Hanging on the Infection Peg
By Jessica Fostvedt (Waitrose CTP Student)
Survival demands that all living things must adapt to their environment or perish. However, an alternative strategy is for an organism to alter the environment to suit its own needs. For fungal plant pathogens, the surrounding plant tissue provides a generous home… and one that is just as prone to manipulation. Some pathogens actively attack cells and disrupt plant defences, but others simply ‘wait’ for their environment to become more favourable, and they do this during a stage called pathogen quiescence.
The processes behind pathogen quiescence were summarized in a 1996 review by Prusky, while looking at a specific fungal disease of avocados called Stem End Rot. This condition causes the fruit to decay from the stem button, once infected by one of the numerous fungal culprits. In this case, it is the cut stem of the avocado which provides a weak point primed for invasion by pathogens. Severe cases of stem end rot will progress into the vascular tissue and eventually necrotize the entire fruit. This rotten disease really all begins with a peg.
The infection peg is a structure which germinates from a fungal spore known as a conidium. When this fungal spore lands on an avocado fruit, it produces specialized hyphae, called appressoria, which grow into the skin of an unripe avocado to form an infection peg. Once the peg has pierced the skin, it sits dormant until the conditions for fungal growth are optimal. As the fruit ripens it releases a chemical called ethylene, which causes the peg to break dormancy and develop into the fungus, ready to consume the fresh fruit. So, whether it is possible to prevent fungal spores from infecting avocados, or prevent the fungus from developing into the fruit, curbing avocado Stem End Rot hinges on a peg.
Prusky, D. (1996). Pathogen Quiescence in Postharvest Diseases. Annual Reviews of Phytopathology, 34, 413–434.
Image Credit: Prusky, D., Alkan, N., Mengiste, T., & Fluhr, R. (2013). Quiescent and Necrotrophic Lifestyle Choice During Postharvest Disease Development. Annual Review of Phytopathology, 51(1), 155–176. https://doi.org/10.1146/annurev-phyto-082712-102349.