Witches' broom resistance screening of seedlings and clones in Ecuador : Some comparisons between natural infection in the field and artificial inoculations

The search for resistance against witches' broom caused by Crinipellis perniciosa is still handicapped by the complexity of the pathosystem and the lack of studies that establish a reliable relationship between artificial inoculation in the laboratory or nursery with natural infection in the field. The CFC/ICCO/IPGRI project has provided opportunities to test a large number of accessions under different circumstances and with different methodologies. The results herein reported, though still considered as preliminary, show some of the promises and constraints encountered with field observations and artificial inoculation of clones and seedlings in Ecuador. In the first study, the levels of natural infection of the accessions of the International Clone Trial (ICT) in the budwood garden and in the ICT were compared. Artificial inoculations of the same clones were carried out in the shade house with the belt spray method and in the budwood garden by using agar blocks. The natural infection in the budwood garden was correlated with the results in the ICT, suggesting that natural infection of a relatively low number of trees can already give quite reliable results. The most resistant clones appeared to be SCA24, LCTEEN46, MXC67, EQX3360-3, GU255V, CCN51, PA107, SCA6 and IMC47. The results of artificial inoculations showed low infection levels of the clones both in the shade house and in the budwood garden, and there was no clear relationship with field results. In the budwood garden and sometimes also in the shade house inoculated shoots became necrotic some time after inoculation. Improvement of the conditions for inoculation of clonal plants is necessary. In the second study, 14 seedling progenies were inoculated twice with the belt spray method. A group of 139 symptomless seedlings was established in the field besides an equal number of seedlings from the same progenies that had not been inoculated. The average number of brooms per tree observed in the field, when corrected for plant size, was correlated (r=0.57) with the percentage of infection of the progenies obtained with the belt spray method. However, the number of brooms for symptomless inoculated seedlings and non-inoculated seedlings did not show any difference. This suggests that the belt spray method is possibly efficient for estimating the average level of field resistance of seedling progenies but not so for selection of more resistant individual seedlings within seedling progenies. The above results are discussed in the context of the complex C. perniciosa x Theobroma cacao pathosystem. Besides improved methods for inoculation of clones, there is a need to better understand how the variation in the pathogen relates to the resistance of the cocoa genotypes.

Bibliographic Details

Main Authors:	Suarez Capello, Carmen, Delgado, R., Del Pozo, Patricio, Vasco, Alfonso, Zambrano, Jorge, Eskes, Albertus, Amores, Fredy, Amores, F.M.
Format:	book_section biblioteca
Language:	eng
Published:	CFC
Subjects:	F30 - Génétique et amélioration des plantes, H20 - Maladies des plantes, U30 - Méthodes de recherche,
Online Access:	http://agritrop.cirad.fr/537707/ http://agritrop.cirad.fr/537707/1/ID537707.pdf
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