Soft Rot of Citrus Fruit Caused by Penicillium Digitatuaa and P. Italicuaa1

Peel tissue of citrus fruit rotted by Penicillium digitatum Sacc. and P. italicum Wehmer was similar in degree of softness and relative water content. Pectolytic enzymes produced by these fungi degraded pectin of the cell wall middle lamella resulting in loss of cell coherence. This pro cess is considered important in aiding hyphal penetration. Rate of development of the decay was not determined by the macerating ability of the pectolytic enzymes but ap parently due to the growth rate of each organism. Penicillium digitatum Sacc. (green mold) and P. italicum Wehmer (blue mold) are 2 important postharvest fungal pathogens of citrus fruit. In Florida, green mold is by far the more important decay of the 2 types. Infection of citrus fruit by these 2 organisms requires an injury which is usually incurred at some time during harvesting and/or during postharvest preparation (4). Rot caused by the 2 organisms is similar in appearance and texture and may spread to contiguous healthy fruit in packed cartons. iFlorida Agricultural Experiment Stations Journal Series No. 2708. Proc. Fla. State HarL Soc. 93: 1980. The importance of pectolytic enzymes produced by various pathogens in the development of soft rots of other fruits is well documented (3). The enzymes degrade pectic substances of the cell wall middle lamella causing loss of tissue cohesiveness and death of cells. The following discussion reports similarities in proper ties of the 2 decays and the identity of pectolytic enzymes produced by each pathogen. Materials and Methods Inoculations. Mature 'Valencia' oranges (Citrus sinensis (L.) Osbeck) were washed and inoculated through a punc ture, 5 mm deep, into the albedo with spores (approx. 106 spores/ml) of either P. digitatum or P. italicum in water containing 0.01% Triton X100. The fruit were incubated at near 100% relative humidity and 25°C. Pectolytic enzyme analysis. Methods used in the extrac tion and characterization of the pectolytic enzymes in the decayed tissues have been published (1, 2). Organic acid analysis. The organic acids in the decayed tissue were extracted according to the procedure of Fernandez-Flores et al. (8) as modified by Ting (unpublished, Florida Dept. of Citrus, Lake Alfred). The decayed peel was homogenized in water, centrifuged and the supernatant was adjusted to 80% ethanol and centrifuged again. Satu rated lead acetate solution was added to the supernatant, centrifuged and the pellet was serially washed with ethanol, acetone, diethyl ether and then vacuum dried. The organic acids in the pellet were silylated with Tri-Sil (Pierce Chemi cal Co., Rockford, IL 61105) and analyzed with a HewlettPackard gas chromatograph, Model 5736, equipped with a flame ionization detector and an SE 30 column. A tempera ture program of 70 to 210°C at a rate of 4°C per minute was used.