Spanish Researchers Introduce Three Olive Cultivars Bred to Resist Devastating Soil Fungus

After nearly two decades of breeding work, researchers in Spain have introduced three new olive cultivars designed to combine resistance to Verticillium wilt with the oil quality and agronomic traits required by modern olive growing.

The idea behind this breeding program was to develop new cultivars combining high levels of resistance to the disease with good agronomic performance.– Lorenzo León, researcher, IFAPA

“Some traditional cultivars such as Frantoio were already known to exhibit high levels of resistance, but also some disadvantages for growers,” Lorenzo León, a researcher at the Andalusian Institute of Agricultural and Fisheries Research and Training (IFAPA), told Olive Oil Times. “The idea behind this breeding program was to develop new cultivars combining high levels of resistance to the disease with good agronomic performance.”

The new cultivars represent the latest stage of a long-running breeding effort to address one of the olive sector‘s most persistent soil-borne diseases. Verticillium wilt, caused by the fungus Verticillium dahliae, can severely reduce productivity and eventually kill infected trees. Once established in the soil, the pathogen is notoriously difficult to eradicate, leaving growers with few effective options beyond prevention and replanting.

The disease has become a significant problem in parts of southern Spain, where intensive olive cultivation and decades of agricultural activity have contributed to the spread of infested soils. Picual, by far the most widely grown olive variety in the region, is particularly vulnerable.

The three new cultivars — Cástula, Urgavona and Iliturgitana — take their names from ancient Iberian and Roman settlements in present-day Jaén province, where much of the breeding and evaluation work was conducted. The naming reflects the project’s deep connection to Andalusian olive-growing traditions and the region most affected by the disease.

How the fungus spreads

The infection process begins underground. Dormant microsclerotia in the soil become active after detecting nearby olive roots, penetrating the root system and migrating toward the xylem — the tree’s internal water-transport network.

From there, the fungus spreads through the canopy while progressively blocking water circulation, eventually causing the branch dieback, wilting and decline that characterize Verticillium wilt.

According to León, the IFAPA program was designed from the outset to move beyond the limitations of previously known tolerant cultivars.

“Frantoio was the parent of two of these three new cultivars,” he said. “But it also produces a long juvenile period in the progeny, which makes most seedlings useless from a breeding perspective.”

A staged selection process

For a successful olive breeding program, resistance alone is not enough if the resulting trees are poorly suited to commercial farming. Excessive vigor, delayed fruit production, harvesting difficulties, low oil content or poor oil quality can all render otherwise promising trees unsuitable for growers.

To address that challenge, the IFAPA team adopted a sequential selection strategy that prioritized agronomic performance before any disease resistance screening.

“In our program, unlike other breeding initiatives for Verticillium wilt resistance in olive, initial selection was carried out for short juvenile period and high oil content,” León explained. “This ensured promising agronomic performance before any disease resistance evaluation.”

Researchers then progressively narrowed the breeding population through multiple evaluation stages, discarding trees that failed to meet increasingly demanding criteria.

“Additional evaluations were conducted sequentially, including an increasing number of traits on a progressively reduced set of genotypes, discarding those showing any critical agronomic disadvantages or undesirable oil quality traits,” León said. “This is what we call the ‘cruelty’ of the breeder.”

The process included testing across a range of experimental conditions — from growth chambers and artificially inoculated microplots to commercial orchards naturally infested with Verticillium dahliae.

“Regardless of the mechanism involved, after several cycles of evaluation under different experimental conditions, these three new cultivars have demonstrated low disease incidence,” León said. “Indeed, much lower than our control ‘Picual’ — the main cultivar in the area that growers need to replace in highly infested zones.”

The growing expansion of intensive and super-high-density orchards across Mediterranean producing regions, combined with increasing disease pressure and the need to replant in already infested soils, has accelerated demand for new olive genetics better suited to modern production systems.

Still, researchers remain cautious about classification. “We still need to conduct additional research to fully understand to what extent resistance and tolerance mechanisms are operating in these new cultivars,” León concluded.