Check roses for signs of blackspot, aphids and leaf-rolling sawfly damage.
Viburnum beetle grubs start nibbling holes in the leaves this month, giving plants a tattered appearance. Inspect V. tinus and V. opulus regularly and spray or pick of the grubs by hand.
Phytophthora root rots can cause die back on mature trees and shrubs. Wet winter weather followed by a hot spring and summer can encourage this problem on susceptible woody plants.
Inspect sick looking box and holly trees for signs of blight.
Check for damage or cankers on deciduous trees.
Caterpillars, aphids and other fly pests can all be problematic at this time of year. Early infestations can be managed by hand removal, but approved insecticides are necessary for more serious attacks.