Tree Disease: How to Identify Issues

Yellowing leaves. Thinning canopies. Dead branches where there shouldn’t be any. Sometimes tree problems announce themselves clearly; other times, it’s just a sense that something looks off. Recognizing those early warning signs and understanding what they may be telling you is the first step toward protecting tree health and preventing larger issues down the line.

The challenge is not all diseases manifest the same way, nor do they show up in the same season. While one issue might be visible in spring, come summer, a new host of issues may arise. Regardless of the season, the ability to identify tree diseases early and plan thoughtful, effective care can help stop spread and mitigate damage.

Quick Links:

• What Is Tree Disease?
• Best Season to Identify Common Tree Diseases
• Tree Health Best Practices
• Protecting Your Urban Forest or Managed Landscape from Tree Disease

What Is Tree Disease?

When we talk about tree diseases, we’re really talking about a broad category of issues that interfere with a tree’s expected growth, function, and long-term health. Some problems are caused by pathogens such as fungi or bacteria, others by insects and pests, and some, such as iron chlorosis, are driven by soil conditions rather than infection.

What makes tree disease management especially tricky is that many of these issues overlap, and some pests actively carry or create entry points for disease.

Understanding the basic categories of tree disease helps make seasonal inspections far more effective, since many symptoms are easier to see once leaves are gone, and insect activity has slowed.

Fungal Tree Diseases

Fungal tree diseases are among the most common issues affecting both ornamental and shade trees. These diseases often thrive in moisture, poor air circulation, or stressed trees, and many overwinter on bark, buds, or fallen debris.

Common fungal concerns include:

• Shot hole fungus (CO, UT, PNW) (pictured)
• Anthracnose (PNW, occasionally in CO/UT with wet spring)- pictured above
• Powdery mildew (CO, UT, PNW)
• Rust (PNW)
• Peach leaf curl (CO, UT, PNW)
• Plant cankers (CO, UT, PNW)
  

Bacterial Tree Diseases

Bacterial tree diseases tend to spread through wounds, pruning cuts, weather events, and sometimes insects. Unlike fungal infections, bacterial infections can spread rapidly through vascular tissue, making early identification especially important.

Examples include:

• Fire blight (CO, UT, PNW)- (pictured)
• Pseudomonas infections (CO, UT, PNW)

Tree Pests

While tree pests aren’t diseases, their impact on tree health can be just as severe. And, in many cases, they can introduce or spread disease.

In some cases, pests carry disease-causing organisms, which can quickly turn a pest problem into a disease problem.

It’s hopefully clear at this point that tree diseases are rarely isolated events. They’re often the result of multiple stressors working together, from soil conditions and pathogens to pests and environmental pressure.

When it comes to prevention and treatment, regular monitoring and inspections are critical. Catching a problem early provides the time you need to address problems before pruning, environmental conditions, or even insect activity accelerates their spread.

Best Season to Identify Common Tree Diseases

Tree diseases don’t all reveal themselves at the same time or even in the same ways. Some issues are easiest to spot when leaves first emerge, others during peak growth, and some only become obvious once trees are dormant. Understanding which season reveals which problems helps diagnose issues more accurately and respond at the right time.

Spring: Early Warning Signs as Trees Wake Up

Spring is one of the most important seasons for identifying leaf-related and early-season diseases, when new growth is most vulnerable.

• Shot Hole Disease (late spring)
  Look for small holes or lesions in newly emerged leaves, especially on stone fruits and flowering ornamentals.

• Peach Leaf Curl
  Distorted, thickened, or puckered leaves shortly after bud break are the hallmark sign. Once visible, the infection has already occurred. (Pictured)

• Rust Diseases
  Early leaf spotting or discoloration may appear as leaves expand, particularly on hosts like serviceberry.

• Anthracnose (mid spring)
  Leaves may emerge deformed, brown, or die shortly after leaf-out. They may also fail to fully leaf out at all. Cool, wet springs often worsen symptoms.

• Iron Chlorosis
  Interveinal yellowing becomes noticeable on new growth as leaves expand, especially on susceptible species in high-pH soils.

Some spring symptoms are dramatic, and others fleeting, which makes close observation during leaf-out especially important.

Summer: Active Disease & Stress Responses

Summer reveals diseases that thrive during warm temperatures and active growth, as well as damage from earlier infections.

• Powdery Mildew
  White or gray powdery growth on leaves and stems, often appearing during warm days with cool nights. (Pictured)

• Fire Blight
  Wilted shoots, blackened leaves, and “shepherd’s crook” branch tips are most visible during active growth.

