One fungal effector defoliates cotton and olive

Two vascular pathogens, two important crops and a very similar symptom: sudden shedding of leaves. A study of Verticillium strains attacking cotton and olive pointed to a secreted effector protein able to trigger defoliation. Effectors are molecules a pathogen introduces into the surroundings or into host cells in order to change how the plant behaves. In this case a single mechanism helps to explain a particularly destructive variant of wilt disease.

The gene was present in two copies in strains causing defoliation. Removing both copies deprived the fungus of the ability to cause the characteristic leaf drop and strongly reduced its pathogenicity. When the researchers introduced the gene into a strain that did not normally cause defoliation, the new symptom appeared. That combination of experiments — loss of function and gain of function — gives a stronger causal argument than a mere correlation between the presence of a gene and disease.

Analysis of the protein's structure revealed a previously unknown fold. More interesting still was the position of the genes in the genome. The authors connect their movement with Starship elements, large mobile stretches of DNA able to carry packages of genes between fungal lineages. Horizontal transfer may allow a pathogen to acquire a new trait faster than gradual evolution through mutations inherited from generation to generation.

This does not mean that every olive or cotton plant shedding leaves has been attacked by a strain carrying this gene. Defoliation is also caused by water stress, temperature, other pathogens and damage. A genetic test has to be interpreted together with the symptoms, isolation of the agent and the growing conditions. Nor does the presence of a mobile element prove that transfer is happening now in a particular field; the reconstruction describes the evolutionary history of the genomes studied.

The discovery nonetheless offers a useful target for diagnostics and resistance breeding. Monitoring the effector may help to distinguish more aggressive lineages, and understanding its target in the plant may show how to block the symptom without relying solely on fungicides. It also shows why surveillance of plant disease should cover whole genomes, not just a species name. Two isolates of the same Verticillium may differ by a small package of DNA that entirely changes the consequences of infection.

Before a marker enters routine use, it has to be tested on a wider collection of isolates from different regions. A pathogen may lose a gene, change its expression or use another route to pathogenicity. Diagnostics based on a single target is fast, but a negative result should not automatically rule out a dangerous strain. Plant phenotype and genomic data have to check each other across successive seasons.

Evidence status — 29 August 2026

The paper was published on 21 June 2026. The authors went beyond genomic correlation: deleting both copies of the D-effector gene abolished pathogenicity and defoliation in cotton and olive, restoring the gene recovered the trait, and purified protein induced wilting and leaf drop. That is strong functional evidence in the strains and plants tested. The conclusion that Starship elements repeatedly mediated horizontal transfer rests instead on phylogenomics and genomic context. The result identifies a useful target for diagnostics and resistance breeding, but it is not yet a field-ready control method.