Plum Tree Problem

I'm hoping someone will be able to help me with my plum tree.  The tree has five branches and two of them had lots of plums this year.  I hadn't been to my allotment for a while but when I did I found that the two branches and all the plums had shrivelled up and died.  My husband has been there regularly to water but didn't notice anything was wrong.  The three branches without plums are fine.  Does  anyone know the likely cause please?

Have checked and can't see anything obviously wrong with it.