Effect of cacao black pod rot screening method on disease resistance determination

Authors: Puig, Alina; Irish, Brian; Ayala Silva, Tomas; Wurzel, Sarah; Gutierrez, Osman

Submitted to: Chemistry Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2022
Publication Date: 2/10/2022
Citation: Puig, A.S., Irish, B.M., Ayala Silva, T., Wurzel, S.B., Gutierrez, O.A. 2022. Effect of cacao black pod rot screening method on disease resistance determination. Chemistry Proceedings. 10(1):71. https://doi.org/10.3390/IOCAG2022-12215.
DOI: https://doi.org/10.3390/IOCAG2022-12215

Interpretive Summary: Black pod rot, caused by oomycetes in the Phytophthora genus, is a devastating disease of Theobroma cacao. It is responsible for major production losses of cacao around the world and resistance to this disease is a priority for breeding programs. Currently, host resistance to the pathogen is determined using the proportion of diseased pods present in the field and the size of lesions following artificial inoculation. This study examined whether artificial inoculation and natural incidence screening methods produced different conclusions. The resistance ranking of clones differed between screening assays. Results of this study will be used to identify new sources of resistance among accessions in the USDA-ARS Cacao germplasm collection, through the use of both field evaluation and artificial inoculation and benefit breeders and farmers by assisting in the identification of new sources of disease resistance and more effective long-term disease control.

Technical Abstract: Black pod rot, caused by several species of Phytophthora, is responsible for greater losses than any other disease affecting cacao. Breeders use various approaches to screen material for resistance to Phytophthora spp., however, the method used to assess disease resistance can influence outcomes. To determine how screening methods affect results, disease reactions of four cacao clones (BE 10, HY 271419, RIM 15 [MEX], and EET 236 [ECU]) were compared using incidence under field conditions, and lesion area following artificial inoculation. Disease incidence differed significantly among clones (p<0.0007), ranging from 6.1% for BE 10 to 24.0% for HY 271419. Differences among clones were also detected based on lesion area (p< 0.032), however, their relative ranking differed: BE 10 (53.9 cm2), HY 271419 (80.64cm2), RIM 15 [MEX] (95.7cm2), and EET 236 [ECU] (102.4cm2). These apparent differences observed in disease reaction among clones when comaparing methods may be due to interactions with environmental conditions or differences in the pathogen species/isolates present. The improved understanding of the how screening methods used can affect the disease reaction determination and breeding outcomes in cacao germplasm will benefit breeders and farmers.