Oak branch canker and dieback—Diplodia corticola and Diplodia quercina
At least two Diplodia spp. fungi cause branch cankers and dieback on oaks in California. Hosts include California black oak, coast live oak, English oak, and valley oak.
Trees affected by Diplodia have completely killed branches with all brown, dead leaves. This differs from the widely scattered twig dieback with small groups of brown leaves caused by oak anthracnose and twig blight fungi. There are many other common causes of cankers, including other species of canker fungi.
Diplodia quercina kills small branches (usually those less than 1 inch, but up to 4 inches in diameter), causing leaves to turn brown, wilt, die, and remain attached in the canopy. Oak branch dieback outbreaks follow years of below-average rainfall.
Diplodia corticola (named Botryosphaeria corticola during its sexual stage) causes crown thinning (leaves that are pale and smaller and less abundant than normal) and bark bleeding, cracking, and staining on the main trunk of coast live oak in southern California. Trunk symptoms of D. corticola infection (called bot canker) resemble damage caused by goldspotted oak borer.
Disease caused by Diplodia spp. often can be distinguished by the masses of tiny, roundish, raised, black fruiting bodies (pycnidia) on bark of infected hosts. Peeling off bark around fruiting bodies and cankers reveals dark brown to black dead wood, commonly with a well-defined border between dark diseased and light-colored healthy wood.
The fungi may enter branches through wounds and possibly natural openings. Asexual fruiting bodies (pycnidia) are produced within 24 weeks and continue to produce spores for about two years. On branches with bark, pycnidia are in black masses of fungal tissue (stromata). Spore production and germination are favored by moisture.
Even drought-adapted oaks may require supplemental irrigation to reduce stress if rainfall has been well below normal. However, irrigation of native oaks should generally be done during the normal rainy season to supplement inadequate natural rainfall. Exceptions include planted oaks adapted to (raised with) summer irrigation from the start and in certain situations where soils have been disturbed. The specific amount and frequency of irrigation vary greatly, depending on factors such as environmental and soil conditions. Be aware that frequent irrigation during the dry season promotes Armillaria root disease (oak root fungus) and Phytophthora root and crown rot.
Protect trees from other pests and injury and provide proper cultural care. No other specific management is known for D. corticola. For D. quercina, the disease is not a major problem in most years, and control is usually not needed.