Pustule disease of pear

The causal agent of the pear pustule disease is the viroid known as Apscaviroid pustulapyri. Viroids are the simplest known pathogens, consisting of a short, single-stranded circular RNA molecule that does not encode any proteins, yet they effectively reprogram the host plant's metabolism to facilitate their own replication.

The pathogen specifically infects pear trees (Pyrus species). It is a systemic pathogen, meaning that once a tree is infected, the viroid particles move through the phloem to colonize all parts of the plant, making eradication from an infected individual tree virtually impossible.

Typical symptoms include the development of pustule-like eruptions on the bark of shoots and branches. These eruptions can rupture, leading to localized necrosis, stunting of overall tree growth, leaf chlorosis, and a significant reduction in fruit set and quality. In severe cases, the infected trees may show signs of decline and dieback.

Transmission occurs primarily through vegetative propagation. Grafting infected scions onto rootstocks or using infected rootstocks is the most common way for the disease to spread. Furthermore, mechanical transmission via contaminated pruning equipment, such as saws and shears, facilitates the spread of the viroid within an orchard.

Because there is no cure for viroid infections in fruit trees, preventive measures are the only way to manage the disease effectively:

• Utilize only certified virus-free (or viroid-free) nursery stock for planting.
• Implement strict sanitation protocols for all pruning and grafting tools.
• Monitor trees regularly and perform rogueing (removal) of infected symptomatic trees to stop spread.
• Avoid trade of propagation material from unknown or suspect sources.