Pear scab, caused by two fungal species, Venturia nashicola and V. pirina, is the most serious disease affecting pears worldwide. V. nashicola is only pathogenic on Japanese and Chinese pears and V. pirina is only pathogenic on European pear (Tanaka and Yamamoto, 1964). Sivanesan (1977) included V. nashicola as a synonym on V. pirina. In recent years, however, Ishii and Yanase (2000) concluded that V. nashicola was distinct from V. pirina on the basis of the pathogenicity tests and morphological characterization of perfect and imperfect states that ascospores of V. pirina were longer and wider than those of V. nashicola and the conidia of V. nashicola were significantly shorter than those of V. pirina (Fig. 1).

Isolation of V. nashicola on selective media is subject to limitations since it grows slowly and could be masked by overgrowth of other faster growing fungi (Koh et al., 2013). Especially, the artificial cultivation of V. nashicola is difficult using common fungal growth medium in the laboratory, since it formed a colony with about 11.02 mm diameter until 60 days after cultivation on potato dextrose agar (PDA) medium but rarely produced conidia (Fig. 1). Although Ryu et al. (2014) found a new culture medium with pear extracts or Ca(NO₃)₂ for better growth of V. nashicola, 60-day-old colony of V. nashicola cultivated on the new medium was 24.35 mm in diameter and conidia were not produced on the culture medium. Therefore, we screened various media with different commercial crop powders to develop a suitable medium for conidia production of V. nashicola, since Maruyama et al. (2014) reported that the medium containing lotus and burdock roots was the most efficient for conidia mass production of Sphaceloma species through screening of various media with different commercial crop powders.

Five isolates of V. nashicola were cultivated on PDA for 60 days at 20°C in the dark for further experiments (Table 1). Various crop or plant extract powder media (1% crop or plant extract powder and 1.5% agar) containing 20 kinds of commercial crop or plant extract powders were prepared (Table 2) and PDA was used as a control. A mycelial disk of each isolate of V. nashicola cut from 60-day-old colony on PDA using cork-borer with 8 mm diameter was placed on the center of the surface of the prepared media and incubated for 30 days at 20°C in the dark. Each treatment contained 5 replications. The number of conidia harvested from the surface of each medium was measured by hemocytometer. Mycelial growths of V. nashicola measured as diameters of colonies with 8 mm agar plug on the media containing 20 kinds of commercial crop or plant extract powders were somewhat slower than those on PDA (Fig. 2). Of the tested 20 kinds of the media, conidia were produced only on the medium containing Cheongah powder (Fig. 3). V. nashicola grew too slow to produce conidia until 15 days after cultivation on the Cheongah medium but produced gradually conidia thereafter. The number of conidia reached about 4 × 10⁴ conidia/plate 30 days after cultivation on the Cheongah medium containing 1% Cheongah powder.

Since commercial Cheongah powder (300 g/package; Jangmyeong Food Co., Seoul, Korea) consists of major constituents such as 50% carrot powder, 22% apple powder, and 14% mandarin powder and minor constituents such as onion powder, ginger powder, dextrin and glucose, we investigated conidia production of V. nashicola on the medium containing separate constituents of Cheongah powder. Each media containing 1% separate constituents of Cheongah powder (carrot powder, apple powder, mandarin powder, onion powder, ginger powder, and sugar) and 1.5% agar were prepared and a mycelial disk of each isolate of V. nashicola cut from 60-day-old colony on PDA using cork-borer with 8 mm diameter was placed on the center of the surface of the prepared media and incubated at 20°C for 30 days in the dark. V. nashicola grew relatively fast on the medium containing carrot powder (300 g/package; Jangmyeong Food Co.) and followed by apple powder, onion powder, sugar, but it scarcely grew on the media containing mandarin powder and ginger, respectively (Fig. 4). V. nashicola produced about 1 × 10⁴ conidia/plate on the medium containing carrot powder, which were nearly equivalent to the conidia production on the Cheongah medium. On the other hand, V. nashicola produced less than 10⁴ conidia/plate on the media containing apple powder, onion powder, and sugar. However, no conidia were produced on the media containing mandarin powder and ginger powder, respectively. This indicates that the carrot medium is more efficient than the Cheongah medium for conidia production of V. nashicola, since carrot powder is cheaper than Cheongah powder.

We also screened various media with different concentrations of carrot powder to find the optimal concentration of carrot powder in the carrot medium inducing conidia production of V. nashicola in the laboratory. When we cultivated isolates of V. nashicola on the carrot media containing 0.5%, 1%, 2%, and 5% carrot powder, V. nashicola showed a peak production of conidia with 4.5 × 10⁵ conidia/plate 60 days after cultivation on the carrot medium containing 2% carrot powder (Table 3). From the above results, the carrot medium (2% carrot powder and 1.5% agar) is considered to be the best medium to obtain conidia of V. nashicola in the laboratory until now.

V. nashicola was first identified as the causal fungus of pear scab in Korea in 1985 (Cho et al., 1985). Although they described the length of conidia and conidiophores by detaching fungal mass from the diseased leaves of pear using a light microscope, they could not isolate V. nashicola on artificial medium. In recent years, Ryu et al. (2014) observed increased mycelial growth of V. nashicola on PDA media amended with pear extracts or Ca(NO₃)₂, but there has been no report on the techniques of conidia production on artificial medium in the laboratory until now. Conidia of V. nashicola produced on the carrot medium containing 2% carrot powder was pathogenic on pear leaves and mycological characteristics of V. nashicola produced on the carrot medium containing 2% carrot powder were in accordance with those found on pear trees or fallen leaves (Fig. 1).

Although pear scab has been the most serious disease on pear orchards during the past three decades, there has been no progress on the mycological researches on V. nashicola, since it grows slowly and rarely produces conidia on artificial media. Since use of systemic fungicides is the most common method to control pear scab at farmers’ pear orchards but frequent use of some systemic fungicides has been induced emergence of resistant strains of V. nashicola, development of new type of fungicides is necessary to manage the prevalent resistant populations of V. nashicola against routinely sprayed fungicides (Kwak et al., 2015). Use of resistant cultivars is the most promising method to control pear scab, but resistant cultivars have never been bred until now in Korea. Moreover, it is very urgent to develop resistant cultivars to pear scab, since the most common cultivar ‘Niitaka’ accounting for 82% of areas in pear cultivation in Korea is the most scab susceptible cultivar (Won et al., 2013). The development of the carrot medium suitable for efficient conidia production of V. nashicola will contribute to control pear scab through its utilization for screening of new fungicides and breeding of resistant cultivars in the future.