First report of Penicillium ulaiense as a postharvest pathogen of orange fruit in Egypt

Citrus is a major export commodity of Egypt, with production estimated to be 2.5 million metric tonnes per year. The most common and serious postharvest diseases of citrus fruits are green and blue moulds, caused by Penicillium digitatum and P. italicum, respectively (Plaza et al., 2003). During April 2009, oranges (Citrus sinensis) from three Egyptian varieties (Baladi, Sukhary, and Abu-surra) were collected from commercial markets and packinghouses in the Giza Governorate. After three weeks storage at room temperature and high relative humidity, a morphologically distinct Penicillium spp. was observed as a mixed infection with P. digitatum and P. italicum (Fig. 1). The pathogen was isolated on potato dextrose agar (PDA), and identified as Penicillium ulaiense, according to its morphological and cultural characteristics (Fig. 2). P. ulaiense was distinguished from P. digitatum by its blue-grey spore mass and from P. italicum by its ability to form coremia (1-7 mm tall) with white stalks (Holmes et al., 1993, 1994).

An aqueous conidial suspension (10⁵ spores/ml) was prepared from 14-day-old cultures of a monoconidial isolate. Thirty 'Valencia late' orange fruits were washed with 0.5% sodium hypochlorite solution, rinsed, dried and wounded (2 mm wide and 1 mm deep) at the equatorial zone. Wounds were inoculated with 10 µl of conidial suspension or water and kept at 25±2°C and 90-95% relative humidity. After 15 days incubation, symptoms caused by P. ulaiense were observed on inoculated fruit (Fig. 3). The control fruit remained healthy. The causal agent was reisolated, and Koch's postulates confirmed. Isolates were also identified by the polymerase chain reaction (PCR) technique, using a specific primer pair designed to amplify the intergenic spacer region (IGS) of rDNA. According to the available literature, this is thought to be the first report of P. ulaiense causing a citrus postharvest rot from natural infection in Egypt. Since this species can easily develop resistance to the fungicide imazalil (Holmes et al., 1994), its presence in Egyptian packinghouses may represent a threat for long-term storage and delivery abroad.