One way pollinating chums

I have a Prunus Mustang in tree habit I top grafted on it’s leader with Kappa plum cherry (chums)
After grafting it last year, the graft reached over 3.5 ft of growth.

This spring, the Kappa graft was covered with flowers as the Mustang tree. Now there’s a lot of fruits on the Kappa graft but nothing at all on the Mustang tree branches.
Can you tell me why Mustang can pollinate the Kappa but the Kappa can’t pollinate the Mustang ?

This is a mystery for me.

Thanks a lot

I also have a large Mustang chum in its 3rd year that had a ton of blooms this spring. I have a Convoy chum 20ft away that flowered at the exact same time. Not a single plum set on either tree. I even hand pollinated them a few days. We did have a lot of rainy/cloudy days during the bloom time but you’d think a few fruit would have set if they were compatible. I also have a Toka plum that only produced a handful of flowers which I tested hand pollinating the Mustang but that didn’t work either.

I tried finding a chum pollination chart online but there’s not much detailed info out there. I did find one source which said Mustang/Convoy were compatible.

I have a vigorous Myrobalan seeding in it’s second year originally for a rootstock that I was thinking of growing out to see if it will work next year.

Last year, I used Prunus nigra blooming branches in a bucket of water near P. Mustang and it worked.

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Yes I was going to mention trying a Canadian or American plum. But I don’t think they grow in the wild around here. Last year I tried hand pollinating the Mustang with pin cherry, it didn’t work.

Here is the answer I got from Mr Wilbert Ronald from Jeffrey Nursery. He is the breed of the Mustang.

The only explanation or untested theory to explain what you are seeing is “ unilateral incompatibility” where Kappa x Mustang is cross compatible but Mustang x Kappa is cross incompatible. This occurs in some plants between wide crosses but I am not familiar with it in Prunus