranging from 0.4 to 71 percent at a line-root intercept. Disease incidence ranged from 1.3 to 100 percent and severity ranged from 0.1 to 21.4 percent per plant. A linear relationship between incidence and logarithmic transformed severity values provided the best quantitative description of the relationship between severity and incidence for seven independent data sets. It was concluded that disease incidence can be assessed more quickly and objectively than severity and can provide reliable estimates of root rot severity.