The beautiful needles of the Blue Spruce are not only the object of desire for many homeowners but also a target for various parasitic organisms.
The main reason why Blue Spruce turns brown is the Rhizosphaera needle cast disease. To cure Blue Spruce, remove any damaged parts along with the fallen needles and spray with copper fungicide several times.
Unfortunately, the disease is not the only cause of needle discoloration, you will learn more about all the causes and what to do about it below. Also, be sure to read the Colorado Spruce Grower’s Guide to avoid problems in the future.
Rhizosphaera Needle Cast
Rhizosphaera needle cast is a fungal disease. The spores of this disease can get on your spruce from other spruce trees that grow nearby and are not cared for by anyone.
When the pathogen gets on the needles, it begins to germinate and the needles die. The disease spreads most strongly in humid and warm weather. Poor air exchange around the spruce also contributes to its development.
- The needles on two-year-old branches turn brown and fall off. Very often the annual growths remain healthy. It can be said that Rhizosphaera needle cast affects the inner branches while the outer ones look healthy.
- But this is not always the case, sometimes the whole branch is affected at the same time. The needles from the trunk to the tips turn brown and fall off.
- If you look closely at the needles, you can see very small black spots. This is the body of the fungus.
- Cut off the dry branches and collect most of the fallen needles. Burn it all or throw it far away from human homes.
- Spray Blue Spruce with an aqueous solution of copper fungicide. Do this thoroughly and don’t miss any branches. Also spray the trunk inside the crown.
- Repeat the spraying after 2 weeks. This time, use Chlorothalonil-based fungicide.
- Several more sprays may be needed throughout the growing season. Alternate fungicides to get the best effect.
Cooley Spruce Adelgid
Cooley Spruce Adelgid are pests of Blue Spruce. These insects have a rather complicated life cycle and need two hosts.
The female lays eggs near dormant buds in spring. When young branches emerge from their buds, the larvae have already hatched by this time and start to feed on young needles. Next, the larva penetrates the young growth and gall begins to form.
The larva feeds on the branch from within and eventually, the gall will shrivel up and turn brown.
- Young branches begin to get thicker and swell. This is how the gall is formed with the larva inside.
- Over time, the gall will shrivel up and turn brown. A new insect emerges from it.
- Remove all the galls from the Blue Spruce and burn them.
- Spray the healthy needles with horticultural oil.
- For severe infestations, use a systemic insecticide.
The next cause of needle discoloration is mites. These are tiny creatures that parasitize on needles and branches by sucking their sap.
They are most active in dry, warm weather. They can stay on a plant for a long time without being noticed, but as soon as favorable weather arrives, their population quickly increases.
The needles turn brown much slower than in the case of disease, so the mites are not immediately noticed.
- The needles are pale and do not look healthy.
- After a while, it changes color to light pink but does not fall off.
- Next, the needles turn brown and fall off.
- Very fine cobwebs can be seen between the needles.
- Wash the mites off with a hose as they do not like moisture.
- Spray the needles with an aqueous solution of neem oil. Repeat the spraying after 2 weeks.
- If the neem oil doesn’t work, use acaricide (miticide).
White Pine Weevil
White pine weevil is also a common cause of needle browning. Adults usually feed on needles on last year’s branches. Females lay eggs near buds in early spring.
When young shoots begin to grow the grubs penetrate into them and feed on young tissues making tunnels. As a result, the current year’s shoots droop downward. After some time, they shrivel up and turn brown along with the needles.
- This pest usually damages the very top of the spruce.
- Young, immature shoots droop toward the ground.
- The needles turn brown and fall off.
- Remove dead shoots from the tree and destroy them to prevent the grub from becoming an insect.
- Spray the needles with a systemic insecticide. Repeat the spraying after 2-3 weeks.
- Clean all plant debris under the spruce together with the insects that are there.
Browning after transplanting
After planting or transplanting a Blue Spruce, the needles may change color to brown. If you planted at the wrong time (summer), the roots may not provide the needles with enough water and they will change color.
Also, if you move the spruce to a new location, you will change the sun exposure. As a result, it may get more sunlight and get a little burnt. Although the Blue Spruce can withstand full sun, sudden changes in light can cause damage to the needles.
- The needles turn brown after planting in a new location.
- Planting was done in summer or in very hot weather.
- Roots were severely damaged during transplanting.
- Plant and move Blue Spruce only in early spring or mid-autumn.
- When transplanting, avoid damaging the root system.
- After planting, place a shade over the spruce for 3-6 weeks.
- Water when the soil is more than 1 inch dry.