Why does this plum/apricot hybrid look like brains?

Question

Why does this plum/apricot hybrid look like brains?

Notes:

• Here's a photo of what a normal fruit on the tree looks like.
• Location is Zone 5, east of Toronto.
• There are about 20 other fruits on the tree. This the only fruit on the tree that has this problem.
• The date is June 26, 2019 (although plants have been delayed by about 3 weeks this year due to unusually cool and rainy weather).
• I have had lots of disease problems on my prunus this year due to the wet weather.

Answer 1

It is difficult to say, but it is likely due to disease. Many plant diseases have the effect of convincing plant tissues that they are some other organ than what they actually are, which leads to deformations.

I was not able to find anything that looked as dramatic as the image that you show, but there are similar diseases in stone fruits that deform fruit:

1. Plum pox potyvirus: http://download.ceris.purdue.edu/file/1424
2. Plum pocket fungus/"Bent banana disease": https://pnwhandbooks.org/plantdisease/host-disease/prune-plum-prunus-spp-plum-pockets, http://shropshireprunedamson.com/a-bad-case-of-bent-banana-disease/

I hope that this is helpful.

Answer 2

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.