Evaluation of detached strawberry leaves for anthracnose disease severity using image analysis and visual ratings

Authors: MILLER-BUTLER, MELINDA - Jones County Jr College; CURRY, KENNETH - University Of Southern Mississippi; BLYTHE, EUGENE - Mississippi State University; Smith, Barbara

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2019
Publication Date: 12/1/2019
Citation: Miller-Butler, M.A., Curry, K.J., Blythe, E.K., Smith, B.J. 2019. Evaluation of detached strawberry leaves for anthracnose disease severity using image analysis and visual ratings. HortScience. 54(12):2111-2117. https://doi.org/10.21273/HORTSCI14321-19.
DOI: https://doi.org/10.21273/HORTSCI14321-19

Interpretive Summary: Disease resistant cultivars are the most desirable method to control diseases such as anthracnose crown rot of strawberry; however, their development requires evaluation of the disease response of thousands of seedlings. Various rating methods have been used to assess disease response including visual rating scales and scales based on image analysis of diseased tissue. Visual ratings may quickly assess disease severity. and electronic image analysis precisely measure percent lesion areas. In this study anthracnose symptoms on inoculated detached strawberry leaves were assessed by both visual ratings and image analysis. The visual 0 to 5 disease rating scale used was shown to efficiently separate susceptible strawberry germplasm from resistant strawberry germplasm. Image analysis gave a more precise disease rating based on percent lesion area: however, it took considerably more time. We concluded that initial screening of large strawberry seedling populations for anthracnose resistance can be accomplished with preliminary screening using by a visual scale, but before releasing a new cultivar, a breeder might choose to base final decisions of anthracnose resistance or susceptibility using electronic imaging. The choice of a disease rating method depends on the purpose of the research and the time, practicality, cost, and accuracy of the ratings. This information will be used by breeders, plant pathologists, and horticulturists when evaluating disease response of strawberry and similar crops.

Technical Abstract: Inoculation of detached strawberry leaves with Colletotrichum species may provide an accurate, rapid, non-destructive method of identifying anthracnose resistant germplasm; however, the strength of any disease screening protocol depends on the reliability and validity of assessing disease severity. The purpose of this study was to compare two methods of evaluating disease severity of strawberry germplasm screened for anthracnose resistance. Detached leaves of 77 susceptible and resistant strawberry clones were inoculated with one Colletotrichum gloeosporioides and two C. fragariae isolates. Anthracnose disease symptoms on each leaf were assessed quantitatively via computer-based image analysis to determine percentage lesion area and qualitatively by two independent raters using a visual disease severity rating scale (0 = no symptoms to 5 = entire leaf dead). The image analysis and visual assessments were compared statistically. The two visual raters' average disease severity ratings (n =3413) were in substantial agreement with a weighted Kappa of 0.80 (95% CI 0.79–0.82). There was a strong positive correlation between percent lesion area determined by the computer analysis and the visual disease scores of the two raters (rp = 0.79). Image analysis provided a precise measurement of percent lesion area of infected leaves while visual assessment provided rapid results. Our results indicate that detached leaf inoculations can be used as a rapid preliminary screen to separate anthracnose susceptible from resistant germplasm in large populations within breeding programs.