Shothole disease

The disease is characterized by small, reddish-brown spots on the leaves. As the infection progresses, the dead tissue within these spots dries and falls out, leaving small, circular holes.

Shoots and branches develop brownish-red lesions that may turn into deep cankers. These cankers are often accompanied by gum exudation, which is a stress response of the tree to the fungus.

Infected buds often turn dark, glossy, and die before they can open in the spring, significantly reducing the potential yield and overall health of the tree branch.

Fruits can also be affected, showing small red or purple spots that later become sunken lesions, often causing the fruit to deform or rot if the weather remains humid.

Defoliation is a major symptom in severe cases; the trees lose their leaves prematurely, which disrupts photosynthesis and forces the tree to expend energy prematurely.

The fungus Plowrightia circumscissa (syn. Clasterosporium carpophilum) is the primary pathogen responsible for this condition in stone fruit trees. It is a persistent fungus that survives in infected plant tissues.

The mycelium resides in infected buds, twigs, and bark cracks throughout the dormant season, waiting for favorable spring conditions to restart its lifecycle and produce new spores.

During the growing season, the fungus produces conidia, which are easily disseminated by wind, splashing rain, and even insects, ensuring rapid spread across the canopy.

The pathogen is highly adaptable and can survive in various plant organs, which makes it particularly difficult to eradicate completely from a commercial or home orchard.

Understanding the biology of Plowrightia circumscissa is essential for effective timing of fungicide applications, specifically targeting the moments of high spore production.

High humidity, frequent rainfall, and moderate temperatures are the most critical environmental triggers for the germination of Plowrightia circumscissa spores.

Poor orchard sanitation, such as failing to remove infected wood or allowing fallen leaves to accumulate, provides an ideal reservoir for the fungus to persist from one year to the next.

Dense planting schemes that restrict airflow through the canopy create a microclimate with high relative humidity, which is highly favorable for the spread of fungal spores.

Prolonged periods of fog or damp spring weather are known to facilitate mass outbreaks of the disease, making trees vulnerable to infection even with some preventative measures.

Poor tree vigor, often resulting from inadequate fertilization or water management, makes the host tree significantly more susceptible to infection and severe damage.

The premature loss of leaves weakens the tree significantly, affecting its ability to store nutrients for the winter and resulting in reduced growth in the following season.

Cankers on the limbs can lead to dieback of entire branches, which drastically reduces the structural integrity and lifespan of the cherry or plum tree.

Fruit infection reduces the quality of the harvest, rendering the yield unmarketable and susceptible to secondary decay caused by other opportunistic fungi or bacteria.

The continuous need for the tree to heal lesions and recover from stress drains the plant's resources, leading to decreased flower bud formation and lower yield.

The financial impact includes the cost of regular chemical treatments, labor for pruning, and loss of revenue due to reduced fruit quality and potential tree mortality.

Effective management begins with strict sanitation: pruning out all infected twigs and branches, and promptly burning or disposing of the debris to eliminate inoculum.

During the fall, cleaning up all fallen leaves under the tree canopy is essential, as these leaves are the primary source of overwintering fungal spores.

Protective spraying with copper-based fungicides is recommended early in the spring, specifically before the buds swell and again after the petals have fallen.

• Remove and burn infected branches during the dormant period.
• Apply systematic fungicides at the onset of primary symptoms.
• Improve orchard aeration through regular and proper pruning.

Maintaining strong tree vigor through balanced fertilization and proper water management helps the trees naturally withstand the pressure from the pathogen more effectively.