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Yellow halo around a Cordana leaf spot lesion on a banana leaf (Musa sp.), caused by the plant-pathogenic fungus, Cordana musae.  These spots typically are surrounded by a yellow halo. Such yellow halos are associated with toxin production by fungal plant pathogens such as Cordana.

Purple halo surrounding a disease lesion on a green (immature) coffee berry (Coffea arabica).  The disease, known as berry blotch, is caused by the plant-pathogenic fungus, Cercospora coffecicola, which also attacks coffee leaves, where lesions are surrounded by yellow halos.

Leaf spots on coffee (Coffea arabica) caused by the plant-pathogenic fungus, Cercospora coffeicola, are typically surrounded by yellow halos.

Definition

A halo on plants is a differentiated zone (usually yellow or reddish) surrounding a central zone or object (usually a lesion).

Etymology. 1563, from L. halo (acc.), from Gk. halos "disk of the sun or moon, ring of light around the sun or moon" (also "threshing floor" and "disk of a shield"), of unknown origin. Sense of "light around the head of a holy person or deity" first recorded 1646.

Discussion

Chlorotic halos may indicate the presence of a plant toxin produced by a plant pathogen and their presence is helpful in daignosing certain foliar plant diseases, such as those casue by the fungal plant-pathoegnic genus, Cercospora.