Possible Root Infection of Cercospora beticola in Sugar Beet

A potential primary infection site of the foliar pathogen Cercospora beticola in sugar beet is described. Sugar beet seedlings of the susceptible cv. Auris were grown in a standard soil for 14 days. A monoconidial culture of a C. beticola isolate was grown to produce conidia. In experiment 1, roots were immersed in a conidial suspension of isolate code IRS 00-4, or in tap water (control), for 2 days. After incubation seedlings were potted in a peat – fine river sand mixture and placed at low relative humidity (RH) (<80%) or high RH (≈100%). Twelve days after infection, seedlings at high RH showed more disease incidence (90%) than seedlings grown at low RH (disease incidence = 25%), whereas no disease symptoms developed in the control seedlings. Cercospora leaf spots (CLSs) developed on the cotyledons, leaves, petioles and stems of the seedlings. In experiment 2, roots were immersed in a conidial suspension of isolate code IRS 00-2 for 5 h. Thirty-four days after infection at high RH, 100% disease incidence was observed in the treated seedlings and one CLS in the control treatment. First indications of leaf spot development were observed as reddish purple discolouration of individual parenchymatic cells. Because splash dispersal and symptoms due to infested soil were excluded, we showed that it is possible to obtain CLS symptoms in sugar beet seedlings when their roots were immersed in conidial suspensions of C. beticola, thus demonstrating that roots can be a primary infection site.