Identification and characterization of Colletotrichum fructicola causing black spots on young fruits related to bitter rot of pear (Pyrus bretschneideri Rehd.) in China

Highlights

• We identified Colletotrichum fructicola responsible for pear black spots disease.

• We showed that black spots is a quiescent stage of bitter rot on pear fruits.

• Morphological and molecular features of the fungus strain were characterized.

• Optimum temperature and pH conditions for growth of the strain were determined.

Abstract

In recent years, Pyrus bretschneideri Rehd. var. ‘Suli’ has been damaged by a disease characterized by the presence of black spots on young fruit in China, which was always followed by severe bitter rot on matured fruits. The etiology of these symptoms and their relationship with the bitter rot was unknown. A colony was routinely isolated from young and matured ‘Suli’ pear fruits showing black spots and rot symptoms, respectively. This fungal colony was identified as Colletotrichum fructicola based on morphological characteristics and DNA sequence data of 7 regions. C. fructicola was confirmed to be capable of eliciting both black spots and bitter rot symptoms by completion of Koch's Postulates. The results suggested that the disease characterized by the presence of black spots on young fruits is indeed an early stage of the bitter rot observed on matured fruits of ‘Suli’ pear. Variety resistance analysis indicated an obvious resistance variation possessed by the pear varieties generally cultured in China, with black spots only occurring on some pear varieties. Our results also indicated that wounding can break the quiescent infections, enhance the infectivity of C. fructicola, and lead to more rapid rot of young and matured fruits. These results provided a clear clue that C. fructicola can invade some varieties of P. bretschneideri Rehd. directly without entry via wounds, but subsequently turned into quiescent infection and causing black spots on the fruit surface. The quiescent infection acts as a pathogenic factor responsible for bitter rot of matured fruits. The influence of temperature and pH conditions for growth and colony morphology of C. fructicola were also evaluated. The characterization of C. fructicola causing bitter rot of pear (P. bretschneideri) is expected to provide useful information for controlling this economically important disease.

Introduction

‘Suli’ pear (Pyrus bretschneideri var. ‘Suli’, also known as ‘Dangshansuli’), cultivated for more than 500 years, is the most important commercial Asiatic pear variety in the world with a production at over 4 million tons per year (Wu et al., 2013). Considering its significance, it was chosen for whole genome sequencing of pear (Wu et al., 2013). In recent years, ‘Suli’ pears cultivated in Dangshan County, in the Anhui Province of China, have exhibited a severe disease symptom charactrized by black spots on the fruit. These symptoms can be observed during the fruit growing period until the fruit reaches maturity. This disease is commonly known as “black spot disease” in China. It is characterized by the emergence of small black spots of about 1.0 mm in diameter on the surface of young fruits. At the maturation stage, numerous fruits rot, while no obvious sign indicates that black spots develop into fruit rot when the fruit reaches maturity. Severe occurrences of this disease were observed in the areas of Dangcheng, Gedian, Xuanmiao, and Liangli in Dangshan County. In the orchards surveyed in this study, fruit losses reached 60–90% in most orchards with normal fungicide control and 10–20% in orchards with heavy application of fungicides. This disease spread rapidly across ∼40,000 ha of pear orchards in those regions, causing economic losses of about 150 million dollars in 2008 (Li et al., 2013). The rot symptom, known as bitter rot of pear, was previously identified to be induced by Colletotrichum fructicola, characterizd by morphological and molecular studies (Li et al., 2013). However, the etiology of the black spot symptom and its relationship with the bitter rot in matured phases of the fruit has remained unknown.

Bitter rot is a common fruit disease of apple identified in the United States in 1867 (Sutton, 1990). The disease is considered as one of the most important diseases, and it can cause crop losses as great as 50% in temperate growing regions with high rainfall (Shi et al., 1996). Three taxa, including Colletotrichum gloeosporioides (Penz.) Penz. Sacc., Colletotrichum acutatum J. H. Simmonds, and Glomerella cingulata (Stoneman) Spauld. and H. Schrenk, have been associated with the bitter rot disease of apple (Chung et al., 2006, Freeman et al., 2001, Giaretta et al., 2010, González et al., 2006, Shi et al., 1996) and pear (Pyrus pyrifolia) (Cannon et al., 2012, Chung et al., 2006, Sreenivasaprasad and Talhinhas, 2005, Wan et al., 2007, Weir et al., 2012). Colletotrichum spp. cause diseases on a wide variety of woody and herbaceous plants. Additionally, they seriously affect cereals such as maize, sugar cane and sorghum, and fruit trees, such as strawberry, mango, citrus, avocado and banana resulting a great loss in fruit production (Cannon et al., 2012). The genus Colletotrichum was ranked the eighth most important group of plant pathogenic fungi in the world, based on perceived scientific and economic importance (Dean et al., 2012). C. fructicola was recently reported for the first time as the causal agent of bitter rot on P. pyrifolia (Weir et al., 2012) and P. bretschneideri (Li et al., 2013); however, the strains isolated from pear are currently far from being well characterized.

Pear is the third most important temperate fruit crop after grape and apple, and is widely cultivated in six continents with major production in China, USA, Italy, Argentina, and Spain. Prevention and control of the disease would significantly decrease economic loss to growers. The objectives of the current study were to identify and characterize the pathogen responsible for black spot disease and determine its relationship with pear bitter rot in order to provide further information on effective means of controlling the disease.