Needle cast is a fungal disease of coniferous trees that causes the inner needles to turn brown and fall off the tree (cast off). The disease typically begins on the lower and inner branches, where there is more shading, less air flow and more moisture. The disease then gradually works its way up the tree. After repeated infections, trees begin looking thin and sparce, with bare twigged branches, especially towards the bottom and on the northern side. If the disease is severe all but the current seasons growth will eventually be shed, and lower branches will die completely.
There are many genera responsible for needle cast, each with a different life cycle and host preferences. The 3 most common fungal genera, responsible for needle cast disease in spruce trees in North America are Rhizosphaera, Stigmina and Lophodermium.
Rhizosphaera Needle Cast
Rhizosphaera is comprised of several species and are known to infect various species of a variety of conifers including Spruce (Picea), True Fir (Abies), Douglas Fir (Pseudotsuga), Pine (Pinus), Cedar (Cedrus), Hemlock (Tsuga) (Hudelson B., (Revised: 03/03/2021)) and larch (Larix) (Brazee N.J., (Updated: April 2022)). The species Rhizosphaera kalkhoffii is the one most encountered on spruce, especially Colorado Blue Spruce. To positively identify Rhizosphaera and its species samples will need to be sent to a laboratory for microscopic evaluation, as other needle cast diseases or conditions appear very similar.
Signs and Symptoms
The first symptom of infection is a faint yellow band or mottling on infected needles along with small immature fruiting bodies. To the untrained eye this initial infection is difficult to detect. By spring these infected needles die and usually turn a purplish-brown, although they may also appear to be brown, reddish brown or tan in colour. The fruiting bodies of the fungus (pycnidia) will begin emerging through stomatal pores on these discoloured needles and release spores that can be splashed or blown to new foliage. The mature pycnidia are very tiny, but they are readily visible with the aid of a hand lense. They appear as dark, smooth, tiny little balls that are arranged in fine rows up and down the needles. They may or may not have a tiny white cap on their tops, which is the plug from the stomata pores. On a healthy needle you will see fine white lines, which are the stomata (the breathing pours). For regions that can experience 2 infections per year the new growth will turn purplish brown and begin dropping from the tree by fall (University of Maryland Extension, (2021)) otherwise they discolour by spring. With each new infection inner needles continue to die off and drop from the tree making the tree appear thin, sparce and twiggy, especially towards the bottom and on the northern side. In severely infected trees, that have experienced repeated bouts of defoliation, only the new growth may remain on the tree. Eventually the whole tree may die.
Rhizosphaera kalkhoffii has a one-year life cycle (Zeleznik J., Walla J., (Revised June 2019)). With some regions capable of experiencing 2 infections per year (University of Maryland Extension, (2021)). The fungus overwinters on needles that were infected the previous season, or dead needles, either as “(mycelium) inside needles and/or pycnidia on the outside of infected needles” (Thrush P., Taylor N. J. and Peduto Hand F., (2021)). For areas experiencing only one infection period per year needles that were infected the previous season die by early spring and turn purplish brown. In late spring, when the weather is warm and moist, the fruiting bodies of the fungus (pycnidia), begin erupting through the stomata and releasing ascospores. The ascospores can be splashed or blown onto the new growth or spread to new susceptible trees. In order for them to germinate the needles must remain constantly moist for 2-3 days. This condition usually exists in regions that have rainy spring weather, but continuous overhead irrigation will also create the condition. Under these moist conditions the ascospores penetrate needles and begin producing mycelium which will kill the needle by spring. For regions experiencing 2 infections per year the new growth will die and turn purplish brown and begins dropping from the tree by fall (University of Maryland Extension, (2021)).
Management and Control
- In areas where the disease is quite prevalent choose varieties of spruce that are more resistant, such as Norway or white spruce.
- Avoid overhead watering. The key to preventing and managing this disease is keeping the needles as dry as possible.
