Hi Tedwar
Have had a look in my trusty Collins Guide to plant diseases and it is a minefield out there! Your plum may have had bacterial canker, bark split virus or merely suffering from drought. These all include exuding quite copious amounts of amber coloured gum. All these diseases include apricots, peaches and ornamental Prunus species, (although you will be relieved to know it says less importantly in these fruits in the case of bacterial canker). There is also Plum Pox Virus (very virulent in Souther Europe) which also affects apricots, nectarines, peaches, etc. this is more serious and the only recommended treatment is get rid of any infected trees. Clear the ground and plant disease resistant varieties. Sorry if this is doom and gloom, must say when I was doing my RHS course 20 years ago I learn´t enough of this disease stuff to put me off growing fruit forever. However, there is lot of new stuff out there that grows very well. Hope I haven´t discouraged you
