Nuevas asociaciones hospedero-patógeno en cultivos hortícolas del sudeste español

Resumen

The high loss of fruit and vegetable products due to disease makes it necessary to follow an adequate strategy for their management and control. Current climatic changes may modify the survival and development of plant pathogens, as well as their sources and transmission mechanisms, all of which has affected the geographical distribution of species. For new interactions in these areas, a correct diagnosis of the causal agents is crucial. This doctoral thesis presents a study of new pathologies of broccoli, melon and pumpkin in the current context. In the case of broccoli, the oomycete fungus-like Pythium ultimum Trow, (1901), syn. Globisporangium ultimum (Trow) Uzuhashi, Tojo and Kakish, growing together with several bacterial species, was isolated inside of mature broccoli flower stalks. This caused aqueous decay of tissues with symptoms very similar to those produced by bacterial soft rot. The internal stem rot of broccoli has been described as a disease called bacterial soft rot caused by pectolytic bacteria. In fact, the bacterium Pectobacterium carotovorum Jones, (1901) is often associated with internal rot of this vegetable. However, in this study, P. ultimum was isolated causing internal soft rot of tissues, whose symptoms were similar to those of bacterial soft rot. Nevertheless, P. ultimum is mainly described as a pathogen causing damping-off and root rot on many nursery plants, including broccoli. Disease symptoms caused by this pseudofungus were reproduced in broccoli stem sections to verify Koch’s postulates. Bacteria isolated near the affected tissue did not cause the disease, but when in vitro grown with P. ultimum, they did interfere with its development. Phenotypic and genotypic traits of the organisms involved in the pathology were studied with the aim of increasing knowledge of the basic characteristics of this unusual disease and the causal organism involved in it. As regards melon, a high incidence of pathogens was identified on two commercial varieties: galia (Cucumis melo var. reticulatus) and cantaloupe (Cucumis melo var. cantalupensis). Fruits showed development of whitish mycelium and softening pulp. Morphological and molecular analysis were carried out to identify the causal organisms of diseases, which were Fusarium solani and Fusarium brachyggibosum for galia and Fusarium equiseti and Fusarium annulatum for cantaloupe. The molecular analysis distinguished these species into four different lineages within the genus Fusarium, being Fusarium solani (FSSC), Fusarium sambucinum (FSAMSC), Fusarium incarnatum-equiseti (FIESC) and Fusarium fujikuroi (FFSC), respectively. The pathogenicity of isolates was tested on healthy fruit melons to verify Koch's postulates. The first rot symptoms were observed 3 days after inoculations at 28 ᴏC. Infection of healthy fruits only occurred in artificially wounded melons. Symptoms were similar to those observed in the initially supplied specimens, and all four pathogens were re-isolated from wounds. It should be noted that these pathologies appearing during the postharvest period may cause significant economic losses, especially in products whose destination is the foreign market. Regarding butternut squash, the fungus Rhizopus sexualis var. sexualis was identified in as a cause for rot characterized by the softening and decomposition of tissues. The symptoms observed have been frequently associated with Rhizopus stolonifer, the most common pathogen of the genus Rhizopus, but a morphological study of the isolate pointed out some umbeliform upper branching of sporangia not described in R. stolonifer. In addition, the molecular analysis identified multiple ITS sequences caused by a casual heterokaryosis of the species R. sexualis var. sexualis. The sequence of the D1/D2 region of the 28S rRNA gene was found to be similar to that of R. stolonifer. The in vitro growth of R. sexualis var. sexualis indicated that, at 25 ᴏC, there was a higher rate of development and sporangia, while at lower temperatures development was not completed in synthetic culture medium and sporulation was not reached. Inoculation of the pathogen into healthy pumpkins revealed that a wound was necessary for disease development. Finally, the pathogen was re-isolated from affected tissues and identified in order to establish Koch's postulates. In conclusion, the aim of this doctoral thesis was to achieve a high level of precision in the identification of pathogens that cause diseases in fruit and vegetables. Most of these identifications are currently being either incomplete because of their only scope at genus level, or poorly categorised on a practical level, which makes this work crucial if we want to guarantee the success of future phytosanitary treatments.