Plant diseases fall into two major groups: parasitic and physiological. Parasitic diseases are caused by fungi, bacteria, viruses and other microscopic organisms. Physiological disorders are caused by unfavorable environmental conditions, chemical injury, improper fertilizing or other environmentally related situations. Some of the most common plant diseases are listed below.

 

Fire Blight is probably the most destructive disease of trees and shrubs in the rose family in North America. Fire blight occurs sporadically and unpredictably. A severe outbreak can seriously damage or kill mature pear, apple, or crabapple trees in one season. Mountain ash is equally vulnerable to the disease and may suffer the same fate. Other ornamentals such as hawthorn, plum, chokecherry, saskatoon, cotoneaster, and spirea may also be affected.

 

The Cause: Fire blight is caused by a bacterium (scientific name Erwinia amylovora) that enters the tree through blossoms, leaves, or stem wounds. Usually the disease is spread by bacteria that over winter in main stem and branch cankers or in infected twigs. In the spring, just as the blossoms begin to open, these cankers and infected twigs exude drops of bacterial ooze that are spread by rain, heavy dew, or wind-blown mist to the blossoms and young leaves. Fire blight may also be spread by pollinating insects such as bees, by sucking, chewing, or boring insects, or by unsanitary pruning tools. Favorable conditions for disease entry and development include warm temperatures and high humidity. In the spring infected blossoms suddenly wilt and turn brown. Later, twigs and leaves also turn brown, appearing to be scorched by fire; hence the common name. Affected leaves usually remain on trees well into the winter. The branch ends curl over like a shepherd’s crook. Young infected fruits become watery or oily in appearance and exude droplets of clear or amber-colored ooze. This fruit later become leathery and turns dark brown. The shrivelled fruit usually remains attached to the tree. Cankers, which are sunken woody areas, also form on affected branches and exude this sticky ooze.

 

Prevention: Preventative treatment of fruit trees and mountain ash should be undertaken as a matter of course if fire blight is present in your neighborhood. Trees that have previously been infected and pruned of their diseased parts should also be treated to prevent new infections.

 

Blossoms are the part of the plant most susceptible to fireblight. Blossoms can be protected by using a Copper Spray solution following label instructions. Apply this solution with a hose-end sprayer during the early stages of blossom, when 10% of the blossoms have opened. Repeat these spray applications at 4 to 5 day intervals until the late stages of blossom, when only a few blossoms remain on the tree. This will require at least three applications. These applications can only be made when the air temperature is around 18 C (65 F.) The best control of fire blight comes from spraying the entire tree at regular intervals through the summer. Copper sprays may be used until 1 day before picking the fruit. Additional preventative measures that should be taken to eliminate fire blight are to:

 

1. Avoid the use of high nitrogen fertilizers that promote succulent growth readily susceptible to fire blight.
2. Remove root suckers from the base of trees for the same reason.
3. Control leaf hoppers, aphids, and other leaf-feeding insects that may spread fire blight.

Control: There is no chemical that can cure fire blight. The only effective method of controlling fire blight is to prune off diseased twigs and branches. During the dormant season, late fall to early spring, prune out and destroy all diseased twigs and branches. Cut 30 cm. (1 ft.) below the diseased area since bacterial infections such as fire blight can extend beyond the visibly blighted area. (All pruning wounds can be disinfected with a copper spray solution.) During the growing season prune and burn any infected twigs or branches or seal in garbage bags for disposal. Once again, cut 30 cm. (1 ft.) below the infected area. Make regular inspections during the summer to detect and remove new infections but avoid pruning excessively during the growing season. Trees that are severely infected, with large cankers in the trunk, should be removed and destroyed immediately.

 

WARNING: Healthy plant tissue can be infected by bacteria-coated pruning tools. After each cut dip your pruning tools in a disinfectant solution of Lysol at 50 ml/L (4tbs/qt) or household bleach at 100 ml/L(8 tbs/qt) to prevent this.

 

Dutch Elm Disease

Dutch elm disease is a fungi that infects elm trees & inhibits the flow of nutrients throughout the tree. The name Dutch Elm Disease originates from Holland, where the fungi was first identified. The disease was accidentally introduced to North America from Europe in 1930. Since then, DED has spread throughout the continent via the transportation of elm firewood within which the smaller European elm bark beetle breeds. It is believed that this elm bark beetle was introduced to Calgary in this way.

Prevention: To keep DED out of Calgary, the City is monitoring its elms to detect DED infection. Part of this monitoring includes completion of an elm inventory. By knowing where its elms are, Parks & Recreation staff may best care for the City's elms. During summer months, watch for signs and symptoms of DED. An elm with DED displays the following signs; wilting, yellowing leaves and leaves turning brown and falling before onset of fall. If you see a tree displaying such symptoms, contact the DED hotline @ 221-4686. Secondly, be sure to maintain the health of your elm through proper care.

Powdery Mildew: Powdery mildew fungus affects all types of plants including lawns, perennials, annuals, and shrubs. It is particularly prevalent on roses and currants. This disease starts on young leaves as raised blister-like areas that are soon covered with grayish-white powder or mold. The fungus growth develops on the surface of leaves, stems, buds, and flowers. Symptoms of injury are: stunting, distortion of leaves and buds, yellowing of leaves, premature leaf fall, and general decline in plant growth.

