Usually, when there is a problem with your yew (Taxus) the first symptom you will observe is a yellowing of the needles. Yellowing needles however can be caused by a variety of things such as the soil being too wet or too dry, disease, insect pests, winter desiccation, salt damage, animal grazing, animal urine, transplant shock, nutritional deficiency or too high or too low of a soil pH. Properly diagnosing the problem takes a little detective work. To complicate things, it is also quite possible that there could be more than one cause. When a shrub is under distress, whatever that cause may be, it is quite common for insect pests and disease pathogens to move in and attack the weakened plant. Treating the secondary problem will not be effective until the underlying cause is also addressed. So, examine yellowing Yew carefully for all signs and symptoms.

Yew Diseases

Phytophthora Root Rot

Phytophthora root rot (Phytophthora cinnamomi) is a common yet serious disease of yews. It occurs in overly damp soils. These fungus-like organisms (water mold), cause the fine feeder roots to rot and turn black, leading to reduced water uptake to the plant. Infected yews will fail to thrive, needles will yellow or turn reddish brown. New growth will be affected first. As the disease quickly progresses stem cankers may occur, and the entire shrub eventually turns brown and dies.

It is very difficult to treat this disease. If caught early, approved of fungicides applied as a soil drench may help, but once the disease is advanced it is usually best to remove and destroy these plants, including the infected soil.

Yew Needle Blight

Yew Needle Blight (Sphaerulina taxi), is a fungal disease that affects the current seasons growth. Needles turn light green then grey or reddish brown then are cast from the shrub. Black fruiting bodies can be observed on affected needles.

Remove cast needles as best as you can to reduce the fungal spores and avoid overhead watering. Prune out diseased tissue. Disinfect all pruning tools and gloves that come in contact with the plant.

Cryptocline Needle Blight

Cryptocline Needle Blight (Cryptocline taxicola) is another fungal disease of Yew. Symptoms include chlorotic spots or bands, yellowing of needles followed browning and needles being cast. Black fruiting bodies can be observed on affected needles. Old and new growth can be affected.

Copper based fungicides may be applied to help protect uninfected needles. Remove cast needles as best as you can to reduce the fungal spores and avoid overhead watering. Prune out disease tissue. Disinfect all pruning tools and gloves that come in contact with the plant.

Armillaria Root Rot

Armillaria Root Rot is caused by Armillaria spp. of fungus commonly found in soils, especially forests, that primarily serves to decompose tree stumps, dead wood, and roots. These opportunistic fungi however will also infect live tree and shrub roots, from a wide variety of species, especially ones that are under stress, and the infection is usually fatal. Unlike phytophthora root rot this fungus has very distinctive and visible fruiting bodies that appear in the fall as honey-colored mushrooms that are located at or near the base of infected trees and shrubs. Fungal spores are released from these mushrooms and carried on the wind in the fall. Infection can take place in one of 2 ways; by the wind-blown spores coming into contact with wounds on stems or roots or by live roots being penetrated by rhizomorphs (black root-like structures) that have spread from infected trees or shrubs. These rhizomorphs spread like a netting over the decaying wood and through the soil to other trees and shrubs, as much as 20m away, sometimes more. Once inside the root the fungus spreads to the sapwood at the base of the tree, causing decay and eventual death. Infected Yews will begin to decline, growth will slow, and foliage will begin turning yellow then brown. The decline and eventual death my occur rapidly or persists for several years.

There is no cure but removing the mushrooms before they release their spores and removing dead stumps and roots as well as the rhizomorphs will help prevent its spread to other trees and shrubs.

Insect Pests of Yew

Taxus Mealybugs and Grape Mealybugs

Taxus Mealybugs (Dysmicoccus wistariae) and Grape Mealybugs (Pseudococcus maritimus) are 2 common but serious pests of yews. They suck the plant juices out of the needles causing them to yellow and eventually die. Their feeding also creates honey dew, which is a sticky substance which is secreted by the mealy bugs. This honey dew attracts ants and wasps who like to feed on it. Blach sooty mold can also often be found growing on the honey dew. Yews heavily infected with mealybugs will also experience yellowing then browning of needles, twig dieback along with stunted or distorted growth. The insects themselves can be difficult to spot as they are quite small between 5-10mm long by 3-5mm wide, when full grown, and usually well-hidden within the foliage on branches and branch forks. They are however immobile as adults and are covered in white mealy-like secretions that stand out from the dark foliage. The egg masses are also white, appearing as cottony sacs laid at branch crotches and cracks and crevices in the bark. 

