Citrus Black Spot (CBS) is a significant fungal disease that poses a major threat to commercial citrus production across the globe. This condition affects the external quality of the fruit rind, leading to substantial economic losses for growers by making the produce commercially undesirable for the fresh market.
Identifying Citrus Black Spot Symptoms
The most noticeable signs of this disease are the various types of lesions that develop on the fruit’s rind, often as the fruit begins to mature or change color. One of the most typical symptoms is the “hard spot,” which appears as circular depressions, generally 3 to 10 millimeters in diameter, with a tan-to-gray center and a distinct dark red or brown margin. Another form is the “freckle spot,” characterized by small, reddish, slightly depressed lesions that can signify a heavy infection.
A more aggressive manifestation is the “virulent spot,” where irregular, sunken lesions expand and often merge, potentially causing premature fruit drop. Conversely, “cracked spot” lesions are irregular, dark, and often raised with a cracked surface, sometimes forming on green fruit. Despite these unsightly blemishes, the disease is entirely cosmetic; it does not penetrate the fruit’s flesh, meaning the internal quality and edibility remain safe for human consumption.
Understanding the Fungal Infection Cycle
The disease is caused by the fungus Phyllosticta citricarpa, whose life cycle dictates the timing and severity of infection in the grove. This fungus produces two distinct types of spores that facilitate its spread and survival. Ascospores are the sexual spores, which are considered the primary source of infection and are produced in specialized structures on fallen, decomposing leaf litter in the orchard.
When wetted by rain or heavy dew, these ascospores are forcibly ejected into the air, allowing them to travel long distances on wind currents to initiate new infections on healthy trees. The other spore type, conidia, are asexual and are typically spread short distances by rain splash, responsible for secondary spread within the canopy. Fruit is susceptible to infection from the time of fruit set until approximately four to six months later, but the fungus remains dormant.
Symptoms only become visible much later, usually when the fruit is mature or begins to ripen, often months after the initial infection occurred. This long incubation period, combined with the continuous production of spores from leaf litter, makes managing the disease a year-round effort. The fungus thrives in warm, humid conditions, with optimal infection occurring when the temperature is between 77 and 80 degrees Fahrenheit.
Management and Global Trade Restrictions
Effective management requires a combination of cultural practices and timely chemical control to reduce the fungal inoculum in the grove. Cultural methods focus on accelerating the decomposition of leaf litter on the orchard floor, the primary source of infection, often using mulching or chemical treatments like urea. Removing dead wood from trees is also recommended, as it eliminates a site where the fungus can reproduce and spread.
Chemical control relies on the precise timing of fungicide applications to protect susceptible young fruit. Fungicides containing copper or strobilurins are commonly used and must be applied repeatedly, often monthly, beginning in the spring when the fruit is small. Maintaining thorough coverage on the developing fruit is important, as the infection window can last for several months.
CBS is a globally regulated quarantine pest that severely impacts international trade. Regions free of the disease, such as the European Union and certain US states, impose strict phytosanitary requirements on imported citrus fruit. These regulations include mandatory inspections, grove traceability, and the requirement for growers to follow specific, approved management programs. The economic consequences of a single confirmed case in a shipment can lead to the rejection of entire consignments, underscoring the disease’s significance as a trade barrier.