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• According to Kansas State University Research and Extension:

In contrast to peach leaf curl, symptoms of plum pocket are most conspicuous on the fruit. Small, whitish spots develop on young plums soon after blossom. The spots enlarge and eventually cover the entire fruit. Seed fails to form in infected plums; the fruit becomes hollow and enlarges to many times normal size. At this stage, the distorted plums have a red to grayish tint. Young shoots and leaves may be deformed and killed by the disease, but these symptoms are not as common as in peach leaf curl.

• According to Kansas State University Research and Extension:

  In contrast to peach leaf curl, symptoms of plum pocket are most conspicuous on the fruit. Small, whitish spots develop on young plums soon after blossom. The spots enlarge and eventually cover the entire fruit. Seed fails to form in infected plums; the fruit becomes hollow and enlarges to many times normal size. At this stage, the distorted plums have a red to grayish tint. Young shoots and leaves may be deformed and killed by the disease, but these symptoms are not as common as in peach leaf curl.

This is a guess, but perhaps the result of an infection by a fungal plant pathogen related to Taphrina deformans. T deformans infects species of the genus Prunus (i.e. the genus of prunes and apricots), but it's best known for causing peach leaf curl in another Prunus species, peaches.

For example, see this image of T. deformans infecting a leaf in Catalonia, Spain:

^{Source: flickriver; Credit: esat_ahi}

From Wikipedia:

trees infected with T. deformans will experience leaf puckering and distortion, acquiring a characteristic downward and inward curl. Leaves will also undergo chlorosis, turning a pale green or yellow, and later show a red or purple tint

And guess what? According to Wikipedia:

The fungus has higher infection rates following cooler, damper winters.

Are you sure your infected plant part is a fruit and not a grossly infected leaf? (Did you cut open the infected fruit to examine it closer)?

Management:

If it is T. deformans, Wikipedia summarizes some good management strategies:

Because infection depends on a wet environment, appropriate irrigation of crops can help control pathogen dispersal. Although some sources also suggest thinning fruit to control the spread of disease,[3] sanitation and culturing practices alone are insufficient to manage the pathogen.[2] Fungicide is preferred; chlorothalonil and ziram are favored, and copper is an organic option.[4] Fungicide application requires the correct timing and complete coverage of the crop.[11] It is recommended that growers spray fungicides after leaf-fall, or after 90% senescence of leaves. In wetter climates, where multiple sprays may be necessary, spraying is recommended in the late fall and in late winter or early spring.[3] Post-infection spraying of fungicide is inadequate to control the disease.[11]

Update: (Side-note)

I still think T. deformans seems like a possible cause of your plum disease, but...

Apparently, there is a closely related species to T deformans called Taphrina communis, which causes Plum pocket by instead infecting the fruits of plums.

• According to Kansas State University Research and Extension:

In contrast to peach leaf curl, symptoms of plum pocket are most conspicuous on the fruit. Small, whitish spots develop on young plums soon after blossom. The spots enlarge and eventually cover the entire fruit. Seed fails to form in infected plums; the fruit becomes hollow and enlarges to many times normal size. At this stage, the distorted plums have a red to grayish tint. Young shoots and leaves may be deformed and killed by the disease, but these symptoms are not as common as in peach leaf curl.

I couldn't find any pictures that matched your tree's symptoms, and truthfully the symptoms of plum pocket don't quite match what you're describing. However, for completeness sake, I thought I'd add this information here.

This is a guess, but perhaps the result of an infection by a fungal plant pathogen related to Taphrina deformans. T deformans infects species of the genus Prunus (i.e. the genus of prunes and apricots), but it's best known for causing peach leaf curl in another Prunus species, peaches.

For example, see this image of T. deformans infecting a leaf in Catalonia, Spain:

^{Source: flickriver; Credit: esat_ahi}

From Wikipedia:

trees infected with T. deformans will experience leaf puckering and distortion, acquiring a characteristic downward and inward curl. Leaves will also undergo chlorosis, turning a pale green or yellow, and later show a red or purple tint

And guess what? According to Wikipedia:

The fungus has higher infection rates following cooler, damper winters.

Are you sure your infected plant part is a fruit and not a grossly infected leaf? (Did you cut open the infected fruit to examine it closer)?

Management:

If it is T. deformans, Wikipedia summarizes some good management strategies:

Because infection depends on a wet environment, appropriate irrigation of crops can help control pathogen dispersal. Although some sources also suggest thinning fruit to control the spread of disease,[3] sanitation and culturing practices alone are insufficient to manage the pathogen.[2] Fungicide is preferred; chlorothalonil and ziram are favored, and copper is an organic option.[4] Fungicide application requires the correct timing and complete coverage of the crop.[11] It is recommended that growers spray fungicides after leaf-fall, or after 90% senescence of leaves. In wetter climates, where multiple sprays may be necessary, spraying is recommended in the late fall and in late winter or early spring.[3] Post-infection spraying of fungicide is inadequate to control the disease.[11]

Update: (Side-note)

I still think T. deformans seems like a possible cause of your plum disease, but...

Apparently, there is a closely related species to T deformans called Taphrina communis, which causes Plum pocket by instead infecting the fruits of plums.

