Résumé:
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.
