Emerald green arborvitae dying

Arborvitae is a catch-all name for species of evergreen conifers in the Thuja genus. Related to cypress, depending on species, it grows as a tree with scale-like foliage. If you’re looking for a wind or privacy screen you may want to consider one of the taller species, such as the Green Giant (Thuja standishii x plicata ‘Green Giant’)

Whichever you grow, you’ll be rewarded with an easy-care habit with few requirements. This doesn’t mean the tree won’t have problems, however. In fact, arborvitae is susceptible to a number of fungal diseases.

Arborvitae fungal diseases

Blight, including needle blight, caused by Pestalotiopsis funerus, is probably the most common culprit when your arborvitae comes down with the nasties. This pathogen generally attacks plants that are already stressed from other causes, such as by pests.

Then, there’s Botrytis twig blight, which infects plants that are under stress from poor nutrition, age, incorrect light or temperature or air pollution.

The Oriental arborvitae is particularly susceptible to Berckmann’s Blight and it can be quite damaging to the plant.

Symptoms of a fungal problem

Anytime you notice dead spots, browning branch tips or dying twigs, especially on an arborvitae that has a pest infestation, suspect a fungal disease, usually a type of blight.

Shoot blight begins at the base of new twigs, causing wilting and discoloration. Botrytis twig blight appears as a gray fur on the twigs and is most common in regions with high humidity.

Leaf blight begins on the arborvitae’s lower branches, near the trunk, and gradually discolors the plant to the top.

Control

Control of these arborvitae fungal diseases consists of pruning away and destroying infected branches and twigs. Spray the plant with a copper fungicide in the spring and then again, as a preventive measure, in early fall.

Wear protective clothing during the application and follow the label’s instructions and precautions. Don’t apply more fungicide than the manufacturer recommends and make sure all infected parts of the arborvitae are covered with the spray.

Prevent fungal disease on arborvitae

Many arborvitae fungal diseases are restricted to plants under stress. Improved growing conditions helps relieve infection rates.

  • Learn how much and how often to water and fertilize the arborvitae.
  • Spread mulch on the soil around the plant to avoid splashing soil-borne fungal spores onto the plant when you water or when it rains.
  • Protect cold-sensitive arborvitae in the winter.
  • Use caution when mowing around the plant as some pathogens are introduced to the plant through wounds in its bark.

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What’s behind die back, brown patches in arborvitaes?

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Question: I have an arborvitae hedge. The south side of each shrub is turning brown, and there are dead patches/dead spaces in the hedge. What is the problem?

Answer: If the south side of your arborvitae hedge turns brown, it may be caused by some sort of stress. It could be too little water, drying winds, high temperatures, injury to the roots or damage to the trunk. According to the PNW Plant Disease Handbook, die-off of arborvitae from physical causes is quite common in the Pacific Northwest.

One-sided die off on a hedge sometimes can be traced to a history of nearby chemical applications, such as drift from a neighbor’s use of herbicide.

Since the damage to the south side of your hedge is probably more likely from a physical problem than a biological cause such as a disease or insect pest, there’s no silver bullet cure. Here is what the PNW Handbook recommends:

  • Prune the affected foliage to remove the unsightly tissue. Pruning allows more light in and stimulates growth of new foliage.
  • Water regularly at the base of the plant during dry periods.
  • Fertilize in spring to stimulate production of new foliage.

You mention dead patches in your arborvitae hedge as well. When there is die-back of individual branches here and there, it may indicate root problems. It is time to think about what is going on below the soil surface. Root rot, a set of fungal diseases, could be the problem, as they are fairly common with arborvitaes in our area.

Sadly, the only way to tell if a plant has root rot is to dig down and actually examine some fine surface roots. So get your shovel and dig shallowly until you run into fine roots of one plant. What do the new fine roots look like? Cut into one with a sharp knife. Healthy fine feeder roots should be brown on the outside but have a white internal core and white tips. If your fine roots are soft and brown and can be peeled or broken off easily, your hedge may have root rot.

More: Two garden chores you can do mid-winter

Do you water your hedge in the summer? Heavy watering contributes to root rot. Does your hedge get hit with an overhead sprinkler? This also may contribute to the problem. Several fungal species cause a whole shovel full of different root rot diseases.

