Citrus black spot

Citrus Black Spot

Citrus Black Spot on Valencia orange

Citrus Black Spot (Guignardia citricarpa, syn. Phyllosticta citricarpa) is a fungal disease affecting citrus plants. It is primarily known for causing significant economic losses in citrus production due to the blemishes it creates on the fruit, making them less marketable despite not affecting the taste or internal quality of the fruit. This disease is of particular concern in humid, subtropical climates where conditions are favorable for its development and spread.

Symptoms

Citrus Black Spot manifests in various symptoms on the leaves, fruit, and, less commonly, twigs of citrus plants. The most recognizable signs are on the fruit, which include:

• Hard spot (a.k.a. cracked spot) - dark, sunken lesions
• Freckle spot - small, dark spots that do not penetrate the rind
• Virulent spot - large, black lesions that can lead to fruit drop
• False melanose - tiny, raised, dark spots

Leaves and twigs may show lesions, but these are less common and typically do not affect the overall health of the plant.

Causative Agent

The disease is caused by the fungus Guignardia citricarpa. It thrives in warm, wet conditions, particularly in regions with a distinct wet season. The fungus spreads through spores, which can be dispersed by rain, wind, and possibly insects, making management of the disease challenging in affected areas.

Distribution

Citrus Black Spot is found in many citrus-growing regions around the world, including parts of Africa, Asia, South America, and Australia. It is notably absent from most citrus production areas in Europe and North America, where strict quarantine measures have been implemented to prevent its introduction.

Management

Management of Citrus Black Spot involves an integrated approach:

• Cultural practices such as pruning and removal of affected fruits and leaves to reduce the source of inoculum.
• Chemical control with fungicides is often necessary in commercial production to protect fruits from infection.
• Quarantine measures to prevent the spread of the disease to new areas are crucial, including restrictions on the movement of citrus plants and fruits from affected regions.

Economic Impact

The economic impact of Citrus Black Spot is significant, particularly in regions where the disease is endemic. The blemishes caused by the disease reduce the market value of the fruit, and in severe cases, can lead to total crop loss. The cost of management practices and the potential for trade restrictions also contribute to the economic burden of the disease.

Research

Ongoing research aims to develop more effective management strategies, including the breeding of resistant citrus varieties, improved fungicides, and biological control methods. Understanding the life cycle of the fungus and the conditions that favor its spread is key to developing effective control measures.