Pests & Diseases of Trees & Shrubs

Dormant Spray

With our early spring and the buds of plants getting ready to open, now is the time to think about doing a Dormant Spray. Combine Horticulture Oil and Lime Sulphur to kill overwintering insects (such as scale and mites), insect eggs, and diseases on certain hardy landscape plants.

This combination spray can only be applied in early spring before leaf buds show green.  Applying at the incorrect time can burn leaf buds that have started to swell or show any sign of green.

This is best applied to fruit trees, roses, ornamental shrubs like Highbush Cranberry, cotoneaster and Cranberry, evergreens such as Cedars, trees like Hawthorn and Crab Apple & Apple. Do not use a dormant spray on Butternut or Colorado Blue Spruce.

Choose a warm day (above  5°C ) from March thru to April. Spray early in the morning so the plant will be completely dry by evening. Do not spray if there is any chance of frost overnight. It is also best to spray when there is no or minimal wind.

Mix Lime Sulphur and Horticulture Oil (available together as a Dormant Spray Kit) according to the package directions in a hose end sprayer that attaches to your garden hose or in a 1 or 2 gallon tank sprayer.

In all cases, spray the plant starting at the top until it just starts to drip off the branches. If you start spraying from the bottom, you will run out of product before the job’s done.

For roses, be sure to spray the soil around the base of the plant as well to control Powdery Mildew and Black Spot.

Mix only what you can use. You cannot save the prepared solution for later use.

Wear protective clothing, and anti-splash goggles. Wash hands and face after use.

Do not let any of the mixture fall or drift onto such hard surfaces as it may leave a permanent stain. Cover the area with plastic to avoid staining.

Cedar-Apple Rust

Cedar rust is a fungus and has a 2 year cycle. It will move from your Juniper (host) to your Malus or Hawthorn.  If you treasure your Apple or Hawthorn, it is best to get rid of the juniper that is infected. The fungus may be controlled with a fungicide spray, such as copper fungicide, but you will have to stay on top of it. Pruning can also work if it is not too badly infected.


Cedar  rust is caused by a fungal pathogen called Gymnosporangium globosum. This disease can occur on a host plants which is normally cedar or juniper,  crabapple, apple, hawthorns and sometimes on pear, and serviceberry. In order to survive, the fungus must “move” from one type of host to another (e.g., from juniper to hawthorn).


On evergreen hosts, very small (1/8 to 9/16 inches in diameter), roundish galls develop on needles throughout the tree. These galls slowly grow onto the twigs, often becoming flat on the twig side. Immature galls are reddish-brown, while mature galls are grayish-brown in color and scarred. When mature, these galls swell and produce reddish-brown, short, blunt, jelly-like  horns during rainy spring weather.

On deciduous hosts, small yellow spots first appear after infection in the spring. As the spots mature and enlarge, they take on an orange color and develop tiny black dots in the center of the lesion. By mid-summer, tubes  are visible on the undersides of mature leaf lesions. With severe rust, foliage may turn bright yellow and drop prematurely. In addition, fruit and young shoots may become infected.

Life Cycle

From the  horns on the evergreen host, sores are released that infect deciduous hosts. Although most infections occur within several hundred feet of the source evergreen, infection has been reported even when there is more than fourteen miles between the host trees.

About 80 to 90 days after infection (about 10 longer than cedar-apple rust), aecia are produced. Most people only notice this stage after the aecia have split and take on a ragged appearance. Aeciospores, released from the aecia during rain or as morning humidity lowers, become airborne and infect susceptible evergreen hosts from midsummer into early fall.

The following spring, galls (consisting of both fungal and host plant tissues) begin to develop on the evergreen host. These galls continue to grow through the summer, and by fall they are full size (1/8 to 9/16 inches in diameter). Rainy weather during the following spring causes the horns to emerge and release spores that infect the deciduous host. As spring rains subside, the galls become inactive until the following spring. In summary, the complete cycle of cedar-hawthorn rust takes 24 months to complete and requires infection of two different hosts.


Even though sanitation is not perfect – follow good cultural practices and remove as much of the infected twigs, fruit and leaves as possible. Follow recommended fungicide treatments  – contact a reputable garden center, landscaper, nursery or arborist).

Life Cycle

In order to survive, the fungus must “move” from one type of host to another (e.g., from juniper to apple). See description above.

Damage by Rabbits, Mice or Voles

Unfortunately rabbits and mice or voles have not enjoyed the prolonged snow and are looking for food. They have been busy on the nursery and in homeowner yards, eating the bark on the trees.  If your tree looks like this, and is eaten around the entire trunk, it may not survive.  The bark and transplant tissue (phloem) which carries the nutrients from the leaves to the roots have been damaged.    One method that may save your tree is by applying Lac Balsam to the area.  It works like a rubber skin and seals the cambium on the tree so the nutrients can flow again.  For $12.95 for a 150 gr. tube, it is well worth trying out.

Fall Needle Shed

Yellowing of needles in the fall is a normal occurrence for evergreen trees.  Conifers do not keep their growth of needles on their inside branches  and shed them naturally in the fall.  The discoloration, which affects the older needles close to the trunk occurs in late August and can continue until freeze up.   The amount of shedding can be greater if the tree has been placed under stress caused by droughts, flooding or newly dug trees.   These needles are not replaced and this is why evergreens are bare of needles near the trunk and there is usually a carpet of needles under the tree.