Mugo Pine Tree

(Question)

We have a 25 feet mugo pine that is turning brown – this has occurred in the last few days but is progressing rapidly – any suggestions/advice would be much appreciated.

(Answer)

Thank you for contacting the Toronto Master Gardeners with your inquiry.

I am sorry to hear that your mugo pine (Pinus mugo) is suddenly turning brown.  It must have been growing in ideal conditions over the years to reach its height potential of 25 feet – full sun, moderate moisture and soil to its liking.

You have not provided any specific information about the changes that have occurred in the needles – did they fade first, are they covered in white specs, did the needles turn brown from the top or the bottom of the tree, did you notice anything on the candles (new shoots) in the spring?  It is almost impossible to provide an accurate diagnosis without actually looking at the tree.

Mugo pines are susceptible to a number of pests and diseases. You might want to look at the web sites below to help you identify what is wrong with your tree.

The most serious pest that can affect a mugo pine is the Pine Wilt Nematode (Bursaphelenchus xylophilus).  The nematodes are transmitted by the vector sawyer beetles (Monochamus spp.).  Once the nematodes are inside the tree, they destroy the water conducting tissues, thus stopping the flow of resin to the tree.  The tree then dies quickly.  Regretfully there is no cure once the tree has been affected.

Older trees are most susceptible, especially if the growing conditions are not ideal, such  as sudden hot, dry weather.  Once infected, the needles change from green to grayish green, tan and then brown.  The wilt moves from the top of the tree downwards.

https://www.na.fs.fed.us/spfo/pubs/howtos/ht_pinewilt/pinewilt.htm

https://byf.unl.edu/pine-wilt

https://www.apsnet.org/edcenter/intropp/lessons/Nematodes/Pages/PineWilt.aspx

Another cause of  browning needles could be Diplodia tip blight, previously known as Sphaeropsis tip blight. This is a common fungal disease that affects conifers that are older than 15 years and are under stress.  The blight affects the new candles (shoots), they turn brown and fail to elongate.  By the end of the summer the shoots are totally brown and dead.  This disease usually begins in the lower branches.  The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) had an excellent web site with more detailed information:

https://www.omafra.gov.on.ca/english/crops/facts/diplodia.htm#fav

https://articles.chicagotribune.com/2013-10-01/classified/ct-sun-0929-garden-qa-20131001_1_tip-blight-sphaeropsis-pine

My advice would be to contact a certified arborist. He will be able to determine if your tree is suffering from a fungal attack or an insect pest and what will be the best course of treatment.

To find a certified professional arborist to help you with a tree problem, visit the Ontario branch of the International Society of Arboriculture here.

Good luck!