Identification de Pseudomonas syringae dans les cultures de tomate (Lycopersicum esculentum L.) de la région de Mostaganem et étude de la toxicité de quelques fongicides à l’égard du pathogène

Abstract

The compaign 2018-2019 has been particulary damaging to the late-saison tomato crop in the mostaganem region. Indeed, considerable yield losses, reaching 100% in certain fields are recorded. In the field or greenhouse, the symptoms are the same. Brown necrosis are visible in all aerial parts of the plant. On leaves, the necroses are surrounded by yellow halo. The origin of the disease is unknown. The symptomatic study suggests Pseudomonas syringae pv. tomato, responsible of dangerous disease that have an economic impact on the tomato crop. The colonies obtained presented morphologically an identical appearance, namely small roud and smooth colonies, a creamy appearance and cream color. Four representative isolates of sampled sites are characterized by biochemical assays and hypersensitivity on tobacco (LOPAT : L, Levane ; O, oxydase ; P, Pectinase ; A, Arginie deshydrolase ; T, Tabac). The tobacco hypersensitivity test gives a positive reaction, which confirms the pathogenicity of the bacterium. In addition, seedling of 3 or 4 leaves of the Susana variety inoculated with the bacterium (108 CFU/ml) show the same symptoms as those obseved in open fields. The colonies reisolated from the seedlings have the same characteristics as the initial colonies. The postulate of kock is verified. Gram staining indicates that the bacteria is Gram negative, rod-shaped. The results of the chemical tests show that bacterium is levan positive, catalase negative, oxidase positive and has neither Arginine deshydrolase nor nitrate reductase. The Hugh & Leifson test reveals an oxidative metabolism. On the basis of all these results the bacterium tuns out to pseudomons syrigae. This work requires complementary analyses, such as confirmation of the identity of the bacterium by molecular analyzes based on the PCR-amplified 16S RNAr gene using universal primers and identification of the pathovar using specific primers. In addition, we have tested the phytotoxicity of certain fungicides, including copper oxychloride, which is known to be toxic to the bacterium, against pseudomonas syringae isolates. The results of this test show a significant difference between the isolates. Among the fungicides tested, phosethyl aluminium is the most toxic to the bacterium. Copper oxychloride comes in second place.