Q.Paskapoo poplar – leaf colour/slime flux
We have a 12ft paskapoo poplar that is showing signs of stress and I need some advice on what to do. It has had slime flux for at least 4 years and an (uncertified) arborist told us to stop watering it to slow down the seepage. For about the same length of time, by about mid July the leaves start turning yellow the black splotches start forming. It is infested with aphids which I try to hose off every couple of weeks or so
This is definitely infected with a fungal infection. This could be caused by the aphids, or can be why the aphids are there now. You can try adding wettable sulfur and dolomitic lime to the soil around the tree. This can help kill off the infection while keeping the soil pH stable.
I would definitely hold off on any watering, unless it remains dry and the soil is completely dry down to about 2 inches. Overwatering will cause an infection very quickly.
As for the insects... This collection of articles will give you ways to control these along with other useful tips: https://www.gardeningknowhow.com/special/organic