Fungal pathogens in Norwegian lettuce fields

Abstract

Norwegian field production of lettuce has increased considerably since the early 1990s. Disease problems rarely required fungicide applications before 1996, when lettuce downy mildew (Bremia lactucae) caused severe losses. From 2002 to 2004, surveys were conducted to identify fungal diseases in Buskerud, Vestfold and Østfold counties in the south-east and Rogaland County in the south-west, representing the main lettuce production regions of Norway. The distribution and incidence of B. lactucae was highly variable, but this pathogen was the most important due to the destructive nature of uncontrolled epidemics. Septoria lactucae caused severe damage, but was found in only one field. Sclerotinia sclerotiorum was the most widespread pathogen, found in 32% of the fields, but usually affecting less than 10% of the plants. Pythium tracheiphilum was reported from 33% of the fields in south-east Norway, but was not found in the south-west. Disease incidence was usually less than 5%, and a disease incidence of more than 10% was reported in one field only. Other pathogens of potential economic importance in Norwegian lettuce fields are Alternaria spp., Botrytis cinerea and Rhizoctonia solani, although they were sporadically distributed in relatively few fields in this survey. © 2008 Taylor & Francis.