Pear Skin Stain Caused by Mycosphaerella graminicola on Niitaka Pear (Pyrus pyrifolia Nakai)

Abstract

Pear skin stains on 'Niitaka' pears, which occur from the growing stage to the cold storage stage, reportedly negatively influence the marketing of pears. These stains on fruit skin are likely due to a pathogenic fungus that resides on the skin and is characterized by dark stains; however, the mycelium of this fungus does not penetrate into the sarcocarp and is only present on the cuticle layer of fruit skin. A pathogenic fungus was isolated from the skin lesions of infected fruits, and its pathogenicity was subsequently tested. According to the pathogenicity test, Mycosphaerella sp. was strongly pathogenic, while Penicillium spp. and Alternaria spp. showed modest pathogenicity. In this present study, we isolated the pathogenic fungus responsible for the symptoms of pears (i.e., dark brown-colored specks) and identified it as Mycosphaerella graminicola based on its morphological characteristics and the nucleotide sequence of the beta-tubulin gene. M. graminicola was pathogenic to the skin of 'Niitaka' pears, which are one of the most widely growing varieties of pears in South Korea.

Keywords: Mycosphaerella graminicola; Niitaka; pear; pear skin stain.

Fig. 1

Typical symptoms of pear skin stains on the ‘Niitaka’ pear. (A) Symptoms were bright in grey color on the young pear skin in a fruitlet stage. (B) The circular grey-brown colored stains grew on the pear skin during the harvest period. (C) The grey-brown colored stains were changed to dark-brown colored stains during cold storage period. (D) The dark-brown stains were merged with each other during the extended cold storage period.

Fig. 2

Invasion process of Mycosphaerella sp. into the cuticle layer responsible for pear skin stains in ‘Niitaka’ pears.

Fig. 3

Pathogenic test of Mycosphaerella sp. on pear skin. (A) Spraying inoculation using a spore suspension on the skin of pear. (B) Inoculation using wet paper disc on the skin of pear. (C) Inoculation with a pipette drops in damaged skin of pear. (a, b and c) Extension of fungus (in dotted circle) from A, B, and C in each.

Fig. 4

Pathogenic test of Mycosphaerella sp. in a pear orchard. (A) Inoculation of a pathogen with lesion (5 × 5 mm²) from infected fruit skin in June. (B) Typical symptoms of pear skin stains after 20 days of inoculation.

Fig. 5

Effect of temperature on the mycelial growth of Mycosphaerella sp. isolated from the pear skin stains of the ‘Niitaka’ pear. The mycelial growth was measured after three weeks of incubation on potato dextrose agar.

Fig. 6

Fungal colony of Mycosphaerella sp., the causal agent of pear skin stains on ‘Niitaka’ pears. (A) Above view of the formed circular flora. (B) Cross-sectional view of the formed convex center (diameter 4±0.8 cm, height 0.4±0.2 cm).

Fig. 7

Mycelia of Mycosphaerella sp., the causal agent of pear skin stains on ‘Niitaka’ pears. (A) Pear skin on a normal pear. (B) Pear skin on an infected pear. (C) Extension of colony from the skin of an infected pear (A and B:×600, Bar=100 μm, C: ×2.5k, Bar=30 μm).

Fig. 8

Phylogenetic tree using internal transcribed spacer (ITS) sequences showing the relationships among Mycosphaerella graminicola isolated from the ‘Niitaka’ pear and the closely related Mycosphaerella species.

Fig. 9

Phylogenetic tree using beta-tubulin gene sequences showing relationships among Mycosphaerella graminicola isolated from the ‘Niitaka’ pear and the closely related Mycosphaerella species.