• According to Kansas State University Research and Extension:

In contrast to peach leaf curl, symptoms of plum pocket are most conspicuous on the fruit. Small, whitish spots develop on young plums soon after blossom. The spots enlarge and eventually cover the entire fruit. Seed fails to form in infected plums; the fruit becomes hollow and enlarges to many times normal size. At this stage, the distorted plums have a red to grayish tint. Young shoots and leaves may be deformed and killed by the disease, but these symptoms are not as common as in peach leaf curl.

I couldn't find any pictures that matched your tree's symptoms, and truthfully the symptoms of plum pocket don't quite match what you're describing. However, for completeness sake, I thought I'd add this information here.

This is a guess, but perhaps the result of an infection by a fungal plant pathogen related to Taphrina deformans. T deformans infects species of the genus Prunus (i.e. the genus of prunes and apricots), but is is best known for causing peach leaf curl in another Prunus species, peaches.

For example, see this image of T. deformans infecting a leaf in Catalonia, Spain:

^{Source: flickriver; Credit: esat_ahi}

From Wikipedia:

trees infected with T. deformans will experience leaf puckering and distortion, acquiring a characteristic downward and inward curl. Leaves will also undergo chlorosis, turning a pale green or yellow, and later show a red or purple tint

Are you sure your infected plant part is a fruit and not a grossly infected leaf? (Did you cut open the infected fruit to examine it closer)?

Management:

If it is T. deformans, Wikipedia summarizes some good management strategies:

Because infection depends on a wet environment, appropriate irrigation of crops can help control pathogen dispersal. Although some sources also suggest thinning fruit to control the spread of disease,[3] sanitation and culturing practices alone are insufficient to manage the pathogen.[2] Fungicide is preferred; chlorothalonil and ziram are favored, and copper is an organic option.[4] Fungicide application requires the correct timing and complete coverage of the crop.[11] It is recommended that growers spray fungicides after leaf-fall, or after 90% senescence of leaves. In wetter climates, where multiple sprays may be necessary, spraying is recommended in the late fall and in late winter or early spring.[3] Post-infection spraying of fungicide is inadequate to control the disease.[11]

Update: (Side-note)

I still think T. deformans seems like a possible cause of your plum disease, but...

Apparently, there is a closely related species to T deformans called Taphrina communis, which causes Plum pocket by instead infecting the fruits of plums.

• According to Kansas State University Research and Extension:

In contrast to peach leaf curl, symptoms of plum pocket are most conspicuous on the fruit. Small, whitish spots develop on young plums soon after blossom. The spots enlarge and eventually cover the entire fruit. Seed fails to form in infected plums; the fruit becomes hollow and enlarges to many times normal size. At this stage, the distorted plums have a red to grayish tint. Young shoots and leaves may be deformed and killed by the disease, but these symptoms are not as common as in peach leaf curl.

I couldn't find any pictures that matched your tree's symptoms, and truthfully the symptoms of plum pocket don't quite match what you're describing. However, for completeness sake, I thought I'd add this information here.

This is a guess, but perhaps the result of an infection by a fungal plant pathogen related to Taphrina deformans. T deformans infects species of the genus Prunus (i.e. the genus of prunes and apricots), but it's best known for causing peach leaf curl in another Prunus species, peaches.

For example, see this image of T. deformans infecting a leaf in Catalonia, Spain:

^{Source: flickriver; Credit: esat_ahi}

From Wikipedia:

trees infected with T. deformans will experience leaf puckering and distortion, acquiring a characteristic downward and inward curl. Leaves will also undergo chlorosis, turning a pale green or yellow, and later show a red or purple tint

And guess what? According to Wikipedia:

The fungus has higher infection rates following cooler, damper winters.

Are you sure your infected plant part is a fruit and not a grossly infected leaf? (Did you cut open the infected fruit to examine it closer)?

Management:

If it is T. deformans, Wikipedia summarizes some good management strategies:

Because infection depends on a wet environment, appropriate irrigation of crops can help control pathogen dispersal. Although some sources also suggest thinning fruit to control the spread of disease,[3] sanitation and culturing practices alone are insufficient to manage the pathogen.[2] Fungicide is preferred; chlorothalonil and ziram are favored, and copper is an organic option.[4] Fungicide application requires the correct timing and complete coverage of the crop.[11] It is recommended that growers spray fungicides after leaf-fall, or after 90% senescence of leaves. In wetter climates, where multiple sprays may be necessary, spraying is recommended in the late fall and in late winter or early spring.[3] Post-infection spraying of fungicide is inadequate to control the disease.[11]

Update: (Side-note)

I still think T. deformans seems like a possible cause of your plum disease, but...

Apparently, there is a closely related species to T deformans called Taphrina communis, which causes Plum pocket by instead infecting the fruits of plums.

• According to Kansas State University Research and Extension:

In contrast to peach leaf curl, symptoms of plum pocket are most conspicuous on the fruit. Small, whitish spots develop on young plums soon after blossom. The spots enlarge and eventually cover the entire fruit. Seed fails to form in infected plums; the fruit becomes hollow and enlarges to many times normal size. At this stage, the distorted plums have a red to grayish tint. Young shoots and leaves may be deformed and killed by the disease, but these symptoms are not as common as in peach leaf curl.

I couldn't find any pictures that matched your tree's symptoms, and truthfully the symptoms of plum pocket don't quite match what you're describing. However, for completeness sake, I thought I'd add this information here.