Poor drainage, saturated soil conditions and warm temperatures encourage root rot in arborvitae. If your hedge plants are planted too deep, or your soil is compacted or pure clay, this also encourages root rot diseases. Is your hedge mulched thickly? Could be a culprit as well.

Many modern houses are built on terrible soil, subsoil or fill that is heavy with clay. If this is the case, your hedge may be dead within a few years.
Consider planting a row of shrubbery that can tolerate clay soil. Or build a berm of well-drained soil and plant new shrubs into the raised and amended soil.

My all-time favorite clay-tolerant shrubs for our climate include Pacific wax myrtle (Myrica californica). It is evergreen, native and grows thickly. Also, there’s several hardy Viburnum species that make great hedges in clay soils.

More: Give houseplants boost with fertilizer, but not too much

Arborvitae (Thuja occidentalis)-Root Rot

Cause Several. Many samples sent to the OSU Plant Clinic are from poorly drained and wet sites where oxygen depletion to the roots is likely. Occasionally, pathogens such as Phytophthora lateralis or Armillaria ostoyae (may be refered to as A. solidipes in some publications) are associated with rotted roots. The latter may be found on ground recently cleared of native forest vegetation.

Cultivars and species differ in susceptibility to Phytophthora root rot. Western red cedar (Thuja plicata) is most resistant, T. occidentalis ‘Pyramidalis’ is intermediate, and T. occidentalis ‘Smaragd’ is most susceptible.

Symptoms Aboveground symptoms are general discoloration of foliage and eventual death. Roots are discolored below the bark in the region of the cambium. Roots in advanced stages are highly decomposed and break off easily.

Cultural control

  • Avoid reusing pots from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
  • Plant only in deep, well-drained soil.
  • Correct drainage if water puddles near bushes.
  • Use resistant species or cultivars.
  • Permanently removing soil and exposing the crown and main root areas has been effective for Armillaria control in tree fruits grown in California and Australia and may be of benefit for managing infected bushes in the Pacific Northwest.

Chemical control Not recommended unless Phytophthora sp. has been identified. Even then, plants are most likely too far gone for chemical therapy to work. The Group 4 and Group P7 fungicides used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant thereby delaying symptoms that might have developed. Once chemical activity has subsided with time, the organism can resume growth within infected plants. Rotate fungicides from different groups that have a different mode of action for resistance management.

  • Aliette WDG at 2.5 to 5 lb/100 gal water for a foliar application. Group P7 fungicide. 12-hr reentry.
  • Alude at 5 to 10 fl oz/100 gal water applied as a soil drench at a rate of 25 gal solution/100 sq ft. Follow application with irrigation. Use only once per month. Can also be used as a foliar spray at 26 to 54 fl oz/100 gal water at 14- to 21-day intervals. Group P7 fungicide. 4-hr reentry.
  • Empress at 1 to 3 fl oz/100 gal water can be used for seedlings. Group 11 fungicide. 12-hr reentry.
  • Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 7 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
  • Mefenoxam 2 AQ at 0.98 to 1.96 fl oz/100 gal water as a soil drench or at 1.23 to 2.45 fl oz/1000 sq ft followed by at least 0.5 inch rain or irrigation. Group 4 fungicide. 48-hr reentry.
  • MetaStar 2E at 1 to 4 fl oz/100 gal water as a drench. Group 4 fungicide. 48-hr reentry.
  • OxiPhos at 1.3 to 4 quarts/100 gal water as a foliar spray. Group P7 fungicide. 4-hr reentry.
  • Phospho-Jet is registered for tree injections. Rates are based on tree size. Group P7 fungicide. 4-hr reentry.
  • Subdue MAXX at 1.25 to 2.5 fl oz/1000 sq ft, irrigated in with 0.5 inch water. Group 4 fungicide. 48-hr reentry.
  • Terrazole 35 WP at 3.5 to 10 oz/100 gal water as a soil drench. Use only in commercial nurseries and greenhouses. Group 14 fungicide. 12-hr reentry.

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