Citrus black spot (CBS), caused by Guignardia citricarpa, is back in the news and causing some South Florida citrus growers profit loss due to fruit drop and increased handling costs. In some trials, researchers have witnessed up to 80% fruit loss due to CBS. With CBS expected to spread, the industry is focused on determining best management practices for the disease.
Groves where CBS is present are subject to stringent guidelines that increase production and harvesting costs, including requirements such as covering infected fruit with a tarp during transport outside of the infected grove. In addition, CBS renders fruit unfit for fresh market sale because of the symptomatic black lesions caused by the disease. When fruit exhibits these lesions, growers must forsake the fresh market and sell their fruit for juice production. These factors, along with reduced access to key export markets and the need for increased fungicide use, reduce the profitability of groves.
Fungal Fighting Time Factors
April through September, when precipitation rates are at their highest, is when citrus groves are most susceptible to infection. CBS is spread by airborne ascospores present in both fallen leaf litter and in existing fruit lesions on the trees, so preventative measures should be taken to help reduce the spread of the disease.
Now is the time of year growers should begin implementing tactics for the management of CBS. “The main management tactic is regular application of fungicides beginning in April and continuing on 30-day intervals through September,” says John Taylor, an agronomic service representative for Syngenta. “Growers also are encouraged to manage leaf litter in groves and limit movement of infected leaf litter from one grove area to another. In addition, it is recommended to remove weak or declining trees, enhance air movement and leaf drying, minimize dead wood in tree canopies, source clean nursery stock, and sanitize equipment when moving fruit from one grove to another,” he says.
For fungicide protection against CBS, growers can turn to fungicides. The two active ingredients in Quadris Top, azoxystrobin and difenoconazole (Syngenta), are found to provide enhanced disease protection, while the two modes of action deliver resistance management benefits. In a 2012 trial conducted by Dr. Pamela Roberts, professor at the University of Florida, and Dr. Henry Yonce, researcher at KAC Agricultural Research Inc., Valencia oranges treated with a rotation of Quadris Top and Kocide 3000 (copper hydroxide, DuPont Crop Protection) had a low percentage of CBS infection and thus, low fruit drop rates. “Not only did Quadris Top provide excellent control of fruit symptoms, but that control also translated to reduced disease-related fruit drop,” explains Taylor.
To optimize crop protection product performance and reduce the chances of resistance development, Taylor suggests growers apply full rates of fungicides, incorporate all proven effective modes of action, and use no more than two sequential applications of any mode of action. Also, growers should incorporate other methods of disease suppression such as leaf litter management and increased airflow in order to optimize disease control.
A well-executed fungicide program, in conjunction with good cultural practices can help Florida citrus growers reduce the spread of citrus black spot and decrease fruit drop. Incorporating such tactics into existing disease management programs will set growers up for a profitable season.