- When planting blue spruce locate them in a full sun area with good air circulation all around it. Keep in mind it’s size at maturity, and give it plenty of space to mature, in order to avoid the humid conditions that come with overcrowding.
- Keep dead wood pruned out of the canopy. Prune during the dry season and disinfect all pruning tools to avoid spreading the disease.
- Keep weeds and grasses under and around these trees mowed to reduce moisture to lower branches and improve air circulation around them.
- Trees that are under stress are most vulnerable to the disease so try to avoid tree stress by:
- Planting them at the correct depth.
- Cut or loosen encircling roots at the time of planting.
- Keep them watered during periods of draught.
- Fertilize them once a year, early in the season.
- Use lawn mowers and trimmers with care to avoid injury to roots or trunk.
- Monitor the trees regularly for insect pests and disease and manage any found.
- Trees can be treated with “copper oxychloride-based fungicides the only fungicide registered in Canada for controlling needle cast and rusts” (Gov. of Canada, (modified: 2015-08-04). The first application is usually made after budbreak and then reapplied every 10 days until while conditions for infection exist (Gov. of Canada, (modified: 2015-08-04). In areas where fall infections take place applications may be required from mid-August to early September. Chlorothalonil was previously registered for use in Canada against Rhizosphaera but is currently under review and its use is proposed only for greenhouse ornamental uses (Gov. of Canada, (modified: 2022-06-15)). Note copper sprays can damage foliage so use with caution and follow manufactures instructions. Better yet hire professional. Copper is toxic to bees and fish and does not break down in the soil and may eventually become toxic in the soil.
Stigmina Needle Cast
Stigmina needle cast is very similar to Rhizosphaera needle cast, and it is possible for both diseases to be present on the same tree. This fungal disease is caused by the pathogen Stigmina lautii, which infects spruce trees, particularly Colorado blue spruce, but also black spruce, white spruce, Norway spruce (Montana State University, (n.d.)), Oriental spruce and Serbian spruce (Brazee N., (Updated: Dec. 2017)).
Signs, Symptoms and Life Cycle
This fungal disease looks almost identical to Rhizosphaera needle cast, and a laboratory is usually required for accurate identification. There are subtle differences between the two however, such as the dark fungal fruiting bodies of Stigmina lautii appear (under magnification) to have fuzzy or feathery tops (Walla, J.A. and Kinzer, K.M. (2008)), whereas Rhizosphaera kalkhoffii fruiting structures are smooth. Stigmina infected needles are also said to appear to look dirtier than Rhizosphaera infected needles, (University of Illinois Extension, (n.d.)). Microscopically Rhizosphaera kalkhoffii are smaller and aseptate (only have septa at branching points on the hyphae) compared to Stigmina lautii which are a bit bigger and septate (their hyphae are divided into sections by walls called septa) (Walla, J.A. and Kinzer, K.M. (2008)). There is also a difference in their life cycles with Rhizosphaera kalkhoffii having a one-year life cycle (Walla, J.A. and Kinzer, K.M. (2008)) or may have 2 generations a year (University of Maryland Extension, (2021)) and Stigmina lautii having a two-year life cycle (Zeleznik J., Walla J., (Revised June 2019)). One final small distinction is that Stigmina lautii is generally present on 2nd year needles while Rhizosphaera kalkhoffii can be a bit more variable (Walla, J.A. and Kinzer, K.M. (2008)).
Stigmina lautii has a two-year life cycle (Zeleznik J., Walla J., (Revised June 2019)). In the first year of infection the tree to the naked eye, looks unaffected. Under magnification you may be able to see a faint yellow band or bands, running down the needles, and immature fungal fruiting bodies in rows. The following year, these bands become wider, and as the needles die off their colour darkens to brown, purplish brown, reddish brown or tan by fall. At this point the fruiting bodies of the fungus, called sporodochia, may begin bursting through the stomata, or they may wait until spring. In spring during mild rainy weather, they begin sporulating and spores are released to infect new growth and other nearby susceptible hosts. It is possible for sporulation to occur at any point during the growing season as long as the conditions are right, but the majority will sporulate in spring while the new growth is pushing forth. Discoloured needles begin dropping from the tree over the remainder of the year. Infected trees begin looking thin, sparce and scruffy, with bare twigged branches usually developing at first towards the bottom of the tree but working their way up as the disease progresses.
