A Pictorial Disease Severity Key and the Relationship Between Severity and Incidence for Black Root Rot of Pansy Caused by Thielaviopsis basicola

Black root rot of pansy (Viola × wittrockiana Gams ex Kappert) is caused by Thielaviopsis basicola (Berk. & Broome) Ferraris. The synanamorph of T. basicola is Chalara elegans Nag Raj & Kendrick. Black root rot and Thielaviopsis root rot are common names used for disease caused by the same pathogen on multiple ornamental and agricultural crops (14,16). The macroscopic appearance of the disease on infected root sections ranges from a characteristic flat black, for which the disease is named, to degrees of brown necrosis that may include interspersed black areas. The visual appearance of the root disorder results from a combination of the symptom of root necrosis and sign of melanized aleuriospores produced on the surface and internally in infected tissue (2). Black root rot disease typically has been assessed using a measure of severity (the proportional or percent area of plant parts or plants diseased) for an entire root system based on the researcher’s preference of a qualitative descriptive scale (1) or quantitative scale of percent disease (4,12,34). Qualitative descriptive keys, quantitative percentage scales, and standard area diagrams can be helpful for obtaining accurate and repeatable estimates of disease intensity. However, efficiency, reproducibility, and precision of the assessment method will depend on the clarity of the description or scale to the evaluator, the nature and extent of disease symptoms and signs, experience of the evaluator, and the objective of the research (5,18–20,23,24). All of the methods involve some degree of subjectivity. Most assessment methods have been developed for foliar diseases (20). Qualitative descriptive keys and quantitative percentage scales have been used commonly for assessing disease intensity (a generalized quantity of disease) on roots, but few disease rating scales have been developed. The appropriateness of a scale will vary with the host plant and disease. O’Brien and van Bruggen (24) found that a qualitative scale assessed on mature plants was generally the most accurate and precise for assessing severity of corky root of lettuce, but the quantitative Horsfall-Barratt scale was most accurate for roots in the 20 to 80% severity range. A measurement of incidence (the proportional or percentage number of plant parts or plants diseased) generally requires less time except at intensity levels close to zero when detection of early stages of signs and symptoms are important. Estimates of incidence are generally more precise than estimates of severity, and could provide an alternative assessment approach that may be transformed into a severity measurement when a mathematical relationship exists between the two. As with the previously mentioned assessment scales, quantitative relationships between incidence and severity have been shown mainly with foliar diseases (6,9,13,26, 28,30). Mathematical relationships have been identified between disease intensity and soil inoculum density in cotton fields (12), lesions on crowns and subcrown internodes of wheat (31), lesions on lower stems of canola (32), yield of lettuce (24), and host resistance (3). However, the quantitative relationship between severity and incidence has not been reported previously for a root disease. Imprecision of foliar disease assessments has been attributed partly to limitations of human visual acuity, including perception of color and optical skewness of the percent light and dark areas between diseased and healthy plant tissue (18–20). Most scales and standard area diagrams have been developed on relatively uniform plant leaves. Disease is more difficult to assess on leaf organs comprised of multiple leaflets, such as bracken fern (19). Although there is no documentation of problems associated specifically with visual perception of root diseases, a number of inherent features of root diseases make assessment a challenging task. Due to the subterranean growth habit, the often difficult step of physically separating roots from the surrounding growing medium is required to obtain an unobstructed view of the entire root system. Even with the aid of more modern image analysis tools, assessment is complicated by the fact that whole root systems are complex threedimensional organs with no significant planar surfaces. If disease severity is evaluated while roots are interspersed within the growing medium, it may be difficult to distinguish the brown discoloration of diseased root tissue from the naturally brown color of roots of some plant species and the brown coloration of rooting media (e.g., pine bark, peat moss). Healthy roots of some annuals, such as pansy, are small in diameter and can have a ABSTRACT Copes, W. E., and Stevenson, K. L. 2008. A pictorial disease severity key and the relationship between severity and incidence for black root rot of pansy caused by Thielaviopsis basicola. Plant Dis. 92:1394-1399.