Plum pox show no symptoms on leaves or fruit. Not all infection in "Prunus" are characterized by a ring symptom on leaves. Several cultivars show yellowing line patterns and blotches, or necrotic ring symptoms on expanded leaves. Leaf distortion has also been observed. Infected fruit can develop yellow rings or blotches, or brown rings, and some plum and apricot fruit can be severely deformed and bumpy. The seed of many infected apricots and some plums show rings. Many non-"Prunus" species, in at least nine plant families, have been infected artificially with one or more strains of the plum pox virus, and in
Cherry plum cerasifera" var "pissardii" ("P." 'Atropurpurea'). The variety 'Nigra' with black foliage and pink flowers, has gained the Royal Horticultural Society's Award of Garden Merit. "Prunus × cistena" (purple leaf sand cherry) a hybrid of "Prunus cerasifera" and "Prunus pumila", the sand cherry, also won the Award of Garden Merit. These purple-foliage forms (often called 'purple-leaf plum'), also have dark purple fruit, which make an attractive, intensely coloured jam. They can have white or pink flowers. The cultivar 'Thundercloud' has bright red foliage which darkens purple. Others, such as 'Lindsayae', have green foliage. Some kinds of purple-leaf plums are used for