To help control this pest prune out heavily infected branches, spray with horticultural oil in March, hand remove egg masses and adults using a cotton ball soaked in rubbing alcohol and control your ant populations (which can carry mealy bugs to other shrubs). The crawler stage can also be sprayed with insecticidal soap or summer oil. For more information on mealy bugs check out my article Mealy Bug Damage to Yews.

Black Vine Weevil

Black Vine Weevil (Otiorhynchus sulcatus) are a common pest of yew and can be quite damaging. The grub like larvae are the most damaging stage of this insect pest. They live in the soil and feed on the roots which reduces the flow of water and nutrients to the plant. Symptoms appear as decline, reduced growth, yellowing of foliage and dieback. These grub-like larvae are off-white in colour with a brown head and measure about 6mm (1/4″) in length. They are active from mid-summer to fall, overwinter in the soil and become active again in spring. The adults are blackish coloured beetles that measure about 1 cm (1/3″) in length, with a pronounced, but short snout and long antennae. They have hard pitted upper wing covers, that also appear ribbed-like and subtly speckled. The adults are all flightless females who begin emerging from the soil in late spring. They feed on the foliage at night, cutting semi-circular notches into the needle margins. They begin laying eggs in the soil near the base of the plant within 1-2 weeks of emergence and continue for about 2-3 weeks. The grubs begin hatching by mid-summer and the cycle begins again. There is usually one generation per year. Occasionally, depending on weather and climate, the adults overwinter in the soil and egg laying begins earlier. Other common host plants include rhododendrons, azaleas, euonymus, Japanese holly, hemlock, grapes and many other.

To control black vine weevil populations, apply nematodes (Heterorhabditis spp) to the soil when the larvae are actively feeding. Adults can also be hand-picked after dusk if your populations are low.

Fletcher Scale

Fletcher scale (Parthenolecanium fletcheri) are a pest of yew, arborvitae, and to a lesser degree juniper. These soft bodied scale insects can create a lot of damage to yews. Common symptoms include shrub stress and decline, copious amounts of honeydew and black sooty mold, (especially in spring), as well as foliage yellowing or browning. Heavily infested yew will also experience a lot of defoliation and branch die back. All life stages of this pest can be difficult to spot. Fletcher scale over winter as stage 2 instar nymphs, on the undersides of needles and shoots. They are very small, oval, flattened in shape and brownish yellow in colour. As they mature, they become rounder and turn a darker brown. You can find them located on either the needles or twigs. In May the females who have moved inconspicuously to the twigs at the base of new shoots, mature, growing to about 3mm (1/8″). They are mottled with colour ranging from yellowish brown, reddish brown, to dark brown. They partially cover themselves in a thin, transparent film that makes them look shiny. The adult females reproduce parthenogenetically and lay their eggs under their bodies, to protect them. The eggs begin hatching in June through July. The first instar nymphs are called crawlers, although in this species the crawlers are not very mobile. They appear yellowish in colour, oval and flattened, measuring about 1mm in length. You will find them out on the needles as well as on the twigs.

The crawler stage is the most venerable and lasts about a month. Apply insecticidal soap sprays or horticultural oil sprays at this stage. Heavily infest branches can be pruned out. If the population is low hand removal may be an option. Scale usually attacks distressed plants so reducing or eliminating stressors is key to long term treatment. Natural predators include several species of parasitic wasps and green lacewings. European fruit lecanium scales appear very similar but they feed on a wide variety of deciduous trees.

