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Phytosanitary control of witches'broom at cacao seed and seedling levels

Ducamp M.. 1999. In : Efficacité de la production cacaoyère et la qualité du cacao au 21è. Actes. Lagos : Cocoa Producers' Alliance, p. 97-98. Conférence internationale sur la recherche cacaoyère. 12, 1996-11-17/1996-11-23, Salvador de Bahia (Brésil).

The main objective of this work was to develop a method of disinfection which guarantees the absence of Crinipellis perniciosa in cacao seeds when these are transferred from a contaminated country to one which is unaffected by this disease. In 1978, Crowshaw and Evans clearly showed that this fungus can be transmitted by seeds. This means that the fungus thus possesses great potential for spreading over a long distance. A transfer of cacao seeds will, therefore, only bc feasible if methods of eradicating this disease are available. According to work carried out at the Cocoa Research Unit in Trinidad, two systematic fungicides, which have had good results in the control of C. perniciosa at mycelial development and spore germination levels, were chosen, narnely Moncut (flutolanyl) and Bayleton (triadimefon). Seeds of clones sensitive to witches' broom were infected with a measured suspension of basidiospores (200,000 b.s. per mililitre), three days after the onset of germination. Two days after being infected the seeds were treated by soaking with the two fungicides using different protocols. The development of both seedlings and eventual symptoms, in a greenhouse, was observed for a period of 16 weeks. The principal result obtained was that a wide range of treatments using Moncut combining concentration and soaking time was liable to eradicate the bean fungus and produce 100% healthy seedlings. A histopathological study of these seedling stems bas indeed confirmed this eradication capacity and in particular the non-formation of possible dormant spores. Eradication was also achieved with Bayleton. But the range of treatments is much more limited as a result of phytotoxic symptoms developing on seedlings treated with concentrations close to those necessary to eradicate the disease. Because it is possible to confuse the symptoms observed, (the) use of Moncut will, for the most part, be advocated for disinfecting cacao seeds. The second objective of this work was to show that it is possible to protect cacao seedlings from infection by witches' broom for a period of 2 years, by using Moncut treatments selected in the first part of this work. When germination occurred, seeds of sensitive clones were treated, as well as the seedlings, at different times according to the protocols. Infection with a suspension of basidiopores was carried out at bud tip level, after different periods of time. Observations on the eventual development of symptoms continued for 16 weeks. The main outcoine was to show that it is possible to protect seedlings which are theoretically sensitive to witches' broom for 15 months if three treatments with Moncut at 1000 p.p.m. are applied both at the time of germination and 3 and 6 months after germination. New tests were set up in order to supplement these very encouraging results in the control of witches' broom.

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