ISSUE 12 July 29, 2010
DIPLODIA TIP BLIGHT INCREASING ON PONDEROSA PINE TREES?
This past spring, I exchanged several e-mails with colleagues in Dickinson and Bismarck regarding dead branch tips and needles on ponderosa pine trees. Given the highly variable nature of our winter and spring weather, I figured it was just “winter injury”, a catch-all term that encompasses a variety of environmentally caused problems. Recent observations indicate that a biotic agent may be one of the causal agents.
A closer look at some of the cones from a tree with symptoms in Bismarck showed fruiting bodies of a fungus called Diplodia pinea. This pathogen causes Diplodia top blight and this disease was also diagnosed on trees in Fargo and, in previous years, in Carrington and Dickinson. The fungus causes shorter-than-normal needles and dead shoots, and it may eventually kill the entire tree. The fungal fruiting bodies on the pine cone scale (Figure 1) are visible with the naked eye.
Figure 1. Scale of a ponderosa pine cone,
infected with Diplodia tip blight. The black
areas erupting from the surface of the cone
scale are the fungal fruiting bodies.
The key diagnostic features of this disease are the actual spores (Figure 2); by wetting the cone scale, you may be able to see the spores under a 15-20X hand lens.
Figure 2. Spores and fruiting bodies of the
Diplodia pinea fungus from an infected cone scale.
Cones are usually infected
before shoot death becomes serious.
The fungus may also live within the tree for many years without causing
infection, but it may suddenly become active if stresses such as drought or
physical wounding (hail or excessive snow/ice) occur.
Fungal spores are dispersed during moist conditions such as those that
occurred this spring. Because the
fungus mainly infects current-year needles, shoots and cones in the late spring
or early summer, we are likely past optimal time for spray treatments.
Nevertheless, as next year’s new growth begins, protectant fungicides
containing copper hydroxide + mancozeb, propaconizole, copper salts, and
thiophanate-methyl may be used. As
always, read and follow directions and safety precautions on labels.