HPIPM:Foliar Diseases
Determining the Cause of Foliar Damage
Diagnosing foliar damage is not easy if there are no distinct patterns that would suggest insect feeding or if there are no fungal fruiting bodies (such as small black pimple-like pycnidia). Fall needle drop occurs every year in the fall and involves older needles. Needles turn yellow to brown and fall off. If the tree retains less than three years of needles the tree is probably stressed. Insects usually cause discernable damage that looks like mining, chewing, or sucking by piercing sucking mouthparts. Aphids leave so much honeydew that leaves look wet and shinny.
Many Agents Can Cause Leaf / Needle Discoloration or Death
Bacteria, Fungi, and Virus: Various bacteria and fungi can attack leaves and cause spots, complete death and defoliation. Root diseases can also cause leaves to yellow and die.
Insects and Mites: can remove needles; they include scales, leafminers, mites, aphids, and defoliators such as elm leaf beetle.
Abiotic Damages such as frost, road salt, high temperatures, drought, winter drying, and root damage from construction may cause needle drop. We do not have air pollution damage to tree foliage in the region yet.
Fall Needle Drop in Conifers: Older conifer needles die normally each fall; usually a conifer will retain at least 3 to 4 year's growth of needles.
Biotic Causes of Foliar Damage
Fungi and bacteria can cause damage to foliage of broadleaf and coniferous speices of woody plants. Fungi usually cause irregular damage to leaves of broadleafed species, accompanied by various color changes and spots, and may cause complete death needles on coniferous species. If there are not distinct raised dark colored spots (fruiting bodies) on needles there is no easy way to determine if the damage is fungal. The pattern of needle death may help to determine the cuase of the problem, as often one-year's growth is usually affected and usually lower branches are affected. Bacteria usually cause a very dark (brown or black) uniform damage to the leaf. Bacteria can also cause cankers that girdle branches or stems, resulting in death of leaves on the affected branch or stem.
Foliar diseases that remove leaves and thus photosynthetic area usually do not kill the host woody plant, especially if infections occurs for during just one year. Hardwood trees can withstand defoliation better than conifers since they normally do not retain their leaves from year to year. Conifers can be defoliated for one to several years before mortality occurs. However, all trees are stressed by defoliation and other insects and diseases may cause problems for the stressed trees.
Abiotic Causes of Foliar Damage
Abiotic damages usually do not look like an insect or fungus is moving through the tissue and thus have more uniform symptoms such as needles being all reddish brown from winter burn. The distribution of symptoms on trees and among trees in the area can give hints of the cause of damage. Abiotic damages are usually more uniformly distributed on a tree and within an area if it is weather-related etc.
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Biotic Causes of Foliar Damage |
Abiotic Causes of Foliar Damage |