Spruce Bud Scale

Spruce bud scale (Physokermes piceae) prefer spruce, particularly Norway spruce but they will also feed on Yew and occasionally pine. The damage they cause, their life cycle and even appearance is very similar to Fletcher scale. This species unlike Fletcher scale though will sometimes have a bit of a white dusting on them. Treatment is the same as for Fletcher scale.

Cottony Taxus Scale

Cottony Taxus Scale (Pulvinaria floccifera) aka Cottony camellia scale is a common pest of Yew as well as many other ornamentals such as camellia, English ivy, euonymus, holly, hydrangea, Japanese maple, mulberry, rhododendron and many others. The most distinguishing characteristic of cottony taxus scale is the white waxy, cottony looking egg masses that are laid on the underside of leaves in mid- to late-spring. These egg masses are laid directly behind the female and are about twice her length measuring about 6 mm (1/4″) long or more. The life stages, damage caused, and management are similar to the scale pests listed above. This species, unlike fletcher scale see the adult females covering their bodies in a waxy covering that is dull tan to dull brown in colour. The crawlers are pale yellow, and the younger adult females have a stripe that runs down the middle of their back with mottled sides.

Root-knot Nematodes

Root-knot nematodes are parasitic nematodes of the Meloidogyne spp. The nematodes penetrate the small feeder roots which causes the cells in the area to multiply and increase in size, forming swellings (galls) that the insects feed within. The full-grown adult females are very tiny measuring about 0.5-1.0 mm long. They are pear shaped, translucent and whitish in colour. The nematodes feed on the vascular system weakening the plant. Above ground symptoms appear as stunting and yellowing foliage. Nematodes can also cause secondary fungal or bacterial infections. Root-knot nematodes are more likely to occur on moist, sandy soils.

Controlling root-knot nematodes in the ornamental garden is very challenging, prevention is worth its weight in gold. To help prevent root-knot nematodes do not move infected soil around, do not introduce plants to your garden that have come from infected soil, buy your plants from reputable growers, buy you soil from reputable suppliers, practice good sanitation, and manage weeds which can harbor this pest. If you discover root-knot nematodes on your Yew, you may want to remove and destroy it to prevent other plants from being infected. Once all infected plant material has been removed the soil can be solarized or planted with a cover crop such as French marigolds which help to reduce the number of root-knot nematodes in the soil. Alternately adding healthy amounts of organic matter such as compost or composted manure, yearly will help to improve the diversity of soil organisms, some of which will feed on the root-knot nematodes or parasitize their eggs.

Animal Pests

Animal Browsing can cause serious damage to yews even kill them as yews are not very tolerant of wounding. In spite of the fact that yews are highly toxic, animals like to feed on them. Ingesting yews can be fatal to the animals (as well as humans), but some animals seem to be less susceptible, such as white-tailed deer. Install barriers around susceptible yews to prevent the animals from reaching them.

Rabbits

Rabbits are less of a pest than deer, but they like to gnaw away at the lower bark to file their teeth down, which can girdle the branch(es). They also snip off lower branches making a nice clean cut at a 45-degree angle. Feeding damage occurs up to about 45cm (18″) in height. Yews are highly toxic to rabbits and can cause sudden death.

Deer

Deer love to feed on yews and will eat almost all parts of this plant. Feeding damage can occur up to about 2m (approx. 6′) in height. Deer will leave ragged edges on the affected branches as they pull and tear away at the branches when feeding. Install a deer barrier to prevent the deer from accessing the yew. This barrier will need to be quite tall as deer can jump over 2m ((7′) high, although they need a running start to jump that high.

Dog Urine

Large yellowing then dead patches will appear lower down on the shrub. The urine burns the leaves and often also kills the woody tissue. For dog owners this usually become more apparent in the spring after the dog has peed on the shrubs all winter. Cat urine will also damage yews if the cat goes repeatedly in the same area.

Spraying the shrub with water right after the animal pees will usually dilute it enough that little to no damage is done. A barrier can be placed around yews in the winter or the dog can be trained to use a pee post.

Voles

Feed on the roots and chew the bark off of trunks and low-lying branches, damaging the vascular system. Branches die back and if there has been enough damage the entire shrub or tree will die.

