Occurrence of Anthracnose on Highbush Blueberry Caused by Colletotrichum Species in Korea

Abstract

A total of 82 isolates of Colletotrichum species were obtained from anthracnose symptoms of highbush blueberry trees grown in the Gochang area of Korea during a disease survey in 2008. Out of the isolates, 75 were identified as Colletotrichum gloeosporioides and the others as C. acutatum based on their morphological and cultural characteristics. Twenty six of C. gloeosporioides isolates produced their teleomorph Glomerella cingulata in PDA culture. Three isolates of each C. gloeosporioides and C. acutatum caused anthracnose symptoms on the leaves by artificial inoculation, which were similar to what was observed in the orchards. Previously in Korea, only C. gloeosporioides has been reported as causing anthracnose in blueberries. This is the first report that C. acutatum causes anthracnose in the highbush blueberry in Korea.

Blueberries (Vaccinium spp.) are widely grown in the world. Among the varieties of blueberries, the highbush blueberry (Vaccinium corymbosum L.) is the most commonly cultivated and has been introduced into Korea from foreign countries since 2,000. Recently, the plant has become a popular fruit tree in the Korea. However, a disease survey in 2008 revealed anthracnose symptoms frequently observed on leaves, fruits and stems of highbush blueberry trees grown in Gochang area. The incidence of the disease symptoms on the leaves reached as high as 30% in the orchards investigated, and less than 5% on the fruits and stems. The symptoms appeared as brown to dark brown, circular or irregularly shaped spots on the leaves (Fig. 1A and B), and sunken, circular or irregular necrotic spots on the fruits (Fig. 1C). The affected stems turned brown to dark brown (Fig. 1D), became gray, and later died.

Fig. 1.

Anthracnose symptoms on highbush blueberry trees as observed in the orchards. A and B, brown to dark brown, circular or irregularly shaped lesions on the leaves; C, sunken, circular or irregular necrotic spots on the fruits; D, affected stems.

A total of 82 isolates of Colletotrichum species were obtained from samples with symptoms of anthracnose. Out of the isolates, 75 were identified as Colletotrichum gloeosporioides (Penz.) Penz. & Sacc., and the others as C. acutatum J.H. Simmonds based on their morphological and cultural characteristics. Twenty six of C. gloeosporioides isolates produced their teleomorph Glomerella cingulata (Stoneman) Spauld. & H. Schrenk in potato dextrose agar (PDA) culture. The morphological characteristics of C. gloeosporioides (teleomorph: G. cingulata) and C. acutatum examined by the authors were similar to those described by Sutton (1992). The isolation frequency shows that C. gloeosporioides is the predominant fungus responsible for anthracnose of highbush blueberries in Korea (Table 1). C. gloeosporioides was isolated from leaves, fruits and stems of highbush blueberries, but C. acutatum only from leaves and fruits.

Table 1.

Isolation of Colletotrichum spp. from diseased plant parts of highbush blueberry trees

Colonies of C. gloeosporioides on PDA were gray to dark gray and showed aerial mycelium in tufts (Fig. 2A). Conidia of the fungus were unicellular, hyaline, straight, cylindrical, round at ends or slightly narrow at one end (Fig. 2B), and measured 14.0~21.5 × 4.0~6.5 µm. Appressoria were brown to dark brown, ovate, obovate, clavate, sometimes lobed (Fig. 2C), and measured 7.0~17.5 × 5.0~12.5 µm. Perithecia of the fungal teleomorph produced in PDA culture were subglobose. Asci were clavate to cylindrical, 8-spored and measured 52.5~102.5 × 7.5~12.5 µm. Ascospores were unicellular, hyaline, slightly curved fusiform or ellipsoid, and measured 12.5~25.0 × 3.5~5.5 µm.

Fig. 2.

Morphological features of Colletotrichum gloeosporioides (A~C) and C. acutatum (D~E) isolated from highbush blueberry trees. A and D, 20-day-old colonies grown on PDA at 24℃; B and E, conidia; C and F, appressoria. Each scale bar represents 20 µm.

Colonies of C. acutatum on PDA were reddish gray to dark gray and showed orange conidial masses scattered on the surface of cultures (Fig. 2D). Conidia of the fungus were unicellular, hyaline, straight, fusiform, abruptly tapered at each end (Fig. 2E), and measured 10.0~20.0 × 3.5~5.0 µm. Appressoria were pale to dark brown, ovate to clavate, slightly irregular or lobed (Fig. 2F), and measured 5.0~15.0 × 4.5~7.5 µm.

Three isolates of each C. gloeosporioides and C. acutatum were tested for their pathogenicity to leaves of highbush blueberry by artificial inoculation with spore suspensions (2~3 × 10⁶ conidia/ml) prepared from 20-day-old PDA cultures. Inoculation was made by dropping 20 µl of each spore suspension on leaves of the highbush blueberry cultivars Duke, Darrow, and Coville unwounded or wounded by pinprick. The same quantity of sterile distilled water was used as the control. The inoculated leaves were placed in humid plastic boxes (30 × 24 × 6 cm) at 25℃. Pathogenicity of the isolates was rated based on the lesion formation seven days after inoculation. The inoculation test was performed with three replicates.

All the tested isolates of C. gloeosporioides and C. acutatum induced anthracnose symptoms on the leaves of highbush blueberry cultivars tested by artificial inoculation (Table 2), which were similar to what was observed in the orchards. The isolates readily induced the symptoms on the leaves of all the cultivars by wound inoculation, but weakly or not at all by unwounded inoculation, suggesting that wounding may promote anthracnose lesion development on the leaves of highbush blueberry trees in the orchards. The inoculation tests showed that the cultivar Duke is more susceptible to the Colletotrichum species than the cultivars Darrow and Coville. There was no difference in pathogenicity between C. gloeosporioides and C. acutatum isolates tested. The two Colletotrichum species which induced symptoms on the leaves were reisolated from the symptoms.

Table 2.

Pathogenicity of Colletotrichum gloeosporioides and C. acutatum isolates on leaves of highbush blueberry cultivars by artificial inoculation

^aPathogenicity was rated based on the lesion formation seven days after inoculation. ++, above 8 mm of lesion diameter; +, 2~7 mm of lesion diameter; -, no symptom.

C. gloeosporioides (teleomorph: G. cingulata) has been reported as causing fruit rot or anthracnose fruit rot in highbush blueberries (Farr et al., 1989; Milholland, 1995). It was also reported that C. acutatum was identified as a causal agent of ripe rot of blueberry fruits (Milholland, 1995), with Barrau et al. (2001) reporting that C. acutatum caused anthracnose on leaves of the highbush blueberry. In the present study, it was revealed that both C. gloeosporioides and C. acutatum cause anthracnose of the highbush blueberry in Korea. Previously in Korea, only C. gloeosporioides has been reported as the causal fungus of anthracnose of blueberry (Kwon et al., 2008). This is the first report that C. acutatum causes anthracnose in the highbush blueberry in Korea.