A fruit horticulture of quality has for pre-required a correct diagnosis of the diseases, a good knowledge of the pathogens and of the sensibilities of the varieties, as well as a knowledge of the occurrence of resistance to the products used to control the diseases. In Belgium, bacterial diseases present important real or potential risks for several horticultural crops. It is the case in strawberry-plant with Xanthomonas fragariae (photo 1), a quarantine organism described first in the USA and, down to recent times, unknown in Belgium. The bacterium penetrates in our country with latently contaminated plant material originating from Holland and France, where the disease already widely propagated. In the case of the cherry tree, deaths of branches and trees, attributable to Pseudomonas syringae but wrongly interpreted by the professionals, hindered the extension of the culture in Belgium. Problems are regularly observed on other important crops: Pseudomonas syringae in pear tree and plum tree and Erwinia amylovora (photo 2), responsible for the fireblight, in apple tree and especially in pear tree. In our country, the control of these phytopathogenic bacteria comes up against two major problems: - The detection and the accurate identification of the pathogens. Indeed, the diagnosis of bacterial diseases is complicated by possible confusions with damages from other origins. Besides, the accurate identification of a bacterium can be difficult. - The lack of means for disease prevention. At present, the use of antibiotics is forbidden in fruit horticulture. With the exception of the copper salts, which have harmful effects for the plant as for the environment, the producers do not have alternatives to fight a bacterial disease. Besides, resistances to copper salts were already observed in several regions of the world. Thus, in the Walloon region, the bacterial diseases are relatively badly known and the identifications of the pathogens are rarely realized. The knowledge of possible variability's within the bacterial species is also very incomplete. The deficiency in methods of disease control against the bacterial diseases gives an increased interest in the resistance of the varieties. However, here also, lacks of knowledge exist in certain cases.
Objectives
Optimalise the diagnosis of the damages caused by bacteria in the orchards in order to better estimate their importance, and to improve and rationalize the fight against these pathogens: - Set up a center of identification and of information in the service of the producers; so, create a center of expertise in the Walloon region. - Study the possibilities of development of kits of detection in situ (on the vegetable), certain steps of which could be realized by the producers. - Study the internal variability of these pathogens to establish a good knowledge of their ecology; this knowledge is necessary for the later elaboration of new methods of disease prevention, effective and respectful of the environment. - Realize an inventory as regards the existence of resistances to copper salts among the populations of phytopathogenic bacteria encountered in the Walloon region. - Clarify the sensibilities of varieties when they are not known.
Expected results
Ending of the identifications and characterization of the bacterial strains collected in the Walloon region. Clarify certain sensibilities of varieties. Our laboratory intends to obtain an accreditation as regards the detection of the quarantine organism Erwinia amylovora and Xanthomonas fragariae.
Results obtained
The techniques of identification of the pathogens, from pure cultures or from enriched media, are workable for several important groups. This expertise is put in the service of the horticultural sector. As a result of 235 visits in Walloon orchards, 647 strains of Pseudomonas syringae and 41 strains of Erwinia amylovora were identified and put in collection. The used tests allowed to characterize the strains of Pseudomonas syringae as regards the phytotoxins and pyoverdines produced. Strains strongly resistant to copper were detected in various groups. Pseudomonas syringae (photos 3 and 4) is present in all its diversity within the analyzed orchards, but also Pseudomonas viridiflava. The observations allow to pull certain conclusions as regards the study of various litigious cases in orchards. In certain cases, evaluations of sensibilities were realized among varieties having an agronomical interest.
Partners
Dr. R. Oger and Ir. V. Planchon, Centre wallon de Recherches agronomiques. Ir. A. Robbe and Ir. C. Melin, Groupement des fraisiéristes wallons (GFW asbl.).
