Pathogen
Tubakia leaf blotch was previously attributed to the actions of Tubakia dryina (Sinclair and Lyon 2005). However, more recent studies have revealed a high level of species diversity in genus Tubakia and other closely related genera (Braun et al. 2018). In eastern North America, at least six species of Tubakia are known to occur on Quercus (Braun et al. 2018; Harrington and McNew 2018). Tubakia species are widespread across North America, Europe, and Asia (Braun et al. 2018; Zhu et al. 2022).
Hosts
Numerous native and non-native oaks (Quercus spp.) serve as the primary hosts in the region. Secondary hosts include maple (Acer), hickory (Carya), chestnut (Castanea), beech (Fagus), and ash (Fraxinus), among others (Sinclair and Lyon 2005). Based on samples submitted to the UMass Plant Diagnostic Laboratory, Tubakia has been found on a variety of species in both the red oak group (Q. coccinea, Q. palustris, Q. rubra, and Q. velutina) and white oak group (Q. abla, Q. bicolor, Q. cerris, Q. frainetto, Q. macrocarpa, Q. mongolica, Q. phellos, and Q. robur).
Symptoms & Disease Cycle
Tubakia leaf blotch symptoms often appear in middle to late-summer across southern New England, with the first appearance typically in early to mid-July. Symptoms first appear as water-soaked, circular lesions that become reddish-brown to dark brown over time. The leaf spots often coalesce to create large necrotic patches. By late August, a significant percentage of the foliage can be consumed. Infections are believed to initiate in the spring from sexually produced spores that develop on the previous year’s foliage (Sinclair and Lyon 2005). Tubakia species are known endophytes, and the fungus appears to have a latent phase following infection before symptoms are visible. Once symptoms appear, Tubakia creates large volumes of asexual spores on necrotic tissue that are readily blown and splashed to create new infection centers. This is how the disease rapidly builds up in late summer (Sinclair and Lyon 2005).
Tubakia can readily colonize foliage stressed by drought, insects, and other diseases (Sinclair and Lyon 2005). For example, Tubakia is readily found co-occurring with oak anthracnose (Apiognomonia errabunda), which causes injury early in the growing season. Additionally, when there are outbreaks of the jumping oak gall wasp (Neuroterus) on white oak, Tubakia can be abundant on the resulting leaf spots. When marginal scorch develops due to drought stress, Tubakia readily colonizes and expanding blotches can cause premature leaf shedding. Tubakia can also colonize twigs but is not known as a significant cankering pathogen of oak (Sinclair and Lyon 2005). However, this may represent another source of overwintering inoculum in the canopy.
Management
Typically, Tubakia leaf blotch is not a serious threat to the overall health of infected oaks because the symptoms develop later in the growing season, when growth is mostly complete for the season. Therefore, only minor growth losses are expected from outbreaks. Cultural disease management should focus on removing discarded leaves during the autumn season to reduce overwintering inoculum at the site. For oak species that exhibit marcesence (incomplete leaf abscission), like pin oak, foliage may need to be removed in the spring. Tubakia species are native and widespread in the environment; therefore, eradication is not possible. Chemical management is often unnecessary since symptoms are most severe late in the growing season. However, if oaks are stressed by defoliating insects, drought, fungal cankering, and oak anthracnose, fungicide application to reduce disease pressure may be warranted. Several fungicides may have utility against Tubakia and include copper hydroxide, copper sulfate, mancozeb, metconazole, myclobutanil, propiconazole, triadimefon + trifloxystrobin, trifloxystrobin, and thiophanate-methyl. Fungicide application should take place as soon as new shoot and leaf development begins in the spring and on regular intervals until leaves are fully developed, should wet conditions that are conducive to spore production persist. By limiting the primary infection in the spring, asexual spread later in the growing season may be reduced.
Literature Cited
Braun U, Nakashima C, Crous PW, Groenewald JZ, Moreno-Rico O, Rooney-Latham S, Blomquist CL, Haas J, and Marmolejo J. 2018. Phylogeny and taxonomy of the genus Tubakia s. lat. Fungal Systematics and Evolution, 1(1): 41–99. https://doi.org/10.3114/fuse.2018.01.04
Harrington TC and McNew DL. 2018. A re-evaluation of Tubakia, including three new species on Quercus and six new combinations. Antonie van Leeuwenhoek 111(7): 1003–1022. https://doi.org/10.1007/s10482-017-1001-9
Sinclair WA and Lyon HH. 2005. Diseases of Trees and Shrubs, 2nd edn. Cornell University Press, Ithaca, NY.
Zhu Y-Q, Jiang N, Dou Z-P, Xue H, Piao C-G, and Li Y. 2022. Additions to the knowledge of Tubakia (Tubakiaceae, Diaporthales) in China. Journal of Fungi 8(11): 1143. https://doi.org/10.3390/jof8111143