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Phacidiopycnis Rot

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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.

Casual 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.

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. 1). 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 1 for comparison). Phacidiopycnis rot can also be mistaken as Sphaeropsis rot. The differences between these two diseases are summarized in Table 2.

Figure 1.  Symptoms and signs of Phacidiopycnis rot caused by Potebniamyces pyri on pears.
Table 1. Comparison between Phacidiopycnis rot and gray mold on pears
Phacidiopycnis rot Gray mold
Texture spongy; decayed tissue not separable from the healthy tissue spongy or firm; 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 light brown to dark brown; color similar across the decayed area
Signs of pathogen white mycelia under high humidity; black pycnidia may form on decayed fruit at advanced stages fluffy white to gray mycelia; sporulation under high humidity; gray to brown spore masses; sclerotia may form at advanced stages
Color of internal flesh translucent, clear at the margin light brown to brown at margin
Odor mild, distinct odor generally not detectable

 

Table 2. Comparison between Phacidiopycnis rot and Sphaeropsis rot on pears
Phacidiopycnis rot Sphaeropsis rot
Texture spongy firm
Color of decayed area initially watersoaked, then light brown to brown, later black; color varies with age brown to dark brown; advanced decayed area may turn black
Signs of pathogen white mycelia under high humidity; black, immature pycnidia may form on decayed fruit at advanced stages white mycelia under high humidity; pycnidia may 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 brown; decay advances along the vascular tissue of the fruit; decayed vascular tissue brown
Odor mild, distinct odor strong, distinct “bandage-like” odor

 

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.

Controls

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.

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.

Contact

Ashour_Amiri

Dr. Achour Amiri,
Postharvest Pathology
Tree Fruit Research and Extension Center
Wenatchee, WA
a.amiri@wsu.edu
509-293-8752

Washington State University