• Cankers (late spring into summer)
  Sunken, cracked, or oozing areas on branches and trunks may become more obvious as surrounding tissue declines.

• Pseudomonas Infections
  While infection often occurs during winter, symptoms may persist into summer, including dieback associated with earlier black lesions or damaged tissue.

Summer inspections help confirm which issues are actively progressing versus those that began earlier in the season.

Winter: Structural Clues & Overwintering Evidence

While many diseases reveal themselves through leaves, winter strips trees down to their framework, which means it the best time to spot structural damage, pest evidence, and chronic issues.

• Galls
  Swellings on branches—especially on oaks, cottonwoods, and ornamental cherries—are often most visible once leaves drop.

• Exit Holes & Pest-Associated Decline
  Look for holes, bark splitting, woodpecker damage, and thinning branch structure that signal insect activity such as borers.

• Overwintering Damage
  Dead wood, cankers, cracked bark, and past infection sites are easier to identify without foliage.

Winter may not show leaf symptoms, but it often reveals the story of long-term stress, decline, and ongoing or repeated disease pressure. 

The truth is no single season tells the whole story. Effective tree disease management relies on seasonal observation, pattern recognition, and understanding when specific diseases are most visible. Knowing what to look for and when, can help identify problems earlier, so the response is timely and appropriate.

Tree Health Best Practices 

While some seasons are best for spotting issues, the best time for tree health is year-round. It's one of the primary reasons we strongly recommend tree inventories and educating teams and communities, enlisting volunteers, prioritizing inspections, and simplifying reporting of issues.

• Prune at the appropriate time for each species
  Dormant season pruning is essential for many deciduous and shade trees, but timing still matters. Some trees should be pruned in mid to late winter, while others are better addressed earlier or after dormancy. Understanding species-specific timing helps reduce disease risk and protects buds and future growth.
  
  
• Remove dead, damaged, or diseased wood
  Winter makes it easier to see dead branches, cankers, and structural defects. Removing compromised wood reduces the chance of disease spread and improves overall tree structure.
  
  
• Practice proper winter watering
  Ensuring trees have enough water, especially during dry seasons like winter is essential. Even dormant trees need moisture. During dry winters, especially in Colorado, Utah, and other arid regions, deep watering during warm spells helps prevent drought stress and supports root health.
  
  
• Protect the root zone
  Maintain mulch around trees to moderate soil temperatures, retain moisture, and protect roots from freeze-thaw cycles. However, it’s essential to avoid piling mulch against the trunk; leave a few inches between the trunk and mulch.
  
  
• Avoid soil compaction
  Limit foot traffic, equipment use, and storage over root zones. Compacted soils restrict oxygen and water movement, contributing to long-term decline and, in some cases, a tree’s ability to absorb nutrients.
  
  
• Inspect trees regularly
  Given the nature of tree diseases, there's no one season for inspection, though it is helpful to know what to look for and when. Keep track of what you find in a tree inventory so you can track trends and monitor potential problems.
  
  
• Plan for spring and fall planting, care, and tree services/maintenance
  The concerns you document allow you to schedule inspections from certified arborists and plan treatments or planting adjustments before the busy growing season begins.

Consistent care helps trees enter spring healthier, more resilient, and better able to withstand disease, pests, and environmental stress. 

Protecting Your Urban Forest or Managed Landscape from Tree Disease

Protecting trees from disease is rarely about a single treatment; instead, it’s about consistent care, observation, and proactive decision-making. Following best practices year-round can help reduce risk and catch issues before they turn into costly or irreversible problems that may require tree removal and replacement.

Regular tree inspections and monitoring are essential. Many municipalities, HOAs, and large properties benefit from maintaining a living tree inventory, a working document that tracks species, condition, maintenance history, and emerging concerns. This allows patterns to be spotted over time and helps prioritize care where it’s needed most.

Being proactive matters. Addressing concerns early, whether that’s pruning, soil correction, or pest management, can prevent disease from spreading and protect the broader canopy. When something looks off, calling in certified arborists and experienced professionals sooner rather than later often saves both trees and budgets.

It’s also important to remember that canopy diversity is a powerful tool for disease prevention. A varied mix of species reduces the likelihood that a single disease or pest will impact an entire landscape, helping urban forests remain resilient and adaptable.

Healthy landscapes start with healthy trees. Working with an established, reputable nursery like SuperTrees ensures trees are grown, monitored, and selected with long-term success in mind.

We start with high-quality trees, but also provide access to certified arborists and professional tree services to support you at every stage, from planting to ongoing care. We work from seed to service, and we’re here to help. Reach out today to connect with our team!