Oak twig blight—Cryptocline cinerescens
The fungus sporadically causes twig blight (dieback) in coast live oak, interior live oak, and valley oak. Trees with heavy infestations become unsightly and lose vigor.
Various anthracnose and leaf spot fungi, including Apiognomonia errabunda (=Discula umbrinella) and Septoria quercicola, can cause similar damage. Unlike oak branch canker and dieback, anthracnose and leaf spot fungi and oak twig blight do not kill large branches or entire sections of the crown.
Cryptocline cinerescens kills leaves and twigs from the current-season's growth. Infected shoots turn white or tan and die and remain on the tree, typically in scattered patches throughout the canopy. Cryptocline cinerescens appears to cause more damage on trees infested with oak pit scales.
Twig and foliar blight diseases tend to be more severe in years when frequent rains coincide with when new leaves are being produced. The fungi infect the current season's growth and cause the shoots to die.
Symptoms are most noticeable in the summer and fall (July to October). Leaves on blighted shoots turn a straw-brown and typically remain attached to the shoot for several months. In some trees, only a few shoots are affected; in others, almost all shoots may be blighted. A general thinning of the crown is obvious in the year following infection.
Provide infected trees with adequate cultural care, especially appropriate watering. Unless they were raised with irrigation (e.g., planted oaks), avoid irrigating native oaks during the dry season; irrigate during the winter, if needed, when rainfall has been below normal. Avoid applying fertilizer, which can favor excessive shoot growth, leading to a denser, slower-drying canopy more susceptible to these diseases.
Pruning may help to control the disease, but this may be feasible only for a few small or specimen trees with limited infections. Prune infected twigs during dry weather in the summer or early fall; make cuts properly in healthy tissue below infected twigs.
Some systemic fungicides provide control if applied within 1 week after pruning. Fungicides alone are not as effective. Dead twigs and leaves remain on the tree, and the tree's appearance does not improve until these drop or are pruned out. Other pathogens and certain insects cause similar damage, so correctly determine the cause before attempting any control.