Alternaria Leaf Spot, also known as Brown Spot, is caused by Alternaria alternata, has been a serious problem on mandarins. Alternaria brown spot attacks young fruit, leaves and twigs, producing brown-to-black lesions surrounded by a yellow halo. The halo is caused by a fungal toxin which rapidly kills citrus tissue. Leaf lesions are generally circular but will often have a tail, following the leaf vein which gives the lesions an eye-spot appearance. The necrosis extends along the veins as the toxin spreads in vascular tissues. On young leaves, lesions can appear as early as 36-48 hours after infection.