Ah, the sign of the plum curculio, probably the worst pest we have here for stone fruits.
Those scars are where the female slices a hole in the young fruit, where her young develop, eating their way inside the young fruit, until it dies and drops.
I've heard of people using Surround, a clay product that is sprayed onto the fruit and irritates the bugs so they don't stick around long enough to do damage. However it may have to be sprayed multiple times during the periods of highest activity, and it just sounds like it is so much work, and it can be washed off during rains.
To me, spraying one thing usually leads to spraying another. I'm experimenting with increasing the health of the tree to fight off the bugs on its own. It seems like stressed trees attract more of the bugs, while strong trees in healthy and well drained soil can fight a lot of them off.
Hope you get some tasty plums soon Torban, and welcome to permies!
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Thank you Steve.
I researched your answer and it 100% is an exact match to my problem.
Year 1 I had amazing plum fruit
Year 2 (last year) it got these markson the fruit and all the plums fell off the tree way before I could harvest anything.
Year 3 this year. I see it happening all over again. Thanks so much for this information.
I will rush out tomorrow and find some kind of solution to spray like clay to try and save this years crop!
The most important thing is do not let fallen fruit stay on the ground. The larva will come out of the fruit and overwinter in the ground. If you are not squeamish and the fruit ripens with the worm in it you can eat it and that breaks the cycle. otherwise it must be removed and cooked to kill the maggot. Having a trap with a light and fan to blow the insect into a screen or bag will help with the ones coming from other sources. Best solution to the problem is to make it into chicken food.