Management and Control
Use the same management strategies as suggested under Rhizosphaera.
Lophodermium Needle Cast of Spruce
This Spruce needle cast disease is caused by the fungal pathogen Lophodermium piceae. It occurs primarily in North America where it can potentially infect Balsam fir, black spruce, Colorado spruce, Sitka spruce, subalpine fir, and white spruce” (Natural Resources Canada, (Modified: 2015-08-04)). Lophodermium piceae has also been found to be an endophyte, where it can live inside spruce needles for long periods of time without causing disease. It has also been found to “play an important role in needle-litter decomposition in boreal forests” (U.S. Department of Energy (DOE) Joint Genome Institute (JGI), (n.d.)).
Signs and Symptoms
The first signs of an infection are yellow spots appearing on 2-year-old and older needles. The center of the spots turns brown leaving a yellow halo. Then whole needles begin to die. Dark fungal fruiting bodies (ascomata) form on dead needles which appear oval in shape with a slit down the middle. They release their spores to infect more needles, then the needles begin gradually dropping from the tree giving the trees a thin, sparce look. After repeated infections inner foliage will have dropped away leaving only the new growth.
It appears this fungus can live symptomless within spruce needles for several years before completing its life cycle, at least in Norway spruce in Europe (Osorio M., Stephan B. R., (1991)). Infections occur in the fall during cool wet weather (University of Illinois Extension, (n.d.). By spring, infected needles will appear to have brown spots with a yellow halo, then needles will completely brown. Dark fungal fruiting bodies (ascomata) form on these dead needles and release ascospores, to infect more needles. The dead needles then begin falling off the tree. The newly infected needles may harbour the fungus for several years, without exhibiting any symptoms. When the fungus resumes its life cycle it produces asexual fruiting bodies conidiomata and conidia; then the sexual fruiting bodies (ascomata) form on the dead needles and release their ascospores to infect more needles (Osorio, M. & Stephan, B. (2007)).
Other Spruce Needle Cast Pathogens
Lophodermium uncinatum is primarily damages true fir trees, but it can also infect Sitka spruce, and white spruce.
Lirula macrospora can infect Black spruce, Colorado spruce, Engelmann spruce, red spruce, Sitka spruce, white spruce (Natural Resources Canada, (modified: 2015-08-04)).
Winter injury can look similar to a needle cast disease. Needles will turn brown or purplish brown over winter, but needle browning affects the tips of needles and branches more than interior needles, unlike needle cast diseases which only see interior needles browning. Winter injury often only affects one side of a tree and browning can occur at the top of the tree. Dark fungal bodies will also be absent on needles killed by winter injury.
Spruce needle miner damage can cause needle browning, but the needles are cut off and bound to the branch in small clusters with webbing, rather than dropping from the tree.
Lirula needle blight causes the inner needles of spruce trees to turn yellow then gray. Caused by the fungus Lirula macrospora. Occurs in Colorado blue spruce and white spruce. White spruce, including Black Hills spruce, is more likely to be damaged.
Spruce needle rust caused by the fungal pathogens Chrysomyxa weirii and Chrysomyxa ledicola, can also cause premature needle drop. However, it is the tips that are affected rather than inner foliage, and their fungal fruiting structures are orangish rather than dark brown or black.
Cytospora canker (Leucostoma kunzei) will cause needle browning and needle drop but entire branches die, often randomly throughout the canopy. Cankers can be found on the trunk and branch along with resin flow and black fungal fruiting bodies form on these cankers rather than the needles. Colorado blue spruce and Norway Spruce are the most vulnerable.
Photo credits: all photos by the author
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