As the weather warms, Christmas tree growers and nursery folks are beginning to get back into their fields to select trees for digging or to assess field conditions. Recent samples in the Plant Diagnostic Laboratory suggest it would be prudent to keep an eye out for disease symptoms and signs that would have appeared or intensified over the winter.
On Douglas-fir in particular, both Rhabdocline and Swiss needlecasts are becoming more visible. Be aware that both of these diseases, if left unchecked, can have devastating consequences for the planting. Scout your fields now to identify and remove affected trees and to prepare for preventive fungicide applications.
The fungus Phaeocryptopus gaumannii causes Swiss needlecast. Symptoms of Swiss needlecast appear as needle tip scorch, which resembles drought stress, or can manifest as an overall yellow discoloration of the foliage that might look like a fertility problem. Fortunately for us, the fungus is easy to identify with a hand lens. Phaeocryptopus fruits through the stomata of infected needles. The fruiting bodies appear in the hand lens as distinct rows of black dots on the undersides of the needles.
The fungus that causes Rhabdocline needlecast is Rhabdocline pseudotsugae. The symptoms of Rhabdocline appear as brownish-purple leaf lesions. These lesions ultimately swell and erupt with fungal fruits that ooze orange spores. Unlike Phaeocryptopus gaumannii, the fungus Rhabdocline pseudotsugae is not so easy to identify. We’ve got to wait until mid-May during the bud break period for the fungus to reveal itself. Fortunately, the leaf spot symptom is quite distinct.
While both needlecast diseases cause needles to drop and canopies to thin, Rhabdocline needlecast is easily distinguished from Swiss needlecast based on the symptoms and signs evident on the individual needles. Remember, it is not uncommon for both diseases to be present in your fields or even on the same tree. Take the time for a closer look because with needlecasts, only constant vigilance will help you hold on to your needles!