CANCELLED - Contemporary Status of Witches' Broom In Bahia, Brazil: Fungal Evolution and Disease Resistance

Studies of epidemics on crops have shown that plant pathogens can quickly adapt to their host plant and overcome resistance genes mainly when one source of resistance is consistently used. This seems to be the case in the cacao x witches' broom (WBD) pathosystem. Most varieties resistant to WBD  in Bahia, Brazil has   Scavina as its sole source of resistance genes. We have evidences that the Scavinas’ resistance have been overcome because of pathogen adaptation. In Bahia, QTL studies to detect resistance gene of WBD were carried out using an F₂ progenie from  Scavina 6  x ICS1. From observations in the field between 1996 and 2002, a major QTL, located in LG9, was detected (LOD score and the percentage of variation explained by the QTL were respectively: 10.5 and 51%).  From new field observations  between 2003 to 2008, the significance of this major QTL gradually decreased  to be no significant from 2006 and in the same time others resistant QTLs were detected. Otherwise, using another F₂ progenie from  Scavina 6  x ICS1 recently created (2007), observations after artificial inoculations with an old and recent (2008) inocula of WBD showed that  major QTL of GL9, initially detected,  was only found on the confidence interval of this QTL with the old inocula.  Currently, the CEPEC cacao breeding program  has taken in consideration the pathogen adaptation. This has increased the efficiency of selecting resistant cultivars to WBD, therefore allowing  the association of  distinct WB resistance genes.