Aggressiveness and production of cell-wall degrading enzymes by Pythium violae, Pythium sulcatum and Pythium ultimum, responsible for cavity spot on carrots

This study investigated the relationships between pathogenesis, types of symptoms and in vitro production of cell-wall degrading enzymes by P. violae, P. sulcatum and P. ultimum. The three pathogens, considered as the three Pythium species principally responsible for cavity spot on carrot roots, secreted only low levels of fatty acid esterases activity, suggesting they have limited ability to degrade suberin in the walls of the outer cell layers of carrot tissues. Among the enzymes that degrade cell-wall polysaccharides, only pectate lyases and cellulases were produced by P. violae, and these were produced late and in small amounts: the symptoms caused by P. violae were limited and typical of cavity spot. Conversely, P. ultimum caused maceration of tissues, and secreted polygalacturonases and β-1,4-glucanases earlier and in larger amounts than P. violae. P. ultimum also produced a large diversity of proteins and cellulase isoenzymes. Although secreting all the monitored enzymes in higher quantity than the two previous species, P. sulcatum was responsible for only typical limited symptoms of cavity spot, with a brown colouring. The role of plant reactions induced in response to early pectinolytic enzyme production by P. sulcatum may account for this apparent inconsistency.