# Emergence of Stilbocrea gracilipes associated with canker and dieback in pomegranate and eucalyptus trees and host-specific responses

## TL;DR

A new fungal pathogen, Stilbocrea gracilipes, was found to cause canker and dieback in pomegranate and eucalyptus trees in Iran, with varying levels of host susceptibility.

## Contribution

First report of Stilbocrea gracilipes causing canker and dieback on pomegranate and eucalyptus in Iran and globally.

## Key findings

- Stilbocrea gracilipes isolates caused canker symptoms in both pomegranate and eucalyptus trees.
- Pomegranate cultivar ‘Malas-e-Danesiyah-e-Esfahani’ was highly susceptible, while ‘Kadro’ showed resistance.
- The fungus demonstrated cross-pathogenicity between pomegranate and eucalyptus hosts.

## Abstract

The increasing incidence of canker and dieback symptoms in commercially important fruit orchards and adjacent ornamental trees in Fars Province, Iran, prompted an investigation into associated fungal pathogens. Surveys in 2023 and 2024 yielded several isolates from symptomatic pomegranate (Punica granatum L.) and eucalyptus (Eucalyptus camaldulensis Dehnh.) trees, which were identified as Stilbocrea gracilipes based on morphological, physiological, and multi-locus phylogenetic analyses (ITS, tef1-α, and rpb2 loci). Pathogenicity tests showed that all recovered S. gracilipes isolates (n = 23) cause dark brown lesions and wood discoloration in detached shoots of their respective hosts, with significant variation in aggressiveness. Cross-pathogenicity assessments using the most aggressive isolates on pomegranate and eucalyptus saplings revealed that isolates from original hosts were pathogenic to both plant species, inducing characteristic lesions and internal wood discoloration. Based on an assessment of eight pathogenicity traits, the pomegranate-obtained isolate (PM7-68) demonstrated more aggressiveness compared to the eucalyptus-obtained isolate (EU1-031). Furthermore, inoculated pomegranate saplings exhibited greater susceptibility than inoculated eucalyptus saplings, highlighting the potential threat of S. gracilipes to economically significant pomegranate orchards. Evaluation of pomegranate cultivar susceptibility to S. gracilipes isolates, using principal component analysis (PCA) and heatmap visualization, revealed clustering of disease severity into four distinct groups. The pomegranate cultivar ‘Malas-e-Danesiyah-e-Esfahani’ consistently exhibited high disease severity (level 4), identifying it as highly susceptible, while the cultivar ‘Kadro’ displayed reduced lesion development and slower disease progression (level 1), suggesting relative resistance.

This study represents the first report of Stilbocrea gracilipes associated with canker and dieback on pomegranate and eucalyptus, and the first confirmation of its pathogenicity as a canker-causing fungus on woody hosts in Iran and globally. The ability of S. gracilipes to infect both fruit and ornamental trees, coupled with its cross-host infectivity, poses a potential risk to mixed cropping systems and natural ecosystems.