 

Control: Powdery mildew can be prevented: by planting mildew-tolerant or resistant varieties wherever possible; by not planting susceptible plants in damp shady locations; by spacing plants out to avoid overcrowding and allow air circulation; and by pruning off affected young growth and discarding it. Water plants in the morning so that leaf moisture has a chance to evaporate during the day. In serious cases, apply protective fungicides like Garden Sulphur at the first sign of infection.

 

Leaf Yellowing:

Trees & shrubs often appear to be declining in vigor by showing signs of leaf yellowing in summer. Pale yellow leaves with dry crisp edges that drop prematurely can be equated to a number of causes: leaf scorch resulting from rapid leaf moisture loss during dry conditions, nitrogen deficiency resulting from lack of fertilizer, toxicity resulting from the use of concentrated insecticides during hot dry weather, or from herbicides or soil sterilants used too closely to ornamental trees and shrubs. However, the most common cause of leaf yellowing is iron or manganese deficiency. Leaf veins stay green while inter-veinal spaces become pale yellow. Young leaves are affected first, but this condition, known as iron chlorosis advances throughout entire plants if not corrected. Iron chlorosis can be corrected by adding iron or acidifying soil conditioners such as peat moss, aluminum sulfate or sulfur to the soil. The problem is that iron, although present, is not available in soil conditions common to the prairies because our soil and water are alkaline.

 

Potato Late Blight:

Late blight caused by a fungus (Phytophthora infestans) is the most destructive disease of potatoes worldwide. This disease is responsible for causing the infamous Irish potato famine in the mid-nineteenth century. Late blight has reappeared as a major disease in many potato growing areas of the world, mainly because fungicide-resistant strains of the fungus have developed. The disease affects potatoes & tomatoes equally severely. In Alberta, the disease has been gaining in importance for the last few years. Its resurgence is of concern to potato growers. Late blight is a "community disease"; if any diseased plant exists in an area, there is a good chance that the disease will spread to neighboring fields. A hobby home garden (of potatoes & tomatoes) with infected plants could threaten a nearby commercial field. It is, thus, very important that the community as a whole keeps their plants free from late blight.

 

Prevention:Since the disease can survive in infected tubers, DO NOT use your own potato tubers if they show any kind of rotting. As a precaution, ALWAYS buy "certified seed potatoes" & tomato seedlings from reputable sources.

 

Control: The disease can be effectively controlled with the spray of an appropriate, registered fungicide. Consult your local greenhouse garden centre or other such sources for the fungicides & their use. Always follow the directions on the fungicide label.

 

Black Knot:

Black knot is a common fungus problem on many flowering Prunus species such as cherries or plums. It affects only woody tissue, developing on twigs, limbs, and branches, stopping the flow of sap and nutrients. Growth beyond the knots can be stunted or even dead, resulting in weak, disfigured, and sometimes dead trees. Knots are easily recognized as lumpy, hard black swellings on the wood. Fungus overwinters on infected twigs and produces new spores in the spring. These spores are easily transferred by wind, rain, or the activities of animals. New shoots are susceptible to infection in the spring just before blooming or just after the petals have fallen. Wet spring weather increases the spread of black knot spores.

 

Control: Unfortunately there are no fungicides currently availableto effectively treat black knot. Instead, prune out any growths making sure to cut at least eight inches below the knot. Disinfect the wound with copper spray, clean the pruning tool between cuts with a bleach solution, and either destroy of completely dispose of infected wood.

 

 

Lawn Diseases

 

Snow Mold:

The incidence and severity of damage caused by snow mold depends largely on the nature of the fall weather. If the ground is frozen solid before a permanent snow cover, damage from snow mold may be minimal. If however, a heavy snowfall is experienced before freeze-up, considerable lawn injury can occur. Snow Mold is a common fungi in southern Alberta.

 

Control: Apply a suitable fungicide to the grass just before the arrival of the permanent snow cover, in approximately autumn. Copper spray or Garden Sulphur can be used to control snow mold. Remember to read the directions first before you use either. Control snow mold naturally by reducing watering in the late summer to allow your turf to harden-off, by raking up leaves in the fall, by breaking up piles of snow to speed their melting in the spring, and by picking up grass clippings and/or dethatching the turf regularly.

 

Fairy Ring:

This fungus spreads easily and is therefore very difficult to control. Dark green circles appear in the lawn. As these rings enlarge the center area of lawn dies. The fungus forms a thick impervious mat under the soil surface, preventing water, air and nutrients from reaching the grass roots.

 

Control: Use a garden fork or an aerating tool to poke holes, 7-10 cm (3-4 in.) apart and at least 12-15 cm (5-6 in.) deep into the fairy ring. To prevent the spread of this fungi to other parts of your lawn or garden be sure to disinfect your tools with a solution of household bleach or lysol containing chlorine after you use them to aerate fairy rings. Water the infected area thoroughly. Next, apply a high nitrogen fertilizer, such as 20-3-4, to the lawn area. Water this area deeply throughout the summer. An alternative method is digging the fairy ring out. To do this effectively the grass and soil must be removed from a distance of 12 inches from the outside and inside edge of the ring and 12-18 deep. Take care not to spill any soil since any fairy ring mycellium that fall on the lawn from the soil being removed can start a new ring. Follow the instructions for cleaning the tools after this procedure has been completed.