Diagnosing Tree Problems In The Landscape

Compounding this problem is that trees often take years to show symptoms of a previous injury. For example, if a tree starts showing branch die back it could be from a canker disease this year or it could be the result of root injury from construction damage such as a new patio or driveway put in up to five years ago. The symptoms from these abiotic (nonpathogenic) factors can sometimes be confused with disease or insect damage.

When diagnosing plant diseases it is important to look at the surroundings the plant is growing in and to ask the right questions of clients or people more familiar with the growth of the plant. It is somewhat like being a detective.

In general, abiotic disorders will have uniform symptoms. The whole plant will be affected or the entire leaf margin will be discolored. Diseases typically show random symptoms. Spots are randomly distributed on the leaf surface or only a few leaves or branches are affected.

Diseases are caused by five groups of pathogens; fungi, bacteria, nematodes, viruses and phytoplasmas (formerly mycoplasma-like organisms or MLO). By far, fungi cause the greatest number of diseases in ornamental plants, and most disease control practices and products applied are directed toward fungi control (i.e. fungicides).

Three principle components must be present for a disease to occur. These include a susceptible plant (host), a pathogen and an environment favoring disease development. Disease control relies on breaking this “disease triangle” in some way.

Choosing and growing disease-resistant varieties or not stressing and injuring disease-susceptible varieties can greatly reduce the occurrence of disease. Pathogens can be reduced in the following manner: by removing infected plants or plant parts that could spread the disease, not introducing organisms through contaminated tools, pots, soil mixes, water or hands and by using fungicides. Changing the environment by avoiding over watering and extended periods of plant wetness and increasing air circulation around plants also can reduce disease.

It is difficult to construct a foolproof key for diagnosing plant problems. Even with laboratory equipment, it is often impossible to determine the exact cause of a tree’s decline. For accurate diagnosis of a problem and control recommendations, contact county extension personnel within each state and/or submit a plant sample to state or private disease diagnostic laboratories.

A sample should include at least a 12-inch section of a branch showing the symptoms of the problem. A diagnosis cannot be made on dead wood or dead leaves.

See the key on page 66 and 67 to help with diagnosing common tree problems. The key is by no means comprehensive and other resources may be necessary to supplement your diagnostic process.

Fungal leaf spots and blights are favored by leaf wetness and high humidity. A variety of fungi can cause leaf spots including Alternaria, Septoria and Cercospora. Some common leaf spot diseases are black spot of rose caused by Diplocarpon rosae and scab on crabapple caused by Venturia inequalis.

Typically, fungal leaf spots are tan to black in color possibly with pigmented borders and are concentrated along the leaf margin and veins. Often the spots will grow together (coalesce), sometimes forming concentric rings of dead, brown tissue.

Within the dead tissue, black pimple-like fungal fruiting bodies can sometimes be seen. Leaf spots are termed “blights” when the entire leaf is affected. Often infected leaves drop prematurely. Leaf-spotting pathogens survive on fallen leaf litter and on dead branches or cankers.

Powdery mildew is another type of fungal leaf blight. It is probably one of the most troublesome diseases of ornamentals. Plants are rarely killed, but it can cause premature defoliation under high disease pressure.

Powdery mildew diseases look the same regardless of host, but the fungi are host specific meaning a powdery mildew pathogen on rose is specific to rose and will not infect crape myrtle. Leaves develop patches of frosty, white fungal growth primarily on the upper leaf surface and stem. Often the infected tissue is distorted and discolored. Unlike other fungal leaf pathogens, powdery mildew fungi are not favored by wet leaves. Instead symptoms occur under dry conditions and when humidity is high.

Rust fungi also cause leaf spots and blights. Pale yellow spots appear on the upper leaf surface while pustules containing rusty reddish-brown powdery spores break through the lower and sometimes the upper leaf surfaces. These spores are easily rubbed off with your fingers or paper. Pustules develop as individual spots or as concentric rings similar to a bulls-eye pattern in which an inner ring of rust pustules is surrounded by an outer ring. This type of ring pattern symptom is especially prominent on geraniums, zinnias and snapdragons.

Rust fungi are host specific. Some rusts produce different spores on alternate hosts. For example, cedar apple rust produces leaf spots and pustules on apple and crabapple, but produces hard gall-like structures on cedars. In the spring, the cedar galls rupture producing orange, jelly-like extensions (similar to tentacles) which release spores that infect apple.

Cankers are dead portions of plant stem tissue. When the canker is at the end of a branch or shoot, it is referred to as “die back.” Cankers are brown or black areas that become shrunken with time as the healthy adjacent tissue grows. Within old cankered tissue, black pimple-like fruiting bodies are often seen.

Both bacteria and fungi cause cankers. These pathogens enter the stem through wounds made by hail, insects or mechanical damage from pruning and mowers. As the canker grows, stems are girdled causing wilting and tissue death above the canker.

A common fungal canker is Bot canker caused by Botryosphaeria that infects numerous hosts, in particular Leyland cypress and rhododendron. The most common bacterial canker is fire blight caused by Erwinia amylovora. This disease affects pear, crabapple and pyracantha to name a few.

Vascular wilts occur when the vascular (water- and nutrient-conducting) tissues are infected by fungi and at times bacteria.

Obvious symptoms of these diseases are wilting because of water stress caused by the pathogen or its byproducts produced during the disease process which cause blockages in the water-conducting vessels of infected plants and death of plant sections. Other foliar symptoms include yellow or scorched leaves and stunting.

Sometimes, as with the fungus Verticillium spp., wilt pathogens infect the root and move upward to the leaves producing symptoms that appear on only one side of the plant, one branch at a time, or on one half of a leaf. A lengthwise cut across infected stems shows dark streaks within the vascular tissue. These pathogens are often introduced into landscapes from infected plants. Symptom expression is favored by plant stress associated with high temperatures and drought.

Root and crown rots are caused by numerous soil fungi including Pythium, Phytophthora, Rhizoctonia and Sclerotium. These fungi have broad host ranges, meaning they are capable of infecting a wide variety of herbaceous and woody ornamentals. Most of these pathogens are favored by cool, wet soil conditions.

Pythium and Phytophthora are classified as “water molds.” These pathogens produce a spore (zoospore) that can swim in water. The fungi inhabit soil naturally, and under stressful conditions for plant growth such as over watering, over fertilization and poor soil drainage. Roots are discolored brown or black.

Often small feeder roots are sloughed-off greatly reducing the plant’s ability to uptake water and nutrients. Above-ground foliar symptoms are similar to those produced by an unhealthy root system such as yellowing of older leaves, stunting and general decline of the plant. Often symptoms mimic nutritional deficiencies.