Macroderma curtisii

APPEARANCE OF THE DISEASE
The appearance of tiny yellow spots on holly leaves during late May constitutes the first symptom of the disease. At this stage the spots cannot readily be distinguished from those produced by certain sucking insects ... During early summer the yellow spots slowly increase in area. By the middle of July the central portion of each spot becomes reddish brown and the discolored area gradually enlarges until only a narrow border of yellow tissue remains. By fall the discoloration has deepened to shining black, and flat cushion-shaped stromata have developed beneath the epidermis. Frequently pairs of stromata develop in a single lesion, one near the upper surface and the other near the lower surface, and sometimes elongated boat-shaped ones form independently on the ventral surface of the  midrib. By November the stromata on the upper leaf surface have attained diameters of 1-4 mm., while those on the lower surface are somewhat smaller. Differentiation of the stromata proceeds slowly throughout the winter months, and by April the orange-red apothecial discs are matured and are exposed by the rupture of the outer layers. After dehiscence the surrounding leaf tissues die as a result of desiccation, forming a buff colored necrotic area which bears the exhausted stroma. These necrotic areas may finally drop out, leaving perforations. Infected leaves usually remain on the tree for the normal period of two years, the unaffected portions continuing to function, but occasionally severely infected leaves may become entirely yellow and moribund by the end of the first year or parts or all of the leaf may have died before that time. The fungus does not continue development after the infected tissue of such leaves becomes necrotic ... 

The stromata contain large druses of calcium oxalate which were formed within hypertrophic epidermal and mesophyll cells. A few such crystal-bearing cells are always found scattered throughout healthy leaves, but as a result of infection an inordinate number of these cells is produced. During fall the stromata become differentiated into three layers: (1) An outer layer in which the hyphae are thick-walled and very compactly arranged; this tissue and the adjacent epidermal walls become impregnated with a dark purplish pigment which is responsible for the deep black color of stromata when observed macroscopically, and the cuticle over these areas takes on a yellowish
hue. (2) A median layer composed of a loose plectenchyma of hyaline hyphae. (3) A basal layer made up of partially disintegrated mesophyll cells filled with thick, dark-colored hyphae.

Tar Spot of American Holly