Voles can be trapped and barriers using hardware cloth can be placed around yew shrubs. Most vole species cannot climb).

Environmental and Other Damage to Yews

Nutritional Deficiencies

Yews are not overly demanding shrubs and usually only require fertilizing once a year, in spring. When there is a nutritional deficiency however the most likely suspects are nitrogen, iron and magnesium. A deficiency in any one of these will cause yellowing of the needles.

a. Nitrogen Deficiency is the most common. Symptoms start off with the growth lower down on the shrub beginning to yellow while the new growth remains green. This is because nitrogen is a mobile nutrient within the plant, and available nitrogen is first sent to the newest growth. Compost and composted manures are good sources of nitrogen but there are many options available to you.

b. Iron Deficiency is more common when the yews are being grown in alkaline soil, like many clay soils. It is not that the nutrient is not present in the soil rather the high soil pH prevents the uptake of this nutrient. Applying a chelated iron foliar spray will usually green up the needles again. Symptoms of chlorosis begin appearing on the newest growth, as iron is not a mobile nutrient, and when there is a shortage, the newest growth suffers first. If the deficiency is severe lesions will begin appearing on needles and new growth will become stunted. Alternatively, the yew may show symptoms of overly high pH instead, which appears as overall yellowing. Working on lowering the soils pH will help to bring about longer-term relief, although it is easier said than done.

c. Magnesium Deficiency is more common on yews grown in sandy or acidic soils or when there has been a great deal of rainfall. Symptoms appear as yellowing needles, occurring lower down on the plant. Before adding magnesium to the soil adjust pH level if it is below 6. Ideal soil pH for magnesium absorption is between 6.0 and 6.5. Have your soil tested as excessive amounts of calcium or potassium in the soil can lock up magnesium. If you need to add magnesium to your soil sources include Dolomitic limestone, Epsom salts (magnesium sulfate), K-MAG (sulfate of potash-magnesia). Compost is also a good source of magnesium and also helps to improve sandy soils.

Poor Soil Conditions

a. Overly Wet Soil is commonly due to poor drainage. Soil compaction, heavy clay soils, over watering or overly rainy periods all contribute to this condition. Yews do not like “wet feet” and under these conditions they begin yellowing, dropping needles, and lack vigor. If the conditions prevail, the yellowing intensifies and gradually they begin turning brown and dying. Even one episode of boggy conditions can be enough to seriously damage a yew, even potentially kill it. Planting yews to deeply can also cause them to receive too much water. To correct the problem, ensure that yews are planted in well-drained soil, away from down spouts, avoiding low lying areas of the yard.

b. Overly Dry Soil or Draught can occur when there is too little rain fall or irrigation, when the soil is overly sandy, in windy locations, in full sun locations surrounded by hardscape, in garden beds located near dense tree roots and over hangs from buildings. The newest growth is usually the most vulnerable. Under draught conditions the shrubs growth will decline, needles will begin yellowing, then browning, (due to their inability to adequately photosynthesize), and they will also become more prone to winter injury and insect attacks. If the draught conditions persist to long the feeder roots will begin dying, which will further reduce water uptake to the plant, a condition that is very slow to recover from. One final symptom of draught is nutritional deficiencies. Soil nutrients are taken up in the soil solution, thus with insufficient water the soil nutrients will be unavailable to the plant, even though they are present in the soil.

Although Yews are fairly draught tolerant, they do benefit from regular irrigation during dry periods. If the soil is sandy and drains to quickly add plenty of organic matter yearly to improve the soils water retaining ability. Try to locate yew away from the windier areas of the yard and away from rain shadows and roof overhangs.

c. Soil pH That is Too High or Too Low can affect the health of yews. Yews will grow in a fairly wide pH range from about 5.5 to 7.5. Anything above or below this range will harm the plant. Yews will begin declining, yellowing and gradually dropping needles until the shrub eventually dies. To raise the pH level of an acid soil, add calcium carbonate, ground dolomite limestone or wood ashes and switch to fertilizers and amendments that do not acidify the soil. To lower a soils pH, add agricultural sulfur, and switch to acidifying soil amendments and fertilizers.

d. Soil Compaction or Hard Pan are common in new subdivisions and when the property has had large equipment or work crews on site, especially when the soil is wet. When soil is compacted it drains poorly leading to root diseases and yellowing needles. Roots have a difficult time penetrating compacted soil thus the plants will be stunted, lack vigor and begin dying. Loosen the compacted soil and break up the hard pan with a pickaxe then add plenty of organic matter to improve the soils’ structure. It is often easier to remove the yew, correct the soil then replant it.

