© 2006 Plant Management Network.
Postharvest Fruit Rots in d’Anjou Pears Caused by Botrytis cinerea, Potebniamyces pyri, and Sphaeropsis pyriputrescens
C. L. Xiao, Department of Plant Pathology, Tree Fruit Research and Extension Center, Washington State University, 1100 North Western Avenue, Wenatchee 98801
Xiao, C. L. 2006. Postharvest fruit rots in d’Anjou pears caused by Botrytis cinerea, Potebniamyces pyri, and Sphaeropsis pyriputrescens. Online. Plant Health Progress doi:10.1094/PHP-2006-0905-01-DG.
Winter pears in the United States are produced primarily in the Pacific Northwest, and ‘d’Anjou’ pear is the major winter pear variety grown in the region. Postharvest fruit rot diseases are a limiting factor for the long-term storage of d’Anjou pears in the Pacific Northwest. Phacidiopycnis rot caused by the fungus Potebniamyces pyri and Sphaeropsis rot caused by the fungus Sphaeropsis pyriputrescens are two newly reported postharvest fruit rot diseases of pears in the US (12,13). The symptoms of these two diseases are similar to gray mold caused by Botrytis cinerea. The two newly recognized diseases are often misdiagnosed as gray mold. The objective of this article is to provide a practical guide to diagnosis of these three diseases.
Pears (Pyrus communis L.).
Gray mold; Phacidiopycnis rot; and Sphaeropsis rot.
Botrytis cinerea Pers., teleomorph Botryotinia
Potebniamyces pyri (Berkeley & Broome) Dennis, anamorph
Sphaeropsis pyriputrescens Xiao & J.D. Rogers (13).
Botrytis cinerea is the anamorph of the discomycete Botryotinia fuckeliana (de Bary) Whetzel (3), which is placed in Sclerotiniaceae of Helotiales (7). Most strains are heterothallic and carry one or another allele of the mating type gene (1). The teleomorph state is seldom found in nature (8), but apothecia can be readily obtained in the laboratory (4).
Potebniamyces pyri (anamorph Phacidiopycnis piri) is a discomycete placed in Rhytismataceae of Rhytismatales (7). It was originally described under the name Phacidiella discolor by Potebnia (9). Detailed descriptions of the fungus were given by Potebnia (9) and DiCosmo et al. (2). Apothecia may occur intermixed with pycnidia of the fungus on pear trees in the orchard, but the pycnidial state is much more prevalent in pear orchards (16).
Sphaeropsis pyriputrescens was described in 2004 by Xiao and Rogers (13). It is a pycnidial anamorphic fungus. The teleomorph of the fungus has not been reported.
Symptoms and Signs
Gray mold. Gray mold originates primarily from infection of wounds such as punctures and bruises that are created at harvest and during postharvest handling (Figs. 1 and 2). Stem-end gray mold is also common on d’Anjou pears (Figs. 3 and 4). Botrytis cinerea also invades floral parts of fruit and causes calyx-end rot (Fig. 5). Calyx-end gray mold is seen on pears grown in the US Pacific Northwest but is not very common. The decayed area appears light brown to dark brown, and the color is uniform across the decayed area (Figs. 1 through 6). The decayed area is spongy and diseased tissue is not readily separable from the healthy tissue. Under high relative humidity, grayish spore masses and/or fluffy white to gray mycelium may appear on the decayed area (Figs. 2 and 4). On advanced decayed fruit, sclerotia may form on the lesion surface (Fig. 6). The other form of infection by B. cinerea is fruit-to-fruit spread of gray mold, resulting a nesting of decayed fruit in storage containers (Fig. 7). The internal decayed flesh appears light brown to brown at the margin area (Fig. 8). Generally, there is no detectable odor from gray mold decayed fruit.
