Dothistroma needle blight is a common and serious disease of Austrian and ponderosa pines in windbreaks and ornamental landscapes. Mugo pine also can be infected, but Scots pine and white pine are considered resistant. The disease causes premature needle drop the year after infection. Austrian and ponderosa pines usually retain needles for 3 to 4 years, so the loss of interior needles is a loss of important photosynthetic capacity. Repeated infection over many years can kill a tree. The disease tends to be more severe in crowded plantings.

Symptoms: New infections become apparent in late summer or early fall. Diseased needles develop dark green bands or scattered yellow/tan spots (Figure 4). The spots often enlarge and become red bands that are sometimes bordered by a light yellow region. Dothistroma is sometimes confused with environmental stress or natural needle drop (see below). However, a key indicator for Dothistroma is that the tip of the needle (beyond the red band) turns brown, but the base stays green for several months resulting in a partial needle scorch (Figures 4 and 5). The disease is usually most severe in the lower part of the tree on the interior parts (1-, 2-, or 3-year-old needles). The current needles sometimes show symptoms as well.

In late winter or early spring, the fungal sporeproducing structures (from the previous-year’s infection), visible as black bumps that rupture the needle surface (Figure 6). The green base turns brown, and the needles are shed throughout the spring and summer. The loss of old needles, combined with the growth of the newest needles, gives branches a tufted appearance. Heavily infected trees appear bare in the interior of the lower part of the tree.

Dothistroma can resemble damage by natural needle drop or environmental stress (see below). The heavy loss of older, internal needles plus the fungal spore structures are evidence of Dothistroma. In addition, environmental scorch usually causes all needles to turn brown at about the same length. In contrast, the length of browning from new Dothistroma infections is more variable.

Cause:

The fungus, Dothistroma septospora, overwinters in infected needles and produces small, black, spore-bearing structures (acervuli), which break through the dead, needle epidermis in late winter or early spring (Figure 6). A 10X hand lens may be helpful in viewing these structures. The best time to confirm the presence of Dothistroma needle blight is December through April when these structures are present. Without the structures, the diagnosis is not clear. Getting a diagnosis at this time leaves plenty of time to make management decisions for spring fungicide applications.

Spores are dispersed in water droplets or aerosols during periods of rainfall from mid-May through October. Newly developed needles are resistant at first, but become susceptible by midsummer. Older needles are susceptible throughout the growing season.