Last autumn I wrote a blog post about conifers dropping needles in autumn. There’s a difference between needle drop (a natural process of renewal) and needle cast (a disease symptom), but my post didn’t do enough to make the distinction between the two. So, I’m going to re-address the topic, and hope to be clear this time.
This morning I looked at the 30-foot (10-m) white cedar filling a corner of the front garden, and noticed that a third of its foliage is brown and beginning to fall. This isn’t a reason to panic, because October is the month when conifers shed their oldest needles in preparation for replacement growth next year. Most of the brown needles are on the ground by the end of November, and they make a beautiful mulch under and around the tree. They’re also an excellent soil amendment if you’re digging holes for new plants.
The appearance of conifers with so much brown foliage in October might be alarming, but this is a natural process that allows the plants to renew their needles over three or four years. But if conifers drop copious amounts of brown foliage in spring or summer, and continue dropping needles into fall, this may indicate they are in the grip a fungal needle cast disease.
How can we tell the difference? The autumn needle drop associated with natural renewal is uniform — it appears all over the plant, visible at every level, from top to bottom. In spring, there will be obvious signs of fresh needle growth. However, if the needle drop is associated with a fungal disease, brown foliage will initially develop at lower levels, and progress upward over a period of years. The first symptoms are light green to yellow spots appearing on individual needles, eventually turning to red or brown. As the infection spreads throughout the shrub or tree, branch dieback will cause empty spaces in the plant’s structure. The majority of infected foliage browns and dries up in winter, then begins dropping in spring and summer, and continues to defoliate into fall.
Simple visual observation should be enough to figure out if a conifer is renewing its foliage or if it’s infected. Needle cast diseases rarely kill a conifer outright, but they will disfigure the plant’s form and make them unattractive. Spruce and pine are the most likely victims, and you can read more about the individual disease pathogens affecting them in a fact sheet from the University of Guelph Pest Diagnostic Clinic: http://www.uoguelph.ca/pdc/Factsheets/Diseases/NeedleCastPine&Spruce.htm.
I’m optimistic about the health of my conifers. Some are 75 years old, and others have been here for just a few years. They all get some irrigation in summer, and the clay loam soil in this garden drains well. I suspect poor drainage is an invitation for fungal diseases in conifers; a well-drained site is the best insurance for a healthy plant.