Presence of Phytophthora fragariae var. fragariae in Norway

Abstract

The strawberry red core disease caused by Phytophthora fragariae var. fragariae has until recently not been reported from Norway. During 1995 and 1996, samples from 323 Norwegian strawberry fields were tested in a root‐tip bait test for possible detection of red core. Symptoms of the disease developed in test plants cultivated in growth medium containing root tips from 21 fields at eight locations in four different counties. Disease symptoms included wilting of plants, root rot, red steles in the roots, and oogonia and oospores identical in size and appearance to those of P.f. fragariae developing along the root steles. In V8 juice agar, the mycelium was slow‐growing, thick, fluffy and aerial. Sporangia were large, mainly inversely pyriform, and without papillae. Optimum temperature for mycelial growth was 21°C, and the fungus ceased to grow at 25°C. In infested fields, the most severe attacks were found in low, often poorly drained areas. The means by which red core may have been introduced and spread in the country are discussed, together with preventive action to avoid further dissemination.