Outbreak of postbloom fruit drop of citrus, caused by Colletotrichum acutatum, in Santa Catarina State, southern Brazil

Postbloom fruit drop (PFD), caused by Colletotrichum acutatum, is an important disease of citrus, especially in orchards planted in humid areas of the Americas (Timmer et al., 1994). In Brazil, PFD was first reported in 1979 and causes variable yield losses depending on the weather conditions (Feichtenberger, 1994). Until recently, the disease was not considered serious in Santa Catarina, a southern Brazilian state, in which about 100 km² is planted with citrus. In 2002, disease symptoms of PFD were reported in numerous orchards located in the west area of Santa Catarina. These were reddish brown spots on the petal tissues followed by abscission of the fruitlets at the base of the ovary, with the basal disk, calyx and peduncle remaining firmly attached to the tree. These persistent calyces (buttons) are diagnostic for the disease and may persist for many months after flowering (Timmer et al., 1994). In September 2002, during the main flowering period, the weather conditions were particularly suitable for the disease, as the mean temperature during the month was low (11.1°C) and associated with long periods of leaf wetness. The severity of the infection was estimated by the number of infected flowers and persistent calyces observed on 20 trees randomly distributed in a seven-year-old "Rubi" sweet orange orchard. On each tree, the number of buttons in a 1 m² quadrat was counted. An average of 58.6 persistent calyces per m² was calculated and 100% yield losses were observed. Five isolates of a slow-growing fungus were recovered from calyces plated on potato-dextrose-agar. After 7 days of incubation at 25°C in the dark, there was an abundant production of orange conidial masses. The conidia were mostly fusiform, without setae and were 11.5 ± 1.1 µm long by 4.0 ± 0.5 µm wide. The morphological characteristics of these isolates fit the description of C. acutatum (Agostini et al., 1992, Brown et al., 1996). Flowers of 3- to 4-year old potted sweet orange trees in a screenhouse were inoculated with a conidial suspension (10⁵ conidia per ml). A control treatment was sprayed with distilled water only. The characteristic reddish brown lesions on the petals were observed on the inoculated flowers and C. acutatum was reisolated from the infected tissues fulfilling Koch's postulates. This is the first report of a significant outbreak of PFD for the Santa Catarina State, Brazil.