Phacidiopycnis rot. Phacidiopycnis rot causes three types of symptoms on pears: stem-end rot (Figs. 9 through 11), calyx-end rot (Figs. 12 and 13), and wound-associated rot (Fig. 14) originating from infection at the stem, calyx, and wounds on the skin of fruit, respectively. The decayed area is spongy. In the early stage of symptom development, the decayed area appears water-soaked (Figs. 9 and 12). The color of the decayed area varies with age. As the disease progresses, the aging decayed area turns brown and later black, but the margin of the decayed area continues to have a water-soaked appearance (Figs. 10, 11, 13, and 14). Under high relative humidity, the fungus forms white mycelium, and pycnidia (fruiting bodies) of the fungus are formed on the decayed area at advanced stages, starting from the infection sites (Fig. 11). Pycnidia formed on decayed fruit under commercial fruit-storage conditions are usually immature, and only microconidia are present in immature pycnidia. Phacidiopycnis rot can also spread to the surrounding healthy fruit through fruit contact, but fruit-to-fruit spread of Phacidiopycnis rot is less extensive compared with gray mold.
In the early stage of symptom development, Phacidiopycnis rot can be misdiagnosed as gray mold because of the similarity in symptoms, but the fruit flesh at the margin of Phacidiopycnis rot appears translucent and water-soaked (Fig. 15), whereas internal decayed flesh of gray mold usually appears brown (Fig. 8). Phacidiopycnis rot decayed fruit produces a distinct odor, whereas gray mold decayed d’Anjou pear fruit generally does not produce a detectable odor.
Sphaeropsis rot. Stem-end rot (Fig. 16) and calyx-end rot (Fig. 17 and 18) are the two major types of symptoms of Sphaeropsis rot. Sphaeropsis rot can also come from infection of wounds on the surface of fruit, but this type of symptom is less common. The decayed tissue is firm or spongy and the decayed areas appear brown. As the disease advances, S. pyriputrescens may form pycnidia in the decayed areas, usually starting from infection sites (Fig. 18). The pycnidia are black, superficial or partially embedded in the decayed tissue. The skin of decayed areas generally remains brown or dark brown but sometimes appears darker in aged decayed areas. Under high humidity, the entire fruit is frequently covered by mycelia and pycnidia of the fungus. The internal decayed flesh appears brown (Fig. 19). Decayed fruit caused by Sphaeropsis rot have a distinct "bandage-like" odor, particularly in the decayed flesh when the fruit is cut.
B. cinerea causes diseases on many plant species, including Pyrus communis L. (European pear) and Pyrus pyrifolia Nak. (Asian pear) (3,5). P. pyri has been reported only on Malus × domestica Borkh., Pyrus communis, Pyrus pyrifolia and Cydonia vulgaris (2,10). S. pyriputrescens has been reported on Malus × domestica, Malus spp. (crabapple trees) and Pyrus communis (13,14,17). It is not known whether S. pyriputrescens also infects Asian pear species.
B. cinerea is widespread in the world (3). P. pyri has been reported in Europe (11), India (10), and Oregon and Washington State of the United States (16). S. pyriputrescens has been reported in Washington State (13,14).
The three fungi can be readily isolated from diseased fruit. Decayed fruit are sprayed with 70% ethanol and air-dried. The skin of decayed fruit is peeled off with a sterile scalpel at the margin of decayed and healthy tissues. Then small pieces of the fruit flesh are excised and plated on acidified potato dextrose agar (APDA; Difco Laboratories, Detroit, MI) (4.0 ml of a 25% solution of lactic acid per liter of medium). Plates are incubated at room temperature (20 to 22°C) for 3 to 14 days and examined for culture development. Fungi that grew on or in the agar are transferred onto fresh APDA plates.
To facilitate identification of decay-causing fungi, PDA or oatmeal agar [OMA, 60 g of iron- and zinc-fortified single-grain oatmeal (Gerber, Fremont, MI) with 15 g of agar in 1000 ml of deionized water and sterilized for 90 min] cultures are incubated at 20°C under 12-h alternating cycles of dark and fluorescent light to induce sporulation or formation of fruiting bodies.
Botrytis cinerea. Mycelium of B. cinerea on isolation plates initially appears from colorless to white, either fluffy (Fig. 20) or appressed (Fig. 21), depending on isolates encountered. On 10-day-old isolation plates, gray to brown sporulation may appear (Fig. 22 and 23). Abundant sporulation (gray to brown masses of conidia) can be seen in 10-day-old PDA cultures grown under the conditions described above (Figs. 24 and 25). Conidia are borne in grape-like clusters. Conidia are hyaline, ellipsoid to obovoid, 7.5 to 14 × 6 to 10 µm (Fig. 26). Formation of sclerotia on PDA varies with isolates (Figs. 22 through 25).
