Foliage can be damaged in a number of different ways, but identifying the causes from leaf symptoms is not always easy.
Leaves invariably suffer damage in the growing season, but this can be more extensive after severe winter weather or summer drought.
Generally a number of things have to be taken in to consideration before a proper diagnosis can be made, e.g.
When and where a plant is growing?
Is it a new plant? - new plants can take time to establish.
Are the soil conditions suitable for the particular plant? – e.g. pH, moisture content?
Has the weather been abnormal recently?
Browning of leaf tips or margins, especially on young growth, may suggest drought and a failure to establish.
Brown leaf tips are common when dry weather follows spring planting before new roots have established.
Dry, brown lower leaves are often found on climbers that are too dry.
Blackening of leaves, usually along the vein can be due to water-logging, particularly on heavy soils.
On further investigation you may find the roots are bluish-black and fall apart if teased out.
Water-logging can also reduce variegation or leaf colour, or cause evergreens to lose all their leaves after periods of stress.
Frost damage / Cold Temperatures:
Frost can damage leaf tips and margins on young growth in spring.
Cold winds can burn the foliage of hardy conifers.
Green leaves that turn dark purple, is caused by low night temperatures.
Some will turn dark red after cold winters.
Tender plants put outdoors too soon in spring can turn white and chlorotic.
The leaf margins of plants growing in too bright sunlight may curl or droop, e.g. rhododendrons and some evergreens.
These symptoms are also symptomatic of cold and drought!