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Title: Sclerotinia sclerotiorum: An evaluation of virulence theories

Author
item XU, LIANGSHENG - NORTHWEST AGRICULTURE AND FORESTRY UNIVERSITY
item LI, GUOGING - HUAZHONG AGRICULTURAL UNIVERSITY
item JIANG, DAOHONH - HUAZHONG AGRICULTURAL UNIVERSITY
item Chen, Weidong

Submitted to: Annual Review of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2018
Publication Date: 6/29/2018
Citation: Xu, L., Li, G., Jiang, D., Chen, W. 2018. Sclerotinia sclerotiorum: An evaluation of virulence theories. Annual Review of Phytopathology. 56:311-38. https://doi.org/10.1146/annurev-phyto-080417-050052.
DOI: https://doi.org/10.1146/annurev-phyto-080417-050052

Interpretive Summary: The fungal necrotrophic plant pathogen Sclerotinia sclerotiorum causes white mold diseases on more than 400 plant species including almost all dicotyledonous plants. Its pathogenic mechanisms are still not fully understood. The pathogen invariably accumulates oxalic acid in culture and in diseased plants and oxalic acid was always associated with disease. Previous research using ultraviolet light (UV) induced mutants that lost oxalic acid production and virulence concluded that oxalic acid is required by S. sclerotiorum to cause disease. However, we have generated genetically defined mutants that completely lost oxalic acid production but accumulated fumaric acid. These genetically defined mutants could cause considerable disease on host plants such as cabbage, canola, common bean, faba bean, gourd, pea. potato, sunflower and tomato. The experiments showed that oxalic acid is not necessary for S. sclerotiorum to cause disease on many host plants, contradicting the notion that oxalic acid is responsible. In order to reconcile the different opinions regarding the role of oxalic acid in virulence of S. sclerotiorum, we evaluated the evidence available in the literature and discovered that the UV-induced mutants harbored previously unrecognized genetic defects in growth and response to acidic pH, solving the mystery why the UV-induced mutants are non-pathogenic. This research opened the door to search for true pathogenicity factors to advance our understanding of pathogenic mechanisms for better management of Sclerotinia diseases.

Technical Abstract: Oxalic acid production in Sclerotinia sclerotiorum has long been associated with virulence. Research using UV-induced, genetically undefined mutants that concomitantly lost oxalate accumulation, sclerotial formation and virulence supported this concept, and concluded that oxalate is an essential pathogenicity determinant required by S. sclerotiorum to cause disease. However, recent investigations using genetically defined mutants that lost oxalic acid production but accumulated fumaric acid could cause disease on many host plants, and concluded that it is acidic pH, not oxalic acid per se, that sets the optimal condition for disease development. In assessing the different opinions regarding the roles of oxalate in virulence, critical evaluation of available evidence showed that the UV-induced mutants harbored previously unrecognized genetic defects in growth and response to acidic pH that confounded oxalate deficiency. Because of the confounding factors in the UV-induced mutants, conclusions drawn based on the UV-induced mutants must be reevaluated.