Differential aggressiveness of Austropuccinia psidii isolates from guava and rose apple upon cross-inoculation

Myrtle rust, caused by Austropuccinia psidii, has been associated with more than 480 plant species belonging to the family Myrtaceae. Intraspecific variability in pathogenicity has been reported among isolates of A. psidii from different hosts. However, there are few studies that have comparatively quantified the disease in guava (Psidium guajava) and rose apple (Syzygium jambos). The objective of this work was to quantify the pathogenic variability of A. psidii isolates collected from guava and rose apple in Brazil and to investigate the mechanisms of infection and colonization of the pathogen at the cellular level. The monocyclic components of the rust disease were evaluated on young leaves of rose apple and guava plants cross-inoculated with isolates from rose apple and guava. Pathogenic specialization of both A. psidii isolates for their respective hosts was confirmed in this work. The guava A. psidii isolate was able to infect and colonize rose apple leaf tissues but no sporulation occurred. Similarly, the rose apple A. psidii isolate infected guava plants but did not sporulate. Confocal laser scanning microscopy revealed that lobed haustoria were present within rose apple leaves at 2 days post-inoculation (dpi) with both isolates, which resulted in intense mesophyll colonization for both interactions at 9 dpi. The latter is remarkable because infection of rose apple with the guava A. psidii isolate did not result in mature rust pustules.