First Report in Sweden of Downy Mildew on Parsley Caused by Plasmopara petroselini.

Abstract

During September 2004, downy mildew of parsley caused by a species of Plasmopara was observed in an experimental field of parsley (Petroselinum crispum subsp. crispum L. cv. Gigante d'Italia/Hilmar) in Borgeby in southern Sweden. The summer of 2004 was exceptionally wet and humid. Disease became widespread throughout the field in just a few days. Local growers reported that symptoms consistent with downy mildew had appeared in their parsley fields every year since 2001. Plasmopara, under P. nivea, has been reported on parsley in Europe since the middle of the 19th century (4). In recent years, this disease has caused severe damage to parsley grown in several European countries, e.g., France, Germany, Switzerland, and Belgium (1,3). The first symptoms appeared as faint chlorotic spots on the upper surfaces of the leaves. On the corresponding lower surfaces, mycelium and sporangiophores grew profusely and developed a white mat that in part turned dark gray. Eventually, the leaves and stalks became necrotic and died. The sporangiophores were monopodially branched, 248.4 ± 13.36 μm long (n = 17), each branch ending in 2 to 5 ultimate branchlets tapered toward the tip. The trunk diameter measured 7.0 ± 0.77 μm (n = 9) above the basal part and 6.1 ± 0.81 μm just below the first branch. The sporangia were broadly ellipsoidal to ellipsoidal, hyaline, 22.5 ± 0.73 μm long and 16.6 ± 0.48 μm wide (n μ 40). They were mostly nonpapillate when young, although exit pores 4.8 ± 0.32 μm (n = 10) were visible. Mature sporangia exhibited a dehiscence apparatus and a plug in the exit pore. On the basis of the characteristics above, the pathogen was identified as P. petroselini (= P. nivea pro parte [2]). Independent verification of the identity was done by O. Constantinescu at the Botanical Museum, Uppsala, Sweden. A voucher specimen was deposited at the Herbarium UPS, in Uppsala under the number UPS F-118873. To our knowledge, this is the first report of P. petroselini on parsley in Sweden.