Spanish Scientists Advance Understanding of Devastating Olive Disease

A team led by researchers at the University of Córdoba’s depart­ment of agron­omy has pub­lished one of the most com­pre­hen­sive stud­ies to date into Colletotrichum, the fun­gus which causes anthrac­nose, or ​“soapy olive.”

Anthracnose in olive fruit is highly vir­u­lent and can cause crop losses of up to 100 per­cent. In addi­tion, a toxin pro­duced within the rot­ten fruit can weaken the trees them­selves by caus­ing the dieback of branches, thereby reduc­ing future yields even after suc­cess­ful treat­ment. In Spain, the dis­ease is respon­si­ble for an aver­age annual crop loss of 2.6 per­cent.

In the study, a total of 185 iso­lates col­lected over a period of more than two decades were ana­lyzed. Samples were taken pri­mar­ily from Spain and Portugal, which are two of the world’s largest olive oil-pro­duc­ing coun­tries. However, many other sam­ples were col­lected from Australia, Brazil, California, Greece, Italy, Tunisia and Uruguay.

While much prior research exists, mol­e­c­u­lar iden­ti­fi­ca­tion of iso­lates had not pre­vi­ously been car­ried out.

“In the case of Colletotrichum, mor­pho­log­i­cal char­ac­ter­is­tics do not allow us to dif­fer­en­ti­ate between dif­fer­ent species, so we must resort to DNA sequences that tell us how sim­i­lar some iso­lates are to oth­ers,” said Juan Moral, one of the lead­ing researchers.

After using seven spe­cific gene regions, 12 dis­tinct species of Colletotrichum were iden­ti­fied.

Samples from other sus­cep­ti­ble crops such as almonds, sweet oranges and straw­ber­ries, also were included in the study, and the fun­gus was found to be highly adapt­able and oppor­tunis­tic.

Isolates from Australian olive sam­ples showed the high­est Colletotrichum diver­sity by far, but with the two species dom­i­nant in Spain, Portugal, Greece and Italy being entirely absent. This adds weight to the hypoth­e­sis that native Colletotrichum species are able to rapidly jump to new hosts.

This abil­ity of the fun­gus has prac­ti­cal impli­ca­tions for the pre­ven­tion of the dis­ease, as demon­strated by a case of cross-con­t­a­m­i­na­tion at a nurs­ery in north­east­ern Spain where cit­rus plants host­ing the species C. fruc­ti­cola are sus­pected to have infected olive plants which then showed necro­sis of the leaves, a rare but poten­tially deadly symp­tom of anthrac­nose.

Given the pathogen’s dev­as­tat­ing eco­nomic impact, var­i­ous species were sub­jected to both beno­myl and cop­per-based fungi­cides to deter­mine their sen­si­tiv­ity and resis­tance.

“We have seen dif­fer­ences in sen­si­tiv­ity to fungi­cides between species and when we inoc­u­lated dif­fer­ent vari­eties we also found dif­fer­ences in vir­u­lence between these iso­lates,” said Antonio Trapero, a University of Córdoba researcher.

Copper-based fungi­cides have become one of the most com­monly used in recent years, due in part to their lower costs. However, results vary widely.

For exam­ple, the team observed that while the Spanish C. gode­tiae iso­lates from olive-grow­ing regions where cop­per-based fungi­cides are fre­quently used by farm­ers were more tol­er­ant to cop­per than C. nymphaeae iso­lates, sam­ples from Portugal showed the oppo­site results.

“Having iso­lates from many coun­tries shows how even iso­lates of the same species behave dif­fer­ently depend­ing on the geo­graph­i­cal area they come from,” researcher Carlos Agustí said.

The University of Córdoba said explor­ing the biol­ogy and bio­di­ver­sity of anthrac­nose-caus­ing pathogens in such depth should help advance the cre­ation of more effec­tive con­trol meth­ods.

The Spanish and Andalusian gov­ern­ments share this goal and both pro­vided sig­nif­i­cant fund­ing for the research.

• University of Córdoba
• Journal of Fungi