In Brazil’s São Paulo citrus belt, infections of fruit by black spot usually occur from petal fall, in September/October, until the end of the rainy season, usually in April. Black spot can reduce production by up to 85%.
Some studies have shown that the amount of fungicide applications to control black spot can be established according to the orange variety. Earlier varieties, like Hamlin, need fewer applications. Later ones, like Valencia and Natal, require fruit protection until the end of the rainy season.
Observations have also shown that older orchards (generally over 12 years) require fungicide protection throughout the rainy season.
In the current harvest, November and December 2022 and January 2023 were marked by heavy rainfall. In this critical period for the occurrence of black spot, all orchards should be protected with strobilurin and mineral oil.
As of February, conditions generally become less critical for the disease to occur in early varieties and younger orchards. The focus now shifts to the management of older orchards and late-maturing varieties, in which black spot tends to cause severe fruit drop if control is not carried out correctly. In these orchards, it is important to continue with control until April, when the rainy season usually ends.
Fundecitrus researcher Geraldo Silva Jr. said the recommendation for older orange orchards and Pera, Valencia, Natal and other late varieties is that the petal fall be controlled until April.
Newer Hamlin orange groves and other precocious orchards that do not have a history of black spot occurrence will not require protection after February. Since they are harvested from June to August, infections that may occur in fruits of these early varieties from March onward will not result in high rates of symptoms and fruit drop.
“The most important thing is to pay attention to disease monitoring and correct management of the different citrus varieties, with a focus on older orchards with a history of the disease,” Silva said.
The adjustment in the number of applications depending on the variety and age of the orchard also helps to reduce the amount of strobilurin used and to avoid the selection of resistant fungi.
Strobilurins are usually associated with mineral or vegetable oil at a dose of 0.25%. However, for early varieties and in young orchards, they proved to be efficient even without adding oil or with lower doses of up to 0.15%.
The interval of strobilurin applications must not exceed 35 to 42 days, mainly from November to February.
Another important factor is to observe the incidence of citrus canker in the orchards and manage black spot and canker together using fixed coppers every 21 days, until fruit reaches 5 centimeters.
Sprayers must be properly maintained and calculated. Orchards must be pruned so that the equipment can travel through the planting lanes without the spray nozzles touching the branches of the plants.
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