ABSTRACT
Despite its association with important agricultural crops, Phytophthora clade 8b is a poorly studied group of species. The clade currently consists of three officially described species (Phytophthora porri, P. brassicae and P. primulae) that are host-specific pathogens of leek, cabbages and Primula spp., respectively. However, over the past few decades, several other clade 8b-like Phytophthoras have been found on a variety of different host plants that were all grown at low temperatures in winter seasons. In this study, a collection of 30 of these isolates was subjected to a phylogenetic study using two loci (the rDNA ITS region and the mitochondrial cox1 gene). This analysis revealed a clear clustering of isolates according to their host plants. To verify whether these isolates belong to separate species, a detailed morphological study was conducted. On the basis of genetic and morphological differences and host specificity, we now present the official description of three new species in clade 8b: Phytophthora cichorii sp. nov., P. dauci sp. nov. and P. lactucae sp. nov. Two other groups of isolates (Phytophthora taxon castitis and Phytophthora taxon parsley) might also represent new species but the data available at this time are insufficient for an official description. This brings Phytophthora clade 8b to a group of six species that are all host-specific, slow-growing and specifically infect herbaceous crops at low temperatures.
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. References: (1) E. Bèliard and J. Thibault. Phytoma 554:2, 2002. (2) O. Constantinescu. Taxon 54:813, 2005. (3) C. Crepel and S. Inghelbrecht. Plant Dis. 87:1266, 2003. (4) A. de Bary, Ann. Sci. Nat. Bot., Sér. 4, 20:5, 1863.
