Research Reveals How Deadly Pathogen Infects Olive Trees

Using a new method of in vitro analy­sis, researchers from the University of Córdoba’s agron­omy depart­ment have proven how fluid emit­ted from olive tree roots allows the ger­mi­na­tion of the fun­gus that causes Verticillium wilt.

The researchers hope the find­ings, pub­lished in Plant and Soil, will pave the way to more effec­tive con­trol mea­sures for the soil-borne fun­gal dis­ease that affects a wide range of plants, includ­ing olive trees.

“The best way to estab­lish con­trol mea­sures in a ratio­nal way against crop pathogens is to know in depth the mech­a­nisms involved,” said Antonio Trapero-Casas, a pro­fes­sor of plant pro­duc­tion at the uni­ver­sity. ​“This study is an attempt to get to know these mech­a­nisms involved,” he added.

Verticillium wilt is cur­rently one of the biggest threats to olive groves world­wide because there is no known method of con­trol­ling it effec­tively.

Though there are resis­tant olive cul­ti­vars, they are less pro­duc­tive and still vul­ner­a­ble. Picual, the most com­mon com­mer­cial cul­ti­var, is also the most sus­cep­ti­ble to the pathogen.

In 2021, the University of Córdoba’s agron­omy depart­ment said its four-year efforts at com­bat­ing Verticillium wilt by graft­ing Andalusian olive trees with resis­tant vari­eties, the most promis­ing strat­egy to date, had failed to pro­duce results in the field.

Once Verticillium dahliae, the fun­gus that causes the wilt, has infected a host, it becomes vul­ner­a­ble to sev­eral other types of fungi, bac­te­ria and pro­to­zoa.

Opportunistic microbes also come into play, such as nema­todes and amoe­bae, which, although not ini­tially involved in the infec­tion, feed on the sub­stances gen­er­ated by the olive tree’s nat­ural defense mech­a­nisms.

A major obsta­cle to con­trol­ling the pathogen is its abil­ity to sur­vive in the soil for up to 14 years until it encoun­ters the roots of its host plants, which, dur­ing growth, secrete sub­stances called exu­dates that influ­ence sur­round­ing microor­gan­isms.

To study the role of these exu­dates, the researchers extracted sam­ples from three olive cul­ti­vars: Frantoio, the most resis­tant; Arbequina, a cul­ti­var with medium resis­tance; and Picual, the most sus­cep­ti­ble.

They found that exu­dates from Frantoio did not sig­nif­i­cantly result in the ger­mi­na­tion of Verticillium microscle­ro­tia, while exu­dates from the sus­cep­ti­ble vari­eties did.

The researchers then ana­lyzed how the bio­log­i­cal con­trol agents applied to the dif­fer­ent olive vari­eties could alter the func­tion of the exu­dates.

This analy­sis showed that in Frantoio, exu­dates from treated plants nei­ther induced nor sig­nif­i­cantly reduced the ger­mi­na­tion of pathogen resis­tance struc­tures. In con­trast, exu­dates from treated Picual or Arbequina plants sig­nif­i­cantly reduced the via­bil­ity of these struc­tures.

Researchers said this result is sig­nif­i­cant because it sug­gests that bio­con­trol agents can mod­u­late the effect of exu­dates, thus decreas­ing the abil­ity of the pathogen to infect sus­cep­ti­ble cul­ti­vars.

The result builds upon researchers’ 2022 find­ing that apply­ing Aureobasidium pul­lu­lans and Bacillus amy­loliq­ue­fa­ciens, two microor­gan­isms, and a cop­per phos­phite fer­til­izer boosted the olive trees’ nat­ural defenses against the fun­gus.

• University of Córdoba