Winter Damage

a. Winter Desiccation (aka winter burn) is a highly common condition of yews in temperate zones. During the winter, evergreens, such as yew, continue to transpire (loose water from their needles). When the ground is frozen, they are unable to take up water to replace that which is lost, and as a result the foliage begins browning, staring at the tips of branches. Damage often does not become apparent until spring. Hold off pruning these damaged shrubs until at least June to see if the dormant buds are still alive and if they will send up new growth. If not prune them back to live wood. To help minimize this type of damage in future do not locate yews in windy areas or on the south or south/west side of buildings. Ensure that the yews are well watered before the soil freezes, and water them again if there has been ground thaw in the winter. Install a protective barrier to protect them from winter sun and wind. Barriers include things like shrub covers, wind breaks and wraps. For young yews an anti-desiccant spray can be applied to the needles to help seal off moisture loss.

b. Deicing Salts used in winter are also a very common cause of browning yew needles. Yews like boxwood are very sensitive to salts. Symptoms usually do not appear until spring when the salt sprayed foliage begins appearing winter burned and when the plant cannot meet its water requirements due to the dehydration caused by the salts in the soil. Do not locate yews in areas that will receive winter deicing salts. In the spring, when the frost is out of the ground, flush the soil with plenty of water to help get the salt out of the root zone. Protective barriers can be installed to protect the foliage from salt splash, but this will not prevent salt buildup in the soil. For more information check out my article on Salt damage.

c. Broken or Damaged Branches Due to Heavy Snow Load. Broken branches will yellow then die. Insect pests or diseases may enter the wounds. Branches may splay out. Gently brush off heavy snow loads from yews. Remove any damaged branches, cutting back to healthy wood.

For other tips on how to protect your shrubs during the winter see Winter Protection for Your Garden

Transplant Shock

Transplant shock is a condition whereby newly planted trees and shrubs experience distress after being moved from one growing environment to another. In the course of the move the plant may have lost part of its root system or received damage. As well the plant has to adjust to a new growing environment, including different soil, different lighting, different wind conditions, different moisture levels, etc. All of these factors collectively can cause the branch tips to wilt and the needles to suddenly begin yellowing then shedding and overall growth to be slow and in decline. Managing this condition involves reducing or eliminating these stressors. Note: Applying root boosting growth hormones to newly planted yew is an excellent way to help them get established more quickly.

a. Planting to deeply is particularly troublesome to Yew. It will deprive the yew of adequate oxygen and also rot the bark. The top of the root ball should be placed level or a few inches above the soil surface. The higher planting height is essential when planting into clay soils or when the soil is loose and will settle. When riding the root ball up a bit, add some soil to the outer edge, and taper it out away from the plant to avoid the roots from drying out.

Weed Trimmer, Lawn Mowers and Other Mechanical Injuries

Mechanical injuries can cause damage to the Yews roots and/or bark. This hinders the flow of both water and nutrients within the plant causing yellowing. If the damage is severe enough it can even girdle and kill whole branches or the trunk. Edge the lawn out further away from your yews to keep these lawn Maintenace tools well back. Other mechanical injuries can come from road, driveway and sidewalk construction that can damage roots. Other types of home construction and stonework can also damage yews when heavy layers of construction dust prevent the leaves from photosynthesizing which will cause them to begin yellowing.

Normal Dieback of Older Needles

Like all evergreens yews shed their oldest needles once a year. For yews it typically occurs in late spring, early summer. Yews typically only hold on to foliage for 3 years before shedding it.

Photo credits: all photos by the author.

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