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Dothistroma Needle Blight
Mycosphaerella pini Rost. in Munk
(= Scirrhia pini Funk & Parker)
(Anamorph = Dothistroma septospora (Dorog.) Morelet)
Ascomycotina, Dothidiales, Dothidiaceae
Hosts: Mycosphaerella pini is found only on pine. In B.C., it has been reported on lodgepole, Jeffrey, ponderosa, Monterey, black, bishop, and maritime pine as well as some hybrid pine species. Elsewhere in North America it has also been found on shortleaf, red, western white, and Scots pine and in rare cases on Douglas-fir, European larch, and Sitka spruce.
Distribution: This fungus is widely distributed throughout the range of its hosts in B.C.
Identification: Trees affected by Dothistroma needle blight tend to have thin crowns with discoloured and dead needles (Fig. 54a). The lower crown is often the most severely affected (Fig. 54b). Infection of needles of all ages occurs throughout the year at the coast and from spring to autumn in the interior, resulting in the development of yellow-brown to red-brown lesions or bands (Fig. 54c). The red colour is due to the presence of a fungal toxin. A characteristic feature of the lesions is the abrupt change from diseased to healthy green needle tissue. Needle tips distal to lesions die and turn brown, while the base of the needles generally remain green (Fig. 54d). Fruiting bodies (stromata) form on dead needles appearing as small black structures breaking through the epidermis. Older diseased needles are shed, often forming "lion's-tail" branches, with only terminal needles remaining.
Microscopic Characteristics: Ascostromata black, multiloculate, subepidermal, erumpent, 200-600 x 95-150 µm; tissues pseudoparenchymatous, cells 7-14 µm diameter; locules globose, in longitudinal order, 40-86 µm diameter. Asci cylindric or clavate, bitunicate, apex rounded, 8-spored, 46-52 x 8-10 µm. Ascospores hyaline, 1-septate, fusiform to cuneate, 13-16 x 3-4 µm.
Conidial stromata linear, subepidermal, erumpent, dark brown or black, 125-1500 µm long, 5-45 µm wide, up to 600 µm high. Conidial locules parallel to the longitudinal axis of stroma, without a distinct wall. Conidia hyaline, 1- to 5- but usually 3-septate, blunt at the ends, straight, slightly curved or bent, 16-64 x 3.5 µm. Conidiophores numerous, approximately the same size as the conidia, hyaline or amber, dense, unbranched, producing conidia at their tips.
Damage: Where environmental conditions favour infection, this disease can spread rapidly and cause significant damage. Trees can be defoliated within weeks, and mortality is common with repeated attacks.
Remarks: This disease is also commonly known as red band needle blight. The life-cycle is completed in 1 year at the coast but requires 2 years in most other parts of the continent. The fungus causing the disease is most commonly observed in its conidial state (Dothistroma septospora).
References:
Funk, A. 1985. Foliar fungi of western trees. Can. For. Serv., Inf. Rep. BC-X-265.
Figures
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Figure 54a: Lodgepole pine plantation affected by Mycosphaerella pini in east-central B.C.
Figure 54b: Lower crown discoloration typical of Mycosphaerella-infec ted western white pine.
Figure 54c: Red banding symptoms on infected needles.
Figure 54d: Mycosphaerella-infect ed needles with discoloured tips and green needle bases.