Pines are most susceptible
Diplodia tip blight is a disease caused by the fungus Diplodia sapinea that affects conifers and is especially harmful to pines. The pathogen targets new emerging growth, leading to browning and die-back at branch tips. Loss of lower branches, and even full tree decline, may occur depending on the tree species and the severity of the infection.
In much of North America, it is common to find diplodia tip blight on Austrian, ponderosa, mugo, red, and Scots pines. Most other pines and spruces are also susceptible to some degree, along with a selection of cedars, firs and other conifers.
Considered an opportunistic disease, trees under stress (such as drought or poor soil conditions) are much more likely to be infected by diplodia tip blight. The pathogen also tends to affect trees planted beyond their natural ranges – making it a problem primarily in landscape trees, rather than forests.
Diplodia tip blight spreads in moist weather conditions throughout the growing season via spores that are released from fruiting structures on cones and dead needles. New growth and wounded tissue are most prone to infection. The disease may progress internally well beyond the point of entry in susceptible hosts, causing more substantial damage.
Easily visible damage from diplodia tip blight typically begins to appear in the spring with the death of new growth at branch tips. The young needles turn yellow and then brown. In late summer, small black dots appear on dead needles and some cones – these are the fruiting structures of the fungus. Depending on the severity of the disease, other symptoms may include cankers/lesions on branches, dead branches, and staining of woody tissue that can be seen by cutting into an infected stem.
Since diplodia tip blight is primarily an issue for stressed trees, keeping conifers healthy is the best way to avoid problems with the disease. Proper irrigation and mulching, as well as fertilization to address nutrient deficiencies, all help to maintain tree vigor. To improve appearance, and to minimize reinfection from the fruiting bodies on dead needles, trees should be pruned in periods of dry weather to remove any brown growth. Diplodia tip blight does respond to fungicide treatments, which should start at bud break in the spring for effective control. If these treatments aren't used, then cones should also be removed during pruning because they hold many fungal spores.