We often get worried calls in regards to a white powdery substance on tree and shrub leaves. Commonly affected species include caragana shrubs, flowering crabapple trees and on occasion turf grass. The problem in question is a fungal disease called powdery mildew, the symptomatic white substance covering the leaves is the fungus and its spores.
In most circumstances powdery mildew is a cosmetic affliction of plants meaning that it will not kill the plant despite reducing its aesthetics and appearance. Fungicides offer little control over curing the disease because fungicides work more as a preventative than a cure for fungal diseases. A fungicide may reduce the spread of the fungus but affected tissues will remain symptomatic.
A better means of control would be to implement preventative cultural practices such as thoroughly cleaning up infected debris in the fall and improving airflow around the infected plant. Another cultural method would be plant health care, ensure that the infected plant is receiving adequate water while being careful not to over water so that the tree can build up energy reserves to naturally combat the disease. Improving root health by using wood chip mulch as a ground cover is also another method you can use to improve the health and growing conditions of the root system.
If the affected tree or shrub has a nutrient deficiency or imbalance it could be more susceptible to disease such as powdery mildew. Nitrogen is a food source for most tree and shrub pests, adding a high nitrogen fertilizer can actually make the problem worse. If you are using fertilizer we recommend a balanced fertilizer containing micro nutrients that is also high in potassium such as tomato and vegetable food. Potassium is the last number of 3 seen on product labels and nitrogen is the first. Prior to using any fertilizer or products it is wise to have a detailed soil or leaf tissue analysis. Samples should be conducted by an agricultural laboratory to determine if there are any nutrient deficiencies or specific soil conditions that could lead to nutrient imbalances.
On a final note please remember that powdery mildew is a cosmetic affliction that will not cause the death of your tree or shrub, try to work on the site conditions leading to the problem rather than trying to find a cure.
Once you have a location selected for your tree consideration should be made towards selecting the most appropriate species of tree for the desired area. If the right tree is put in the right place you and future owners of the tree can earn increasing benefits annually from wise decisions made even before the tree was planted. Things to consider are what is the maximum final height and width of the tree, does it flower? Most flowering trees also bear fruit.
A great choice of tree that stays small, flowers but does not drop fruit or sucker aggressively is the Tree Lilac. If you have a yard limited by space, consider planting columnar or dwarf species of trees and shrubs. An amur maple sold as a shrub can actually reach 15 feet tall and 10 feet wide after a long period of time in the right setting. Far too often we see large Colorado spruce trees planted too near a home or building, a better choice would be a columnar blue spruce or a pine where there are many species of that stay relatively small in comparison to a spruce. People often ask for a fast growing tree because they want an immediate benefit from the tree.
In most cases the faster growing species are also the ones that get the largest. If privacy is desired in a tight location it is better to be patient and plant a slower growing species of tree rather than putting a tree in that will outgrow the area. Tree species selected and planted for the best location also have reduced maintenance and plant health care requirements. Take a bit of time to research what types of insect and disease problems the tree you want is prone to, for example avoid species susceptible to black knot. If you are planting in a full sun open space facing south or west avoid dark thin barked trees such as mountain ash, linden and amur cherry because of their susceptibility to sun scald (tree sun burn). Putting the right tree in the right place is the golden rule which a positive multi-generational relationship with the tree is based on.
A very common myth associated to spruce and other evergreen species such as fir and pine is that they acidify and lower soil pH. There are many remedies for raising soil pH due to the presence of evergreens, do not use these products as they may cause more harm than good to your soil.
Research has proven that evergreens do not have an immediate effect on soil pH but over centuries and millennia they may. Another factor in dispelling the myth is that our soils in the Calgary and area have a high buffering capacity. Buffering capacity is the ability to resist a change in pH, meaning if you try to lower or raise soil pH the soil will return to it’s original pH quickly.
Our soil, and water for that matter, are alkaline and have a high calcium content. Calcium is what gives our soil and water a strong resistance to pH change. The myth came about because, grass and many other plants do not grow under or near a spruce. Competition for light and water is the reason why plants don’t grow well or die under evergreens.
Bergenia is a broad leaf creeping perennial plant that grows well in a dry shady area if you are looking for a suggestion for planting. You can raise the tree canopy to allow for more light at a sacrifice for tree balance, health, value and appearance. Every live limb removed from a tree is an injury, removing live growth from a tree should be done with reason and purpose. What do you value more your grass or your tree? Rather than fight a losing battle with a tree, work with it. Adding mulch under and extending the bed or dead ring around the tree is a great solution. A mulch bed makes a natural healthy environment for the roots and a nice edge that is more maintenance free. If you are adding mulch make sure that it is around 2”-4” deep while making sure not to put any around or bury the tree trunk.
Tree Frog Tree Care is proud to be a division of Bartlett Tree Experts, and our staff now has the backing of an international tree-care company and tree research laboratory to help continue providing excellent tree service and shrub care in Alberta.
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