Selection of cocoa progenies for resistance to witches’ broom

Abstract

Witch's broom (WB) and frost pod rot (FPR) are devastating fungal diseases and responsible for severe losses in cacao trees grown in tropical America. To expand and stabilize the resistance to WB in cacao progenies, as well as to incorporate resistance genes to FPR, 17 genotypes were selected from a structured population. These were used in a second selection cycle, in which four gene donors of FPR resistance were included. All but one of the 59 and 52 progenies differed significantly (P < 0.05) from the susceptible controls Catongo and SIC 23, respectively. Nineteen progenies differed significantly (P < 0.05) from the resistance control SCA 6, suggesting high WB resistance. The direct descendants of cacao genotypes from the Brazilian Amazon and from clones carrying FPR resistance showed lower disease index than the resistant control SCA 6, except for some of the progenies of clone ICS 95, which behaved similarly to the susceptible control. The study highlights nine progenies carrying WB resistance genes from several sources and seven of them from parents carrying frosty pod resistance genes.

Resistance to witches’ broom in adult plants and progeny of local varieties of cacao in Southern Bahia

“…In turn, the CCN51 clone selected in Ecuador, is the result of a breeding between ICS-95, IMC-67 and Canelus, and has been considered a carrier of a moderate resistance to witches' broom (Campo and Andía 1997;Boza et al 2014). Finally, the Catongo variety (Theobroma leiocarpa Bernoulli), first found in the mid-1930s in Pirangi, Bahia, is considered an albino mutation of Forastero cacao (Dantas Neto et al 2005) and is often used as a pattern for susceptibility to witches' broom (Pimenta Neto et al 2018;Benjamin et al 2016). Significant effects for DI between 96 cacao genotypes under study were found (p < 0.01).…”

“…The basidiospores were obtained according to the methodology described by Dickstein et al (1987 ) and subjected to a viability test proposed by Frias et al (1995). The inoculation process was performed according to a previously established methodology, in which 30 μL of the inoculum with concentration adjusted to 2 × 10 5 basidiospores•mL -1 was deposited in 0.2% agar-water medium in the apical bud (Pimenta Neto et al 2018).…”

“…Studies of identification and selection of new sources of resistance to witches' broom involve steps developed in a greenhouse and/or field (Benjamin et al 2016;Pimenta Neto et al 2018). The methodologies are equally important and complementary (not exclusive); however, they may require excessive time and resources when performed together.…”

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“…Este mesmo clone e, certamente, alguns descendentes foram utilizados como progenitores de variedades híbridas distribuídas pela Ceplac, e as seleções em fazendas devem, em sua maioria ou totalidade, serem descendentes de Scavina. O clone Scavina 6 tem se mostrado como portador de dois alelos maiores para resistência, cujos efeitos, em diferentes localidades da região cacaueira da Bahia, vêm sendo quebrados quando um alelo atua individualmente, mas preservado quando em associação com outro gene ou alelo de resistência (Pires, et al, 2009;Pires et al, 2012;Gramacho et al, 2012;Pimenta Neto et al, 2018). E mesmo sem quebra de resistência, descendentes de Scavina, apesar de diferirem muito de materiais tidos como não resistentes em termos de perda de frutos por vassoura de bruxa em níveis médios de pressão de inóculo, diferem pouco quando sob alta pressão de inóculo (Pires, 2003).…”

“…O primeiro destes é um amplo processo de seleção recorrente para produtividade e ampliação do nível e da durabilidade da resistência à vassoura de bruxa e outras doenças, pela associação de diferentes genes ligados a estes caracteres (Lopes et al, 2011;Benjamim et al, 2016;Pimenta Neto et al, 2018;Silva et al, 2010;Rodrigues, Pires & luz, 2020;Yamada et al, 2014).…”