Christmas tree growers are advised to be on the lookout for needle infections by fungal pathogens causing discolouration and defoliation. Severe cases of needle drop not only decrease tree value, but result in poor tree health and vigour. Although most conifers are somewhat susceptible to needle cast diseases, certain varieties of Scots pine, Douglas-fir and Spruce growing in locations favourable for disease may be severely affected.
Scots Pine Needle Casts
Scots pines are widely grown as Christmas and landscape trees. Three important needle blight and needle cast diseases affect Scots pines, especially within North American Christmas tree plantations. Due to there being different timings of management activities for each disease, it is important for growers to identify the problem correctly. Needle blight diseases normally present themselves in shaded parts of plantings on the lower branches, and also on the northern sides of individual trees.
Lophodermium Needle Cast
Lophodermium seditiosum, the causal fungus, can cause the greatest needle cast damage among Scots pine, and red pine trees. In Christmas tree plantings, short-needle varieties of Scots pine with seed origins from Western Europe (especially France and Spain) are particularly susceptible.
In late autumn, small, brown spots with yellow halos appear on the current-year foliage. Needles initially turn yellow, then brown, and are shed throughout the summer. By late summer, only tufts of green current-season needles may be left. Spores of the causal fungus, produced in tiny, black, football-shaped structures in infected needles, are spread by wind and infect current needles during moist periods from August to October. Infected needle symptoms are normally seen the following late winter or early spring. Where the fungus is active, a period of cool moist weather in late summer and early fall can lead to a destructive outbreak.
The photosynthetic capability is reduced and causes the growth of smaller sized trees.
Branches that bear only diseased needles may wither in early spring, and buds that survive produce abnormally small shoots and needles. Thus, Christmas trees may be stunted and disfigured.
Brown Spot Needle Blight
This disease is caused by the fungus Mycosphaerella dearnessii, (also known in the past as Scirrhia acicola) causing needle browning and defoliation of Scots pine. Other species susceptible to this disease include: Austrian, loblolly, mugo/mountain, pitch, red, shortleaf, Virginia, and eastern white pines.
In late summer small, dark spots becoming brown with yellow halos appear on current-season needles. Needles may become resin-soaked, eventually turning brown and begin to drop from the tree. Most needle shedding occurs the following spring after the first case of infection. Dark, oval fruiting bodies are found on needles, and during late spring and early summer spores of the causal fungus initiate new infections. Infections occur readily and the disease favours periods of warm, wet weather.
On pines with dense foliage, infection is most common on lower branches, leaving them bare throughout the winter if the tree has been severely infected. These branches can sometimes survive, producing new foliage in spring that may become infected the next season. Infected trees are not valued as Christmas trees.
Naemacyclus Needle Cast
The causal fungus is Cyclaneusma minus, previously referred to as Naemacyclus. Species susceptible to this needle cast disease include: Scots, Austrian, mugo/mountain, Virginia, and eastern white pines.
Symptoms of the disease begin to appear the year following infection. Normally during late summer, light green spots may be found on second- and third-year needles. Needles soon turn yellow with dark brown horizontal stripes/bands, eventually turning brown and dropping from the tree throughout autumn (fall), winter, and spring. Cyclaneusma minus produces tiny, elongate, protruding, tan-coloured fruiting bodies inside the dead needles. These structures yield spores that initiate infections primarily from April to June but also at low levels throughout the year depending on the weather. The disease favours mild, rainy weather during spring and summer.
Premature yellowing and casting of second- and third-year needles reduce the value of infected trees grown and sold as Christmas trees. Severely diseased trees appear yellow before needles fall. Current-season needles show no symptoms and are retained, even when infected.
Pine Needle Rust
Needle rusts can affect Austrian, Scots, Virginia, loblolly, mugo/mountain, and red pines. The most distinctive feature of these diseases is the prominent white to orange, blister-like, sack-shaped fungal structures on infected needles in spring. Needle rusts may occasionally destroy enough foliage to slow the growth of small trees, but normally they do little damage. The most common needle rust is caused by the fungus Coleosporium asterum.
Douglas-fir Needle Casts
Two fungi may cause important losses in young Douglas-fir plantations. Both cause needle cast, and trees sustain damage due to defoliation. Neither fungus affects pine.
Rabdocline Needle Cast Disease
This disease, caused by the fungus Rabdocline pseudo-tsugae, begins from infections occurring during cool, moist periods in spring. Succulent, young needles are infected from bud break through shoot elongation. By late summer, infected first-year needles begin to show a degree of yellowing followed by noticeable brown banding in late autumn or early spring, giving the needles a mottled appearance. Needles begin to drop in winter and continue falling the next season, with the tree eventually missing most of the previous year’s needles.
By improving ventilation and air movement around the base of the tree, this disease can often be adequately controlled. Removing weeds, improved spacing, and removing diseased lower branches and trees within the plantation also assists in controlling the spread of this disease.
Swiss Needle Cast Disease
This disease is caused by the fungus Phaeocryptopus gaeumanii. Infection often occurs during rainy periods in late spring and early summer. Infected needles may remain green for one or several years, producing spores from inconspicuous black fruiting bodies lined up in rows following the stomata on needle undersides. Diseased needles eventually turn yellow, followed by brown, with the oldest needles being cast off their branches.
Needle Cast of Spruce
The fungus Rhizosphaera kalkhoffii causes significant defoliation of spruce species in the wild and within Christmas tree plantations. Rhizosphaera needle cast is characterised by tiny black, fuzzy, fungal fruiting bodies emerging from the surface of infected green and yellowing needles. Diseased needles turn brown or purplish brown a few months to a year after infection. Infections occur during wet weather throughout spring and early summer. Colorado spruce is very susceptible to Rhizosphaera needle cast. Most other spruces are also susceptible, and even Douglas-fir and Austrian, mugo/mountain, and white pines may become infected.
Control of Needle Cast Diseases within Plantations
- Cut and remove severely infected trees.
- Remove and destroy live, infected branches on stumps of harvested trees.
- Shear trees in healthy plantations before shearing those in diseased plantations to prevent movement of fungal spores on tools from diseased to healthy trees.
- Do not shear infected foliage during wet weather.
- Remove weeds from the plantation to promote better air movement.
- Remove old pine windbreaks.
You can search CAB Abstracts for further information on the diseases covered within this article, for example a search on Lophodermium produces 987 records. In addition subscribers to CAB Abstracts Plus can search for global maps of plant diseases by clicking here. For further details of our products, especially “Distribution Maps of Plant Diseases” please click here.
Sources:
The Co-operative Extension Service Factsheet ID85 by John R. Hartman and Deborah B. Hill, University of Kentucky, College of Agriculture.