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The Most Common Tree Diseases

The Most Common Tree Diseases:

Trees, shrubs, and plants native to Arizona are susceptible to several diseases. As with humans, the sooner these problems are detected and treated, the better the outcome. If you hire an arborist to maintain your property, you can rest assured that signs of plant distress won't go unnoticed. These are the top 8 most common diseases affecting your trees. This writing aims to give you a general idea of what might be happening to your yard and trees. This way, you will know the right questions to ask when an arborist is evaluating your trees.

Bacterial Wet Wood (Slime Flux)

Slime flux is relatively easy to spot and recognize. It starts wet in appearance. (hence the name) It eventually dries into an ooze-like substance. The ooze may be white or dark, slimy, and possess a foul odor. Insects are especially attracted to this sap-like material. These insects can often become a problem for the tree as well. Branches on affected trees may wilt and die back. The slime is toxic to the bark and plants growing under the tree. Bark killed by the ooze, especially around the exit wound, may become loose and may eventually slough from the tree. The bacterium enters the plant through wounds or natural growth cracks. Once inside the tree, the bacteria raise the internal gas pressure. As a result, the bacterium oozes from the tree as the gas pressure forces it out.

Black Spot

Black spot typically develops after periods of rainfall, as it thrives in moist environments. As the name suggests, this fungus causes black spots on leaves. These spots look irregular and often accompany leaf yellowing. It was initially thought it only affected roses, but it is proven to target other plants and trees. Luckily, Black Spot is usually not fatal for plants, but removing it can be a process. Topical treatments and changes in watering practices may help affected plants to recover.

Canker Disease

Canker Disease looks like a legion or cyst on a tree; it often appears as a sappy scab or wound on the bark. It results from a fungal or bacterial pathogen entering the bark of a tree. Canker Disease will eventually cause the tree's vascular system to become blocked, and nutrients cannot flow. Branches may decline if the canker wraps around the trunk.

Dothistroma Needle Blight (Red-banded needle blight)

This blight causes Reddish-brown spots to appear randomly on needles in late summer to fall. The spots can enlarge into reddish-brown bands and encircle the needles. It will also cause the needles to turn brown at the tips. Until, slowly, the entire needle turns brown and dies. Dothistroma needle blight can be fatal if left untreated and is found commonly in Austrian pine and Ponderosa pine.

Fire Blight

You can identify Fire blight by tree branches that look scorched by fire and wilting leaves that turn black. Fire blight infects twigs sporadically so that you may see dead foliage in different areas of the plant. It's most active in warm, moist weather. Remove and destroy infected branches and cut back to uninfected wood to control fire blight. It is crucial to have fire blight treated as soon as possible because it is easily spreadable to other trees and plants. It travels faster in warmer climates and can move through the air, on animals and insects, and from the rain.

Powdery Mildew

Powdery Mildew usually appears as a white or grayish powder-like substance on the surface of leaves and stems. Some people suggest that the fungal spores look like baby powder on foliage. The disease can also appear as felt-like mats or cobweb-like formations. Disease symptoms increase when cooler weather and high humidity are combined, and these signs occur more frequently in areas of a tree that are shaded and lack good airflow. As this mildew spreads, leaves begin to yellow and wilt, and eventually, the entire branch dies. Advanced symptoms of infection also include distorted leaves, premature leaf drop, blemishes on fruit, and buds that won't open.

Root Rot

Usually occurring during warmer months, Texas root rot (also known as cotton root rot.) is a fairly common disease in Mexico and the southwestern United States. The result is a sudden wilt and death of affected plants. It takes around two weeks for a plant with root rot to die. Root Rot happens when a soil-borne fungus attacks the roots of susceptible plants. The dead leaves become stiff but stay attached to the plant. Once the plant dies, its root system looks decayed, giving this condition its name. Since root rot can kill a full-grown healthy tree in a matter of weeks, it is essential to inspect your trees and plants regularly and call an arborist if you notice any unexpected changes.

Verticillium Wilt

As you might expect, wilt is the most common sign of Verticillium wilt. You will notice wilting of the stem and leaves. Wilting occurs due to the blockage of the xylem vascular tissues, which reduces water and nutrient flow. Verticillium can quickly kill the plant in small plants and seedlings. However, the severity can vary in larger, more developed plants.

In some cases, only one side or section of the plant will appear infected; this is because, once in the vascular tissues, the disease moves upward and not horizontally. Other symptoms to look for include chlorosis (yellowing of the leaves), necrosis (tissue death), and defoliation. In addition, internal vascular tissue discoloration might be visible after cutting the stem.

Usually, the effects will only be on the lower parts of plants or localized to only a few tree branches. Verticillium wilt is known to kill older plants. However, the plant can often recover or continue living with the condition, especially with trees.


Hopefully, this list highlights why we must take care of our beloved trees. It is a partial list of conditions to look out for as you admire your yard. Luckily, most situations are noticeable to people who spend time around the Trees. For diseases that aren't as noticeable for the average tree enthusiast, having an arborist look at the trees at least twice a year can help. An arborist can diagnose and treat the tree by building a plan to get it cured and blossoming at full health. If you recognize any of the above conditions on your trees, ask about it while the arborist inspects your yard.


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