noun
1. : a lesion of plant tissue consisting of yellowish, purplish, or necrotic, often concentric rings
2. : any plant disease of which ring spots are the characteristic lesion: as
a. : a virus disease of tobacco and related plants producing chlorotic rings on the leaves
b. : a disease of sugarcane caused by a fungus ( Leptosphaeria sacchari ) and characterized by purplish bordered spots
c. : a disease of the lower leaves of cauliflower and related crucifers caused by a fungus ( Mycosphaerella brassicicola )
d. : an anthracnose of lettuce caused by a fungus ( Marssonina panathoniana )
e. : frogeye a(1)
f. : fairy ring spot