Fir needle cast (Lophodermium lacerum)
- Latin name: Lophodermium lacerum Darker
- French name: Rouge (Lophodermium lacerum)
- Division: Ascomycota
- Class: Rhytismatales
This disease is widely distributed throughout the range of its primary hosts. In the 1960s, Lophodermium lacerum was already considered a weak parasite. In the west, it is mostly found on grand fir, while in the east it is found on balsam fir.
Damage, symptoms and biology
The most conspicuous symptom of needle cast is a red, yellow or brown discoloured needles, which may later turn grey. This discolouration is frequently restricted to 2-year-old of foliage. Discoloured infected needles are often intermingled with a few healthy green needles, unlike frost or drought discolouration, when whole branches or portions of the crown are uniformly discoloured.
Needle cast is caused by a related group of fungi (Ascomycetes), which cause more damage to coniferous foliage in North America than any other group of fungi. See balsam fir needle cast (Isthmiella faullii), needle cast caused by Lirula nervata and Lirula mirabilis as well as Lophodermium piceae. Severe infection can result in significant growth reduction.
Typically, only a few of the many needle cast diseases are known to result in growth loss. Seedlings may be severely damaged if a high percentage of the foliage is infected. Heavily infected trees have an unsightly appearance caused by needle discolouration.
The degree of damage can be influenced by the age of the host tree; older trees are more resistant to damage and infection of older needles is eventually harmless.
Moist conditions influence spore dissemination and germination, and thus the frequency and severity of infection. Needle cast diseases are more common following wet springs. Lower branches and understory trees are frequently the most seriously affected as the more humid conditions favour infections.
Generally two types of fruiting bodies develop during the fungi’s life cycle:
"asexual" fruiting bodies (pycnidiae), which occur as small dots on the upper surface of the needles; and
"sexual" fruiting bodies (producing ascospores), which are generally larger, occur along the mid-ribs on the lower needle surface.
Each fungus responsible for needle cast has a different life cycle and many are incompletely known; that of Isthmiella faullii is presented as an example.
The current year's needles are infected first, but they do not show any damage. The following spring, brown spots appear, eventually covering the entire surface of the needles by mid-summer. The first fruiting bodies form on the upper surface of the needles and discharge spores in late summer or early fall. It is unclear exactly what role these spores play, but they may give rise to the second type of spores. Ascospores form in mid-summer on the needles infected two years earlier. The “sexual” fruiting bodies called hysterothecia (because the ascocarp has a slit-like opening), create a black line on the lower needle surface. During rainy periods, asci will release ascospores that are able to infect new shoots the following spring. The ascospores are dispersed by the wind. Prolonged wet periods contribute to infection.
No control measures are planned in the forest sector. However, Christmas tree growers might consider preventing infection on lower branches by clearing the base of trees and cleaning out grasses growing between rows in order to reduce moisture levels, thus minimizing conditions conducive to new infections. Keeping fir and spruce trees in windbreaks should also be avoided in order to reduce potential sources of inoculum. Lastly, chemical control using copper oxychloride-based fungicides (the only fungicide registered in Canada for controlling needle cast and rusts) applied every 10 days after bud break until wet spring conditions subside could be considered. This spraying should be done by trained personnel who have received the required training.