Sphaeropsis rot is a newly reported postharvest fruit rot disease of apple and pear. This disease first was found on d’Anjou pears, and later it was determined that Sphaeropsis rot can cause even more serious problems on apples. In one instance, 24% of the apple fruit in storage bins was rotted by this disease after several months of storage. Sphaeropsis rot has been observed to occur on d’Anjou pear and on apple varieties including Red Delicious, Golden Delicious, Fuji, and Granny Smith.
Symptoms:The primary symptoms of Sphaeropsis rot are stem-end rot and calyx-end rot originating from infections at the stem and calyx of fruit, respectively (Fig. 4). The decayed tissue is firm or spongy, and the decayed areas appear brown. On Red Delicious apples, decay resulting from infection at the calyx of fruit may appear irregular in shape. As the disease advances, the fungus may form pycnidia in the decayed areas, usually starting from infection sites. The pycnidia are black, superficial or partially embedded in the decayed tissue. The skin of decayed areas generally remains brown or dark brown but may appear dark in aged decayed areas. The internal decayed flesh appears brown. Decay in the fruit flesh develops along the vascular tissue of the decayed fruit, originating from infection of the stem or calyx of fruit. Sphaeropsis rot can be misdiagnosed as gray mold because symptoms of Sphaeropsis rot can be very similar to those caused by gray mold. However, Sphaeropsis rot has a distinct “bandage-like” odor, particularly in the decayed flesh when the fruit is cut (see the comparison between the two diseases in Table 4). This can be used as a preliminary diagnostic indication of Sphaeropsis rot. A further examination of fruiting bodies, if present, or isolation of the causal agent from decayed tissue may be necessary for confirmation of diagnosis.
Causal Organism:Sphaeropsis pyriputrescens Xiao & J. D. Rogers is the causal agent of Sphaeropsis rot. The fungus grows readily on agar media. Mycelium grows at -3 to 25ºC and does not grow at 30ºC but survives at this temperature. Optimum temperature for mycelial growth is between 15 and 20ºC. Conidia germinate at 0 to 30ºC, and a minimum wetness duration of 5-6 h is required for germination at optimum temperature. Conidia can germinate at relative humidity as low as 92%. On potato dextrose agar at 20ºC, S. pyriputrescens initially appears colorless but starts to turn light yellow to yellow after 5 to 7 days of incubation. This yellow pigmentation is a useful characteristic in the diagnosis of Sphaeropsis rot.
Occurrence:S. pyriputrescens causes twig dieback and cankers on apple and crabapple trees. ‘Manchurian’ crabapple trees are very susceptible to twig dieback and cankers caused by this fungus. S. pyriputrescens also appears to be associated with dead bark or twigs on trees. Fruiting bodies (pycnidia) of the fungus are often present in these diseased tissues on the trees. Pycnidia are apparently the main type of inoculum for fruit infection. Infection of fruit occurs in the orchard, but symptoms develop after a period of time during storage. The fungus primarily infects the stem and calyx of fruit and causes stem-end rot and calyx-end rot, respectively.
Control:The inoculum of S. pyriputrescens is apparently present in the orchard. Removal of cankers and twigs with dieback symptoms would help reduce inoculum of the fungus in the orchard. For orchards with a high percentage of crabapple trees as pollinizers, pruning diseased twigs is particularly important to disease control. Research on chemical control of this disease is currently in progress.
Photo Plate: Sphaeropsis RotFig. 4. Symptoms and signs of Sphaeropsis rot (Sphaeropsis pyriputrescens) on apples and pears.
A: Stem-end Sphaeropsis rot originating from infection at the stem of a Red Delicious fruit; decayed area brown, firm
B: Advanced stage of stem-end Sphaeropsis rot on a Golden Delicious fruit
C: Internal flesh light tan to brown; strong distinct "bandage-like" odor commonly associated with it
D: Calyx-end Sphaeropsis rot originating from calyx infection on a Red Delicious fruit
E: Calyx-end Sphaeropsis rot on a Fuji fruit; lesion with irregular margin
F: Stem-end Sphaeropsis rot on a d'Anjou pear fruit; decayed area brown, firm
G: Decay in the fruit flesh advances along the vascular tissue of the decayed fruit
H: Calyx-end Sphaeropsis rot; decayed area brown, firm; black pycnidia formed at aged area
I: Internal decayed flesh brown; strong distinct "bandage-like" odor commonly associated with it
Table 4. Comparison between Sphaeropsis rot and gray mold
|Characteristics||Sphaeropsis rot||Gray mold|
|Texture||spongy to very firm; decayed tissue not separable from the healthy tissue||spongy or firm; decayed tissue not separable from the healthy tissue|
|Color of decayed area||brown to dark brown; advanced decayed area may turn black||light brown to dark brown; color similar across the decayed area|
|Signs of pathogen||white mycelia under high humidity; pycnidia may form on decayed fruit at advanced stages||fluffy white to gray mycelia; sporulation under high humidity; gray to brown spore masses; black sclerotia may form at advanced stages|
|Color of internal flesh||light tan to brown; decay advances along the vascular tissue; decayed vascular tissue brown||light brown to brown|
|Odor||strong, distinct “bandage-like” odor||generally not detectable|
Kim, Y. K., Xiao, C. L., and Rogers, J. D. 2005. Influence of culture media and environmental factors on mycelial growth and pycnidial production of Sphaeropsis pyriputrescens. Mycologia 97:25-32.
Xiao, C. L., and Rogers, J. D. 2004. A postharvest fruit rot in d’Anjou pears caused by Sphaeropsis pyriputrescens sp. nov. Plant Disease 88:114-118.
Xiao, C. L., Rogers, J. D., and Boal, R. J. 2004. First report of a new postharvest fruit rot on apple caused by Sphaeropsis pyriputrescens. Plant Disease 88:223.