Cryptogein is a small 10 kDa elicitor produced by the phytoparasitic oomycete Phytophthora cryptogea. The protein also displays a sterol carrier activity. The native protein crystallizes in space group P4 1 22, with unit-cell parameters a = b = 46.51, c = 134.9 A Ê (diffraction limit: 2.1 A Ê ). Its complex with cholesterol crystallizes in space group C222 1 , with unitcell parameters a = 30.96, b = 94.8, c = 65.3 A Ê and a resolution enhanced to 1.45 A Ê . The large inner non-speci®c hydrophobic cavity is able to accommodate a large variety of 3--hydroxy sterols. Cryptogein probably acts as a sterol shuttle helping the pathogen to grow and complete its life cycle.

Fifteen isolates of Phytophthora parasitica, nine from tobacco (causing black shank disease) and six from other host plants, were compared by root inoculation with regard to their pathogenicity to young tobacco plants. A progressive invasion of the aerial parts over 1 week was observed only with the black shank isolates, while the non‐tobacco isolates induced leaf necrosis within 2 days. Similar necrosis occurred when the roots of tobacco plants were dipped in diluted culture filtrates from non‐tobacco isolates, but not in those from tobacco isolates. The necrosis‐inducing filtrates were shown to contain a c. 10‐kDa protein band which was not present in the other filtrates. This protein (named parasiticein) was purified by ion‐exchange chromatography to homogeneity in SDS‐PAGE and reverse‐phase HPLC. Parasiticein was serologically related to cryptogein, a member of the elicitin family of proteinaceous elicitors previously described from other Phytophthora species. Like the other elicitins, parasiticein induced necrosis in tobacco plants and protected them against black shank. It most closely resembled capsicein in being acidic and in inducing resistance at concentrations (10–100 μg per plant) that caused little leaf necrosis. It is suggested that the absence of parasiticein production by the black shank isolates might be a factor involved in their specific pathogenicity to tobacco.

Phytophthora spp. secrete proteins called elicitins in vitro that can specifically induce hypersensitive response and systemic acquired resistance in tobacco. In Phytophthora parasitica, the causal agent of black shank, most isolates virulent on tobacco are unable to produce elicitins in vitro. Recently, however, a few elicitin-producing P. parasitica strains virulent on tobacco have been isolated. We investigated the potential diversity of elicitin genes in P. parasitica isolates belonging to different genotypes and with various virulence levels toward tobacco as well as elicitin expression pattern in vitro and in planta. Although elicitins are encoded by a multigene family, parAl is the main elicitin gene expressed. This gene is highly conserved among isolates, regardless of the elicitin production and virulence levels toward tobacco. Moreover, we show that elicitin-producing P. parasitica isolates virulent on tobacco down regulate parAl expression during compatible interactions, whichever host plant is tested. Conversely, one elicitin-producing P. parasitica isolate that is pathogenic on tomato and avirulent on tobacco still expresses parAl in the compatible interaction. Therefore, some P. parasitica isolates may evade tobacco recognition by down regulating parA1 in planta. The in planta down regulation of parA1 may constitute a suitable mechanism for P. parasitica to infect tobacco without deleterious consequences for the pathogen.

Gummosis, caused by several Phytophthora species, is one of the most serious constraints to citrus production in Corsica. The current Phytophthora population on citrus trees is mainly composed of P. citrophthora divided into two major groups, namely G1 and G2, with two additional minor groups. Following changes in cultural practices, favourable climatic conditions, and introduction of new hosts, repeated gummosis outbreaks have been observed. The pathogenicity of several Phytophthora isolates of each group collected in citrus groves was assessed for gummosis disease on 20 citrus scion and rootstock cultivars. All isolates tested proved virulent on citrus but extensive pathogenicity differences were observed among and within Phytophthora groups. The data indicate that G1 isolates of P. citrophthora are pathogenic to resistant rootstocks such as Poncirus trifoliata or Carrizo citrange, and moderately aggressive towards scions. In contrast, isolates of the group G2 tend to display a greater aggressiveness to a number of scion cultivars, but are not virulent to P. trifoliata and its hybrids. Implications of these data on disease management are discussed.U. S.

The pathogenicity to tobacco of a large set of Phytophthora parasitica isolates has been assessed using several procedures: root inoculation of young plants, leaf inoculation on detached disks and stem inoculation of decapitated plants, with or without healing. Analysing various aspects of the plantpathogen interaction with this array of tests led to the discrimination between three groups of isolates. In the isolates from hosts other than tobacco, none was truly pathogenic to tobacco, and all but one produced parasiticein, a proteinaceous elicitor of the elicitin family which induces a hypersensitive‐like response in tobacco. Isolates producing parasiticein in vitro induced necrotic fleks on the leaves upon inoculation of roots orof, freshly wounded stems. Most tobacco isolates, including all the highly virulent ones, were characterized by a lack of elicitin production. However, those collected in Australia and Zimbabwe differed in that they exhibited reduced virulence, induced leaf, necrotic flecks and produced parasiticein. The incidence of elicitin production on virulence and the significance of two types of tobacco‐pathogenic strains for tobacco pathology are discussed.