Argentina's citrus sector is worried about the black spot, a disease caused by the fungus Phyllosticta citricarpa that affects citrus crops, because of its complexity and how difficult it is to detect it. Senasa and the Argentine Northwest Citrus Association (ANOC) decided to suspend the export of Argentine lemons to the European Union for the remainder of the season after this disease was detected in various batches of fruit in European ports.
According to Soledad Carbajo –a specialist of the INTA Famaillá citrus farming research group in Tucuman– knowing the biology of the disease is key to establish the most appropriate measures to manage and control it.
In this sense, she stressed the importance of carrying out permanent monitoring to detect the presence of this fungus in time. "The earlier its presence is noticed, the easier it will be to manage it," she said. She also recommended eliminating the fallen leaves or accelerate their decomposition, carry out sanitary pruning, avoid the transfer of branches or any citric material from one place to another, and getting rid of the fruits that have been left unharvested. In addition, she advised maintaining an adequate nutrition and irrigation plan for the plantation.
Blanca Canteros –INTA researcher at Bella Vista, Corrientes– highlighted that the epidemiological aspects that favor the increase of the disease were the long period of leaf fall and release of ascospores, the permanent presence of the inoculum source and susceptible tissue, the long latency period of the disease (manifestation of symptoms), and the difficulty to diagnose it.
Both specialists agreed that "It's impossible to eradicate the black spot once it has established itself in a farm."
How it propagates
It is important to know that the pathogen develops two infectious structures: ascospores and conidia. Both sources of inoculum cause infection and are important in the epidemiology of the disease.
Ascospores develop on fallen leaves or leaf litter and, once the right temperature and humidity conditions are present, they mature and are expelled into air currents. Once in contact with plant tissue, they enter it and remain dormant for a long period. The damage they produce can be seen near or after the harvest.
In turn, conidia form at the center of the injuries on fruits and leaves. They can also develop in leaf litter and dry branches. This source of inoculum needs water to disperse and reach susceptible tissues. They are covered with a mucilaginous layer and are heavier, so they disperse to closer tissues within the plant.