Potebniamyces pyri. On isolation plates, colonies of P. pyri initially appear light white to colorless with little or no aerial mycelia (Fig. 27) and later turn gray to dark starting from the central part of colonies (Fig. 28). Growth characteristics of P. pyri on various agar media were described by Xiao and Sitton (15). Initially only microconidia (spermatia) are present in pycnidia on OMA. Both macroconidia and microconidia are often present in the pycnidia of 4-week-old OMA cultures. Abundant pycnidia are formed in 8- to 12-week-old OMA cultures (Fig. 29), and abundant macroconidia are formed in the pycnidia. Conidia are hyaline, subglobose to obovoid, guttulate as age, 6 to 15 × 5 to 10 µm, and microconidia are oblong, 4 to 6 × 2 to 2.5 µm (Fig. 30).
Sphaeropsis pyriputrescens. On isolation plates, the colonies of S. pyriputrescens initially appear with dense, appressed colorless mycelia (Fig. 31), which later turn light yellow to yellow within 7 to 14 days at 20°C in the dark (Fig. 32). This yellow pigmentation of colonies on PDA is a useful characteristic in the identification of this fungus. Growth characteristics of S. pyriputrescens on various agar media were described by Kim et al. (6). The fungus begins forming pycnidia on OMA after 7 days under 12 h light/12 h dark at 20°C. Abundant pycnidia are formed in 3-week-old OMA cultures (Fig. 33). Conidia are brown, clavate to subglobose to irregular, flattened at the secession end, (11-) 14-17 (-23) × (7-) 8-10 (-13) µm (Fig. 34).
Mycelium plugs of PDA cultures of B. cinerea and S. pyriputrescens or OMA cultures of P. pyri can be stored either in sterile water at 4°C or in 15% glycerol at -80°C.
Examples of successful methods for pathogenicity tests incorporate the following techniques. Fruit that are harvested from orchards neither sprayed with fungicides within 2 to 4 weeks before harvest nor treated with postharvest fungicides are used for pathogenicity tests. Prior to inoculation, fruit are surface-disinfested for 5 min in 0.5% NaOCl, rinsed three times with sterile water, and then air-dried.
The conidial states of the three fungi are the main type of inoculum responsible for fruit infection. In pathogenicity tests, conidia of B. cinerea produced on 10- to 14-day-old PDA cultures, pycnidia of P. pyri produced on 8- to 12-week-old OMA cultures, and pycnidia of S. pyriputrescens produced on 3-week-old OMA cultures at 20°C under 12-h alternating cycles of dark and fluorescent light are used to make conidial suspensions for fruit inoculation. Concentrations of conidial suspensions are adjusted to 1 × 104, 1 × 104, and 1 × 105 conidia per ml for B. cinerea, S. pyriputrescens and P. pyri, respectively. Fruit are wounded with a 4-mm-diameter nail head to a depth of 4 mm and then inoculated by placing 20 µl of conidial suspensions at each wound with a pipette. Inoculated fruit are placed on fiberboard pear trays wrapped in perforated polyethylene liners and stored in cardboard boxes in air at 0°C.
To inoculate the stem and calyx of pear fruit with either P. pyri or S. pyriputrescens, pieces of sterile cheesecloth (2 × 2 cm) are dipped into the conidial suspensions of either pathogen. Prior to inoculation, the end of the stem of each fruit is slightly cut horizontally with a sterile scalpel to make a wound. The end of the stem and calyx are inoculated by covering them with a piece of wet cheesecloth containing the conidial suspensions of either pathogen. For a control treatment, fruit are treated with cheesecloth moistened with sterile water. To facilitate infection at the stem and calyx, inoculated fruit are kept in aluminum muffin trays in plastic containers with a shallow depth of water on the bottom at room temperature (20 to 22°C) overnight. Cheesecloth is then removed and fruit are stored as described above. Decay development on wound-inoculated fruit is evaluated 6 to 8 weeks after inoculation. Stem-end rot and calyx-end rot are evaluated 2 to 3 months after inoculation.
Plant Pathology New Series 0418, Project 0367, College of Agricultural, Human, and Natural Resource Sciences, Washington State University. I thank R. J. Boal, D. Corey, and Y. K. Kim for technical support.
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