Citrus black spot is a fungal disease affecting citrus plants, especially in areas with summer rainfall. The disease primarily damages the fruit’s external appearance by creating distinct blemishes on the rind. While these marks are visually unappealing, the internal quality of the fruit is not affected, and the pulp and juice remain safe to eat. Severe infections can lead to premature fruit drop, reducing the harvest. Because the disease is cosmetic, its main impact is economic; blemished fruit is unmarketable for the fresh market, forcing growers to divert it to lower-priced juice processing.
Identifying Citrus Black Spot Symptoms
The visual signs of citrus black spot are most noticeable as the fruit begins to ripen and change color. A common symptom is “hard spot,” which appears as small, sunken black spots on the fruit’s rind, typically 3 to 10 millimeters in diameter. These spots often have gray centers and a distinct dark brown or reddish border, and they tend to develop on the side of the fruit that receives the most sunlight.
Another symptom is “freckle spot,” characterized by reddish, slightly depressed lesions that are smaller than hard spots, usually 1 to 3 millimeters wide. The presence of freckle spot can indicate a heavy infection. A different symptom, “virulent spot,” consists of larger, irregular lesions that can merge and cover a significant portion of the fruit’s surface, causing it to drop prematurely.
A less aggressive symptom called “false melanose” or “speckled blotch” can appear on green fruit. These are numerous, small, raised, dark brown spots that give the fruit a speckled look. While symptoms are most prominent on fruit, they can also appear on leaves, particularly on lemon trees. Leaf lesions start as small, reddish-brown spots with a yellow halo and develop gray centers as they age.
The Fungal Cause and Transmission
The disease is caused by the fungus Phyllosticta citricarpa. This fungus survives between seasons primarily within fallen, decomposed citrus leaves on the orchard floor. This leaf litter serves as the main reservoir for the pathogen. When conditions become warm and wet, the fungus produces spores that spread the infection.
The primary method of transmission is through spores dispersed by rain splash and wind. Rain dislodges spores from the leaf litter, splashing them onto developing fruit and new leaves. Wind can carry another type of spore over longer distances, spreading the fungus throughout an orchard and to new locations.
For an infection to occur, a prolonged period of leaf and fruit wetness is required, as the spores need several hours of continuous moisture to germinate and penetrate plant tissues. Fruit is most vulnerable to infection from petal fall until it is about six months old. Symptoms often remain latent, only becoming visible as the fruit matures.
Control and Prevention Methods
Managing citrus black spot involves a combination of cultural practices and chemical applications aimed at reducing fungal spores and protecting the fruit. One of the most effective cultural controls is sanitation. Removing and destroying fallen leaves and fruit from the orchard floor helps to eliminate the primary source of overwintering fungus, reducing the number of spores available for new infections.
Proper pruning is another useful technique. By trimming dead wood from the tree canopy, you remove potential fungal habitats and improve air circulation. Better airflow helps the leaves and fruit dry more quickly after rain or irrigation, shortening the wet periods that spores need to germinate. Using irrigation methods that avoid wetting the tree canopy, such as micro-sprinklers, can also help limit the spread of the disease.
Fungicide applications are often necessary for effective control, especially in commercial groves. These treatments act as a protectant and must be applied before or during the period when fruit is most susceptible. Because regulations, recommended products, and effective timing can vary by region, it is best to consult with a local agricultural extension service for specific advice.