Phacidiopycnis Rot

Phacidiopycnis rot has previously been reported only in Europe and India and has recently been reported in North America. In Washington State this disease is common on pears, particularly on d’Anjou pears, and is much less common on apples.

Symptoms:
Phacidiopycnis rot causes three types of symptoms on pears: stem-end rot, calyx-end rot, and wound-associated rot originating from infection at the stem, calyx and wound on the skin of fruit, respectively (Fig. 5). Decayed area is spongy. In the early stage of symptom development, the decayed area appears water-soaked. Color of the decayed area varies with age. As the disease progresses, the aging decayed area turns brown and then black, but the margin of decayed area continues to have a watersoaked appearance. Under high relative humidity, the fungus forms white mycelium, and pycnidia (fruiting bodies) of the fungus are often formed on the decayed area at advanced stages, starting from the infection sites. Pycnidia formed on decayed fruit under commercial fruit-storage conditions are usually immature, and only microconidia are present in immature pycnidia.

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 watersoaked, whereas internal decayed flesh of gray mold usually appears brown (see Table 5 for comparison). Phacidiopycnis rot can also be mistaken as Sphaeropsis rot. The differences between these two diseases are summarized in Table 6.

Causal Organism:
The causal agent of Phacidiopycnis rot is the discomycete Potebniamyces pyri (Berk. & Br.) Dennis, anamorph Phacidiopycnis piri (Fuckel) Weindlmayr. The fungus has two types of conidia, macroconidia and microconidia. Microconidia are considered spermatia in the life cycle of the fungus. Mycelium grows at temperatures from -3 to 25ºC but not at 30ºC. Conidia germinate in two different manners, by developing germ tubes and budding to produce secondary conidia. Low nutrient levels favor budding, whereas high nutrient levels favor germ-tube development. Conidia germinate at 0 to 30ºC but not at 35ºC, with optimum temperature between 20 and 25ºC. Four to 5 and 6 to 8 h of wetness durations at optimum temperature are required for budding and developing germ tubes, respectively. Secondary conidia germinate at 0 to 25ºC in the same manner as mother conidia. Secondary conidia do not germinate at 30ºC. Relative humidity affects the mode of conidial germination. At 20ºC, conidia germinate only by germ tubes at 98% relative humidity. Conidia germinate in both manners at 100% relative humidity, but germination is dominated by germ tubes. In free water the majority of conidia germinate by budding.

Occurrence:
The fungus Potebniamyces pyri is a weak canker-causing pathogen on pear trees. It is commonly associated with dead bark and cankers on pear trees.  P. pyri is present in major pear-producing areas in the U.S. Pacific Northwest and is widespread in pear orchards in the Wenatchee River Valley, the primary production area for d’Anjou pears in Washington State. P. pyri survives as mycelium and pycnidia in diseased twigs and dying or dead bark on the trees in the orchard. Apothecia also occur in pear orchards in north-central Washington but at a low frequency. Pycnidia are the main type of inoculum for fruit infection, and viable pycnidia are available throughout pear fruit-growing season in north-central Washington. Infection of fruit by P. pyri occurs in the orchard, and symptoms develop during storage.  P. pyri infects the stem and calyx of pear fruit as well as skin wounds such as limb rubs and stem punctures created at harvest. The stem and calyx of fruit near harvest are more susceptible to infection by the fungus. Wound infections develop decay symptoms within two months after harvest. Stem-end rot and calyx-end rot symptoms are usually first seen three months after harvest. In north-central Washington, d’Anjou pears are packed soon after harvest and stored in cardboard boxes. At packing, fruit that are infected at the stem and calyx may not have decay symptoms, but symptoms appear on packed fruit after a period of time during storage. Unacceptable levels of decay on packed fruit in cardboard boxes may result in repacking.

Under high relative humidity, the fungus produces fluff mycelium on decayed fruit, which is able to infect surrounding healthy fruit, causing secondary infection through fruit-to-fruit contact.

Control:
Ziram applied within two weeks before harvest provides some control of stem-end and calyx-end Phacidiopycnis rot. A postharvest drench treatment with Mertect (thiabendazole) applied prior to storage is effective to control Phacidiopycnis rot originating from infection of wounds on the fruit skin. Research has been undertaken to develop relevant measures for control of stem-end and calyx-end Phacidiopycnis rot.

Photo Plate: Phacidiopycnis Rot
Fig. 5. Symptoms and signs of Phacidiopycnis rot caused by Potebniamyces pyri on pears.

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A: Early stage of stem-end Phacidiopycnis rot on a d’Anjou fruit; watersoaked appearance

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B: The color of decayed area varies with age; watersoaked at the margin, black at aged area

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C: Advanced stage of stem-end rot showing the change in color across the decayed area

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D: Early stage of calyx-end Phacidiopycnis rot; watersoaked appearance

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E: As the decay advances, the aging decayed area turns brown to black

pha-anjou-calyx-advanced-20

F: Advanced stage of calyx-end rot; black at the calyx-end area; pycnidia may form

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G: Early stage of wound infection commonly associated with limb rubs or punchtures on the fruit created at harvest

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H: Phacidiopycnis rot from wound infection; the infection site turns brown to black as the decay advances

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I: Internal decayed flesh translucent, clear at the margin

Disease Comparison:
Table 5. Comparison between Phacidiopycnis rot and gray mold on pears

Phacidiopycnis rot
Texture: spongy; decayed tissue not separable from the healthy tissue
Color of decayed area: initially watersoaked, then light brown to brown, later black; color varies with age
Signs of pathogen: white mycelia under high humidity; black pycnidia may form on decayed fruit at advanced stages
Color of internal flesh: translucent, clear at the margin
Odor: mild, distinct

Gray mold
Texture: spongy or firm; decayed tissue not separable from the healthy tissue
Color of decayed area: light brown to dark brown; color similar across the decayed area
Signs of pathogen: fluffy white to gray mycelia; sporulation under high humidity; gray to brown spore masses; sclerotia may form
Color of internal flesh: light brown to brown at the margin
Odor: generally not detectable

Table 6. Comparison between Phacidiopycnis rot and Sphaeropsis rot on pears

Phacidiopycnis rot
Texture: spongy
Color of decayed area: initially watersoaked, then light brown to brown, later black; color varies with age
Signs of pathogen: white mycelia under high humidity; black, immature pycnidia form on decayed fruit at advanced stages
Color of internal flesh: translucent, clear at the margin  of decayed area; advanced internal decayed flesh may appear dark
Odor: mild, distinct odor

Sphaeropsis rot
Texture: firm
Color of decayed area: brown to dark brown; advanced decayed area may turn black
Signs of pathogen: white mycelia under high humidity; pycnidia may form on decayed fruit at advanced stages
Color of internal flesh: brown; decay advances along the vascular tissue of the fruit; decayed vascular tissue brown
Odor: strong, distinct “bandage-like” odor

References:
Liu, Q., and Xiao, C. L. 2005. Influence of nutrient and environmental factors on conidial germination of Potebniamyces pyri. Phytopathology 95:572-580.

Xiao, C. L., and Boal, R. J. 2004. Prevalence and incidence of Phacidiopycnis rot in d’Anjou pears in Washington State. Plant Disease 88:413-418.

Xiao, C. L., and Boal, R. J. 2005. Distribution of Potebniamyces pyri in the U.S. Pacific Northwest and its association with a canker and twig dieback disease of pear trees. Plant Disease 89:920-925.