Updated: Nov 11, 2018
Arbor vitae's (or Arborvitae) Latin name is Thuja occidentaliswhich is important because the species has its own, short but important, list of pests. Leyland cypress (Cupressus x leylandii) are often called Arborvitae as well. This is one reason for the Latin naming of plants - to differentiate. Ironwood for example, is the common name of more than five different species of trees including those native to the Sonoran Desert and the Appalachian Mountains.
In the cedars, Thuja, and junipers, there is a pathogen causing a canker disease and I have yet to see a hedge of Arborvitae or Leyland around here without it this spring/summer. It's disconcerting because the two disease causing agents (that's science jargon for known pest- old habits are hard to break) have no known cures. To make the situation worse, the two cultivars which are or were resistant, are now being colonized by the pests. That means these diseases/pests have probably mutated allowing them to infect plants that were at one time resistant (virulent). This was my favorite research subject in graduate school - hybridization and mutation of fungal pathogens allowing them to expand their physical and host range. Great for thinking about but not great for you beloved green fence!
So...What to do???
I've updated this to suggest the best thing to do is to replace your trees. After talking with a bunch of arborists, foresters, and pathologists - this is the best way to combat the spreading disease. It may however, not be the best for your budget. You'll have to do the math on how much it costs to cut down and replace the trees versus bi-annual treatments. I'm guessing removal and replacement will save you money if you consider a 5+ year timeline.
Other options are:
1. Planting the trees farther apart initially or thinning the trees to increase distance between them (tree removal), or thinning the canopy are ways to slow the disease down. These pathogens don't do well in sunny, airy locations so make your trees' inner canopy sunnier and airier.
2. The other issue is the soil quality. I'm recommending my clients add a product I am partial to because it is a living fertilizer with the ability to increase the plant's immune response. That in combination with proper mulching (see that blog post please) and removing the fallen leaves at early spring and mid-summer annually.
3. When pruning the canopy, clean the tools every singlecut, I meanevery, with unscented Lysol. If you're going to be keeping the cut material around your property then spray it with a 10% bleach solutionto reduce the probably of spores floating into the wind and back to your trees.
4. There is a 2011 extension paper that recommends one type of fungicide. It will not cure the plant, but it shows some ability to slow the spread of the disease. Have your arborist apply it annually or bi-annually depending on the density of the trees and their response to the treatment.
5. NEVER water the canopy. The plants need to dry out a bit but under prolonged drought it is good to supplement water them. Soak at the ground-level. If you have sprinklers reduce their height to keep from spreading the disease via water droplets.
6. Remove the fallen duff twice a year (spring and summer). It's a great place for spores to reinfect new parts of the plant. I recommend laying down a cedar mulch and spraying that with a product called cedarcide - that will at least reduce the number of living spores in the mulch and duff around your trees.
Finally - plants resiliant to the disease include our very own Eastern Red Cedar. It's not actually resistant to the disease but because the trees are adapted to local soils and climates its immunity is not compromised. Leylands and other non-natives don't do well in our alkaline & clay soils. It's as like depriving your body of key nutrients - your immune system and ability to fight disease will be depressed. There are a couple of other beautiful evergreens that are disease resistant - please contact me for more information. To read more about the research on this world-wide issue see:
Oh and plant your Eastern Red Cedars 500' apparat to accommodate their growth or plant them closer to get that immediate green wall with a plan to have some removed to avoid the problems already mentioned above!