Description: Diplodia blight is a fungal disease caused by two species of Diplodia: D. pinea and D. scrobiculata. Diplodia pinea is more aggressive and capable of causing more severe damage than D. scrobiculata, but both fungi are capable of causing disease. Diplodia blight is most common and severe in pines that have 2 to 3 needles per bundle (Table 1). Disease is significantly more severe in stressed trees than in vigorously growing non-stressed trees. Pines that have five needles per bundle are highly resistant to diplodia blight, which in Minnesota includes the only native five needle pine, eastern white pine (P. strobus) as well as the non-native bristlecone pine (Pinus aristita).

Other conifers like spruce, fir, larch and arborvitae occasionally become infected. This is most common when trees are stressed and planted near severely infected pine trees. Even though the disease can be found in other conifers, it rarely causes much damage.

Identification

•Needles of new shoots remain stunted, turn straw colored and are glued into place from excess resin.

•Mature needles on branches killed by girdling cankers turn tan and die.

•Sticky clear to white resin or pitch found on killed twigs, the main stem or branches with cankers.

•Infected young, actively growing shoots and dead branches occur most commonly on lower branches but can occur randomly throughout the tree.

•Tiny, black, pimple-like spore producing structures (pycnidia) found at the base of dead needles, on cone scales, or on twig and/or branch bark.