Olives with Higher Phenol Content More Resistant to Anthracnose

Scientists from the University of Córdoba’s olive genet­ics research group have found that changes in the phe­no­lic pro­files dur­ing the olive ripen­ing process play a fun­da­men­tal role in the resis­tance to anthrac­nose.

The eco­nom­i­cally dam­ag­ing olive tree dis­ease is caused by the Colletotrichum fun­gus. The fun­gus causes severe rot in olives, lead­ing to sig­nif­i­cant crop losses.

Olive oil from olives con­t­a­m­i­nated by fun­gus has higher acid­ity and organolep­tic defects. It usu­ally falls into the lam­pante cat­e­gory and is unsuit­able for human con­sump­tion.

“We ana­lyzed six vari­eties for two years, car­ry­ing out analy­ses of phe­no­lic com­pounds and resis­tance tests to the pathogen,” said Hristofor Miho, a PhD stu­dent at the University of Córdoba and first author of the study.

“The results allowed us to observe that resis­tance was greater in vari­eties with high phe­no­lic con­cen­tra­tions and spe­cific phe­nols present in them,” he added.

The researchers selected Empeltre and Frantoio cul­ti­vars, known for their resis­tance to the fun­gus; Hojiblanca and Picudo, known for their lack of resis­tance; and Barnea and Picual, con­sid­ered mod­er­ately resis­tant.

Olives were har­vested from the World Olive Germplasm Bank of Córdoba before they began to ripen and at three ripen­ing stages: green, turn­ing and ripe.

Samples were taken to deter­mine the olives’ phe­no­lic pro­files, and they were then inoc­u­lated using spores of the most com­mon Colletotrichum strain found in Spain and Italy.

While all green olives are immune to the fun­gus, they accu­mu­late inac­tive Colletotrichum infec­tions in the form of appres­so­ria, an organ-like struc­ture that pen­e­trates the fruit.

“This infec­tion remains latent dur­ing fruit devel­op­ment until ripen­ing, result­ing in pathogen reac­ti­va­tion and dis­ease devel­op­ment,” the researchers wrote. ​“Subsequently, olive fruit sus­cep­ti­bil­ity to the pathogen increases dur­ing ripen­ing, while in par­al­lel, there is a decrease in total phe­no­lic com­pounds.”

The researchers also iso­lated seven stan­dard phe­no­lic com­pounds to test their anti­fun­gal activ­ity: hydrox­y­ty­rosol, tyrosol, oleu­ropein, oleu­ropein agly­cone, olea­cein, oleo­can­thal and hydrox­y­ty­rosol 4‑O-glu­co­side.

“Oleocanthal exhib­ited the high­est inhibitory activ­ity, fol­lowed by olea­cein, oleu­ropein agly­cone, hydrox­y­ty­rosol and tyrosol,” the researchers wrote.

Oleuropein, ligstro­side (the pre­cur­sor of oleo­can­thal) and their deriv­a­tives, includ­ing olea­cein, were the most crit­i­cal com­pounds inhibit­ing spore ger­mi­na­tion.

The com­pounds are pre­dom­i­nant in all green fruits regard­less of cul­ti­var and rep­re­sent more than 90 per­cent of total phe­nols dur­ing ripen­ing of the main resis­tant cul­ti­vars.

Meanwhile, sus­cep­ti­ble cul­ti­vars con­verted oleu­ropein, olea­cein and oleo­can­thal into hydrox­y­ty­rosol-4-O-glu­co­side as they ripened, which reduced anthrac­nose tol­er­ance.

“Overall, resis­tant cul­ti­vars induced the syn­the­sis of alde­hy­dic and demethy­lated forms of phe­nols [oleu­ropein, oleo­can­thal and olea­cein], which highly inhib­ited fun­gal spore ger­mi­na­tion,” the researchers wrote. ​“In con­trast, sus­cep­ti­ble cul­ti­vars pro­moted the syn­the­sis of hydrox­y­ty­rosol 4‑O-glu­co­side dur­ing ripen­ing, a com­pound with no anti­fun­gal effect.”

They fur­ther found that a total phe­no­lic con­cen­tra­tion of 50,000 mil­ligrams per kilo­gram in all sam­ples of devel­op­ing olives across cul­ti­vars com­pletely inhib­ited spore ger­mi­na­tion.

The researchers observed that cul­ti­vars sus­cep­ti­ble to the fun­gus expe­ri­enced a 73 per­cent decline in phe­no­lic com­pounds dur­ing ripen­ing, while resis­tant cul­ti­vars expe­ri­enced a 28 per­cent decline.

“The sharp phe­no­lic reduc­tion of the sus­cep­ti­ble cul­ti­vars caused the com­plete reduc­tion of the anti­fun­gal activ­ity,” they wrote. ​“Interestingly, the lesser phe­no­lic decrease of the resis­tant cul­ti­vars did not reduce the inhibitory effect of spore ger­mi­na­tion.”

Juan Moral, who over­saw the research, said the study would help pol­i­cy­mak­ers and farm­ers select new vari­eties to plant and inform researchers about what vari­eties to cross­breed for more resis­tant hybrids.

“Knowing how the phe­no­lic cas­cades [changes in the phe­no­lic com­pounds] behave in the dif­fer­ent vari­eties will allow us to bet­ter select, based on sci­en­tific cri­te­ria, the par­ents that should be used so that the fol­low­ing gen­er­a­tions of olive trees are resis­tant to this dis­ease,” he con­cluded.

• University of Córdoba
• Food Chemistry