Foliar Diseases
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Rhabdocline Needle Cast
Rhabdocline spp.

Host:  Douglas-fir

Figure 189. Figure 189. Rhabdocline needle cast lesions on 1-year Douglas-fir needles.

Symptoms/signs:  Tiny chlorotic spots on one or both surfaces of first year and occasionally older needles arise in late summer or early autumn. These lesions enlarge and darken to purplish brown spots and bands that are conspicuous by late autumn to early spring. Numerous lesions coalesce and involve the entire needle except for a short basal portion. Discolored foliage is most conspicuous the following spring.

Biology:  Rhabdocline species produce apothecia May through June in needles still attached to the twigs. The apothecia appear as swellings up to several millimeters long and open by splitting the needle surface, usually near the midrib. Ascospores are airborne and penetrate developing needles directly through the cuticle. Only one infection period occurs per year, but it may last several weeks. Needle colonization increases during the fall as symptoms become apparent. Infected needles drop during the winter, making missing foliage the only noticeable sign of disease.

Figure 190. Figure 190. Needle cast due to Rhabdocline needle cast.

Effects:  This is the most important needle disease of Douglas-fir. Damage can be severe in Christmas tree plantations and ornamental nursery stock. Seedlings infected in nurseries can become foci of subsequent damage in plantations. Occasional outbreaks in forest stands can appear spectacular, but trees usually recover.

Similar Insects and Diseases:  The fruiting body of Rhabdocline may be confused with secondary fungi.

References:  83, 93