Mugo pine is also susceptible to the disease, but Scots pine is considered resistant. The disease causes premature dropping of pine needles the year following infection. Because Austrian and Ponderosa pines typically retain needles for three or four years, premature loss of the foliage results in a reduced photosynthetic area and a loss of tree vigor. Extensive defoliation over several years may kill the tree. Symptoms of Dothistroma needle blight are evident first in late summer or early fall.
Diseased needles exhibit dark green bands or scattered yellow to tan spots.
The spots often enlarge and develop into red bands that encircle the needle. The red bands may be bordered by a light yellow region. Typically, the disease is most severe in the lower portion of the tree crown. In late winter or early spring, small black fruiting structures of the disease-causing fungus rupture through dead portions of the needle. The green base of the needle turns brown, and dead needles are cast prematurely throughout the spring and summer.