Abstract: Rotted apples collected from six cold stores and two fruit markets in Peshawar and Swat divisions showed attacks of Penicillium expansum (74.2%), Alternaria alternata (11.6%) and Rhizopus sp. (14.2%). However, P. expansum was found to be the most prevalent fungus both in apple stores and markets because of its survival potentiality both at low and high temperatures. These fungi are mostly wound pathogens and require injury for infection. The apple variety Red delicious was more susceptible to the disease than Golden delicious indicating differential reaction of these varieties to apple rot. However, the amount of rotting incited by Rhizopus sp. was greater than the other fungi due to its rapid growth and virulence at 20°C.
Introduction
Post-harvest rot of apples is a major disease in most of the apple growing areas of the world (Eckert and Sommer, 1967; Tepper and Yoder, 1982). Economic losses caused by post-harvest diseases are heavier than is generally realized because fresh apples increase severalfold in unit value while passing from the field at harvest time to the consumer. A major portion of these losses is attributed to diseases caused by micro-organisms. Fungi responsible for the disease are Penicillium expansum, Botrytis cinerea, Rhizopus sp. (Rosenberger et al., 1979); Phomopsis mall (Rosenberger and Burr, 1982); Acremonium irnplicatum, Nodulosponum hinulium, Ceratocystis major (Vyas et al., 1976); Trichoderma harzianum (Conway, 1983); Mucor periformis (Spotts and Cervantes, 1986) and Alternaria alternate (Tak et al., 1985).
Unsanitary conditions in apple storage and packing facilities provide an ideal environment for the survival of post-harvest pathogens. In Pakistan, a considerable percentage of apples are wasted every year due to inadequate picking, packing, transportation, marketing and storage facilities. Apple rot in cold stores and fruit markets has gained a tremendous momentum. However, no systematic work has been done so far on the etiology and control of this disease in Pakistan, and particularly in NWFP. Owing to the complex nature of the disease both in apple stores and markets in NWFP, the present study was carried out to identify the causal organisms of the post-harvest rot of apples so that control measures for the disease could be worked out.
Materials and Methods
Collection of diseased specimens: An extensive survey of the main cold stores and fruit markets in Peshawar and Swat divisions was made to collect rotten apples. About 318 samples were collected from 8 different cold stores and fruit markets.
Identification of the pathogens: The rotten apples were surface disinfected with 5 percent ethanol for one minute, washed with several changes of distilled water, and then blot dried. From the margin of the fruit, small pieces were taken aseptically from under the skin and placed on Potato Dextrose Agar (PDA) medium on sterilized petri-plates and incubated at 20°C for seven days. Mycelial colonies of the different fungi were subcultured to obtain pure culture of each fungus. The isolated fungi were identified with the help of keys devised by Barnett (1956), Ellis (1971), Mirza et al. (1979) and Pitt (1979).
Pathogenicity test: Healthy fruit of two apple varieties Red delicious and Golden delicious were surface disinfected with 5 percent ethanol for one minute, washed with distilled water and then injured with a sterilized pointed needle 2 mm deep. Each fruit received three injuries at different places. Some of the fruit were left uninjured (check). Both types of fruit, injured and uninjured, were dipped in spore suspension (1.5 x 105 spores/ml) of each of the test fungi. Spore counts in sterile water suspension were determined with the aid of a haemocytometer. Inoculated apples were wrapped in paper bags so that they did not touch one another, and were incubated at 20°C for seven days. Data was recorded on the diameter of the rotten area in different treatments at the end of the incubation period. Factorial design was followed, and the treatments were replicated three times. The fungi were re-isolated from the disease lesions and were identified to be the same as isolated from the original specimens of the rotten fruits.
Results and Discussion
Among the several species of fungi isolated from 318 diseased samples from eight cold stores and fruit markets, P. expansum, A. alternata and Rhizopus sp. were the most abundant with a frequency of 74.2, 11.6 and 14.2 percent respectively (Table 1). However, P. expansum was found to be the most prevalent fungus both in apple stores and markets. The rapid development and survival of P. expansum, both at high and low temperatures, caused this fungus to be present on apples at a high percentage rate in all the cold stores and fruit markets surveyed. The results coincide with those obtained by Conway (1983) and Roy (1975).
Pathogenicity test of P. expansum, A. alternata and Rhizopus sp. indicated the importance of injured fruit and apple varieties in the causation of rot (Table 2).
| Table 1: | Percentage of the fungi isolated from rotted apple fruit |
| *Total rotten specimens collected from the above eight cold stores and markets | |
| Table 2: | Pathogenicity test on two apple varieties of P. expansum, A. alternate and Rhizopus sp. incubated at 20°C for one week |
Mean* is the average of three replications, and figures followed by different letters differ from one another significantly at all levels. Mean1 is the average rotting on two varieties, and figures followed by different letters are significantly different from one another at all levels. Mean2 is the average rotting of the four treatments, and figures followed by different letters are significantly different from one another at all levels |
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Wounds on the fruit provided ideal ports of entry for the fungi and hence rotting occurred, while the uninjured fruit indicated no rotting. Blanpied and Purnasiri (1969) reported similar results. Interaction between the pathogens and the varieties was highly significant at all levels. Rotted lesions were larger on the variety Red delicious than on Golden delicious. The fruit of Golden delicious is firmer and crispier which might have caused less decay in this fruit due to its texture (Conway and Sams, 1984). However, acidity is the main factor that could reduce the incidence of rot. A higher level of acidity could contribute in the reduction of susceptibility, whereas an increase in sugar content resulted in increased susceptibility (Kaul, 1984). The amount of rotting caused by Rhizopus sp. was greater than the other fungi due to its rapid growth and virulence at 20°C (Kaul, 1986).