Lettuce ring necrosis, caused by a chytrid-borne agent distinct from lettuce big-vein ‘virus’

Abstract

Ring necrosis is a serious disease of lettuce (Lactuca sativa) with often coalescing necrotic rings and ring-like patterns on middle leaves of plants or groups of plants in glasshouses during winter. Affected leaves may decay and plants rapidly become unmarketable. The disease was shown to be soil-borne and transmitted by the zoospores ofOlpidium brassicae. Symptoms in lettuce do not appear before seven weeks after inoculation via the soil. Additives to the inoculum and chilling of source leaves, inoculum buffer and utensils enabled mechanical transmission of a pathogenic agent toChenopodium quinoa, C. amaranticolor, Nicotiana benthamiana, N. clevelandii, N. hesperis, andN. occidentalis but not to lettuce. TheChenopodium spp. reacted with local lesions, infection was symptomless inN. clevelandii and mostly so inN. benthamiana, butN. hesperis andN. occidentalis reacted with leaf spotting and plant stunting. With zoospores of an originally pathogen-free fungus culture further cultivated on the roots of cuttings from sap-inoculated plants ofN. clevelandii andN. occidentalis, the agent could be transferred back to lettuce and the symptoms of ring necrosis be reproduced. The agent biologically resembles those of lettuce big-vein (LBV) and freesia leaf necrosis and the tobacco stunt virus. In lettuce it often occurs together with LBV ‘virus’ but differs in longer incubation period, type of symptoms and symptom appearance only during winter. It could be separated from a mixture with LBV ‘virus’ by serial transfer always selecting plants without LBV symptoms. So far cultural hygiene, including soil disinfection addressing the vector, is the main means of control.