Washington: Italian plums without fruit

I have two Italian plums, one planted 10 years ago and one planted 3 years ago. Both flower well, mostly in April and after the petals fall off there appear to be tiny green plums starting to grow. However by the beginning of June all of these have fallen off. We have had Mason bees every year. I also have another plum tree with 4 types of plum, 3 of which bloom at about the same time. One, a Beauty plum, flowers earlier and sets lots of fruit. An arborist has looked at the Italian plums and saw no sign of disease. The last 2 years other friends on Vashon Island have also had their Italian plums fail to set fruit, so our rainy Aprils may be part of the problem. However, a plum tree at a house we own in Seattle sets fruit every years. Is this a climate change problem, a pollination problem or something else? Do you have any suggestions? The Beauty plums lack flavor and are not very useful.

 

Your problem seems similar to what occured in Burnaby, BC, last year with all of my stone fruit trees, including a plum with 4 varieties. We had practically no fruit on them due to cold and wet weather during the pollination season. Not surprisingly, the poor pollinating weather affected the pollinators as well; my Mason Bee colony shrank from about 200 to only 8 members. I actually had to buy bees this year, the first time ever. Oddly enough, we had good conditions this year, and I had to thin out over half of the Italian Prune plums to avoid overloading that part of the tree.

So, my guess is that your prune plums bloomed during a spell of bad weather and were not adequately pollinated. I assume that your blooming period is somewhat earlier than ours, and I remember that we had poor weather at the start of our season. The weather improved suddenly while the Japanese plums were still blooming and the Italian Prune was just starting to bloom. As a result we had a decent fruit set of Japanese plums and and excessive one of Italian Prunes.

Perhaps Seattle had somewhat better weather than Vashon Island during that critical period, which would explain why it had a better fruit set.

 

Lots to ponder -

the original post asked why the fruit set THEN fell off the plum tree

Any thoughts - I’m curious too

 

All of the stone fruits (and apples) go through a "June drop" phase after apparently setting fruit. I've often seen most, and sometimes all, of the apparently pollinated fruit fall off at the June drop. There seem to be many reasons for this loss of fruit, but I've always asssumed that it was due to either inadequate pollination or poor weather conditions at some critical period after pollination. Many reports state that the drop is due to the tree's thinning itself to ensure that adequate energy will be available to develop the remaining fruit. However, this does not explain the loss of all or almost all of the apparently pollinated fruit, which happens frequently on my trees.

 

So maybe the « fruit dropping » tree itself is stressed fr previous summer drought or other factors?

It wouldn’t be birds ?

Have to ask some Okanagan orcharist experts - then again their trees are regularly irrigated and tended and probably don’t have the disturbances that a backyard garden plum tree might have (eg new deck or reno on neighbor house or cars parking nearby and compacting the earth etc). I think they also have some spray that keeps early stage fruit fr dropping don’t they - don’t quote me on that - best to call the Ag Canada research station in Summerland BC.