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Tubakia and Bur Oak Blight

Posted on November 24th, 2020 in Disease, Forestry, Forests and Street Trees, How To, Plants, Woodlands | No Comments »

Purdue Landscape Report: Tubakia leaf spot, caused by the fungus Tubakia, is the disease we find more commonly on oak than any other. Throughout the world, there are 11 species of Tubakia known to infect oak, with Tubakia dryina (previously known as Actinopelte dryina) being the most commonly encountered species in our landscapes. Apple, ash, black gum, chestnut, elm, maple, and redbud are all reported as hosts of Tubakia species, but oaks are the most frequently and severely affected. Among the oaks, the red oak group, specifically red, pin, and black oaks, are reported to be more susceptible to infection compared to those in the white oak group.

Figure 2: Oak leaf with circular shaped Tubakia leaf spots along with extensive veinal necrosis.

Figure 1: Oak leaf with irregularly shaped Tubakia leaf spots and veinal necrosis.

Leaf spots appear in mid to late summer (July –August) as small circular to irregular tan, red-brown, to dark brown spots (Figure 1, 2) that expand to approximately the size of a dime overtime, but can coalesce, forming large areas of necrotic tissue (Figure 3) . When a spot reaches a leaf vein it expands very quickly, causing a necrotic streak along the vein, and can cause blighting of most of the leaf (Figure 4, 5). Trees under stress from other causes will frequently exhibit more severe leaf spotting compared to healthier trees. Premature defoliation can occur in these situations.

Figure 3: Marginal blighting due to coalescing spots and veinal necrosis.

Figure 5: Blighting of large leaf area caused by veinal infection by Tubakia.

Figure 4: Typical Tubakia leaf spot symptoms illustrating how the fungus spreads along leaf veins.

The pathogen produces conidia within shield shaped structures called pycniothyria (Figure 6) which can be found on both the top or bottom surface of the leaf and along veins (Figure 7). These structures are very small and can only be seen with a 40x or stronger hand lens. The fungus overwinters on fallen leaves and on dead stems which act as the source of inoculum for the next year. During early spring, spores are spread by wind and water splash dispersal (rain) to healthy new foliage. However, it takes time for symptoms to develop throughout the season, depending on tree stress and environmental conditions (warm wet weather favors spread).

For full article view: Purdue Landscape Report, Purdue Landscape Report.