Abstract
Assessing plant disease severity and pathogen population size is central to epidemiological studies that help to devise disease control practices for crop protection. Among current methods, there is a trade-off between accuracy, defined as the closeness of the estimated value to the true value, and cost, defined as the consumption of resources that have to be spent in order to achieve the appropriate measurement. On the one hand, accurate methods based on counting lesion numbers per plant are time consuming. On the other hand, quick methods based on evaluations of diseased area, are adequate for varietal evaluation, but not sufficient for a quantitative ranking of numerous observations such as those required for an estimation of spore dispersal. A new method based on counting of leaf spots observed during 1 min in a delimitated one square meter area (lesions counted/m2/min, Mac.m2) was tested, using phoma stem canker as a case study in experimental plots and farmers’ fields of oilseed rape. We showed that direct sampling of disease symptoms is feasible, reduces observation time and yields a continuous quantitative variable. We confirmed that: (i) lesions counted/m2/min (Mac.m2) values were correlated with mean number of leaf spots per plant (lesions/plant, Mac.pl); (ii) repeat phoma leaf spot counts of the same sample area by the two methods (Mac.m2 and Mac.pl) were correlated when assessed either by the same or different observers; and (iii) ranking of field plots with different disease severity was coherent among observers. We conclude that the lesions counted/m2/min (Mac.m2) method is equal in performance to the lesions/plant (Mac.pl) method, and was faster. Used with care, this method will facilitate studies requiring disease severity estimates, which were previously hampered by cost and/or time.
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Acknowledgments
We thank Farmers near Le Rheu who allowed us to assess disease in their fields, and the Experimental Unit (UE787) at Le Rheu for their assistance in performing the field experiments. We thank Patrick Vallée for help with disease assessment, and all colleagues who participated in the scoring sessions. We thank the BraCySol Biological Resource Centre for providing oilseed rape seeds. We thank CH Bock and two anonymous reviewers for fruitful comments on earlier versions of the manuscript. All authors declare the absence of conflicts of interest.
This work benefited from the financial support of INRA – the French National Institute for Agronomical Research – and from ANR – the French National Research Agency – programs « Agriculture et Développement Durable » grant « ANR-05-PADD-05, CEDRE » and « AGROBIOSPHERE » grant « ANR-11-AGRO-003-01, GESTER ».
LB, SJ, CD and AR carried out field experiments, HP was responsible for the contact with farmers, LL and RD performed statistical analyses. LB conceived and designed the study. LB LL and RD prepared the manuscript, read and approved by all authors.
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Bousset, L., Jumel, S., Picault, H. et al. An easy, rapid and accurate method to quantify plant disease severity: application to phoma stem canker leaf spots. Eur J Plant Pathol 145, 697–709 (2016). https://doi.org/10.1007/s10658-015-0739-z
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DOI: https://doi.org/10.1007/s10658-015-0739-z