Hi Soho,

Morus (Mulberry's) are somewhat susceptible to a variety of fungal and bacterial diseases and that's certainly what yours is suffering from too.

The spores over-winter in the dead leaves and other litter surrounding the tree and then re-establish once the tree has leafed out in spring, which is the pattern that you have observed.

The attack will severely limit the extent to which the tree can photosynthesise, and so it's overall vigour (in growth and fruiting) will be much reduced.

The answer is multi-fold... Firstly, in autumn/winter it's very important to remove all of the old/dying/diseased leaves from around and underneath the tree as that's where the reservoir of disease will be coming from.

Then, as soon as the tree is starting to leaf out in Spring, you need to treat the foliage with an anti-fungal spray, and then repeat perhaps twice more over the subsequent 4 to 6 weeks, the idea being to break the cycle of disease and prevent it from returning.

There are a variety of fungicidal/anti-bacterial agents to choose between - Copper based (and thus organic) Bordeaux mixture is widely available and effective against Bacterial and Fungal infection.

Opening up the canopy of the tree by selectively removing branches will increase the air flow and so help to prevent the disease re-establishing, and finally giving the tree an intensive feed in early spring will help it's vigour and ability to fight infection.