Trees in Fresno work hard. They shade homes through 100-plus degree summers, anchor front yards, and add real value to a property. Like anything living, they also get sick. Fungal infections, boring insects, root rot, and decay can quietly weaken a tree for years before the damage shows, and by then the fix is often expensive. This guide from the arborists at Cut Rite Tree Service walks through the most common tree diseases and pests we see across Fresno and the Central Valley, how to identify them, what treatment looks like, and when a sick or dead tree needs to come out.
Cut Rite Tree Service is a licensed, family-run Fresno tree service rated 4.8 stars across more than 200 Google reviews (CSLB #1015044). If you are worried about a tree on your property, call us at (559) 320-7723 for a free, no-pressure estimate.
How to Tell If a Tree Is Sick
Most tree problems give you warning signs months before they become an emergency. Walk your trees a few times a year and watch for the following:
- Leaves that wilt, yellow, brown, or drop well before fall, or a canopy that is visibly thinning.
- Dead or bare branches, especially starting at the top of the crown.
- Cracked, peeling, or oozing bark, and soft or hollow spots in the trunk.
- Mushrooms, conks, or fungal growth at the base or on the trunk.
- Sawdust, tiny exit holes, sticky residue, or visible insects on the bark and leaves.
- A lean that is getting worse, or soil heaving up around the roots.
One quick test for a branch you think is dead is the scratch test: scrape a small spot of bark off a twig with your fingernail. Green and moist underneath means that wood is still alive, while brown and dry means that part has died back. Check several branches before deciding the whole tree is gone. Because many diseases produce similar symptoms, and some look far worse than they actually are, the smartest first move is a professional diagnosis before you spend money on fungicides or pesticides you may not need.
Common Fungal Tree Diseases in Fresno
Fungus is behind the majority of tree disease we treat in the Central Valley. Most fungal pathogens get in through wounds, then attack the leaves, vascular system, or roots. Catching them early is the difference between a treatable problem and a removal.
Root Rot
Root rot attacks the roots and stops them from absorbing water and nutrients. The fungus thrives in soggy soil, which makes overwatering and poor drainage the number one cause, a common problem in Fresno yards on automatic irrigation. Signs include prematurely yellowing leaves, a thinning canopy, and slow decline. Because the damage is underground and structural, advanced root rot often means the tree is no longer safe to keep. Correcting drainage and easing back on irrigation is the best prevention.
Anthracnose and Leaf Blights
Anthracnose and similar leaf fungi cause brown or black blotches on leaves, distorted growth, twig dieback, and early leaf drop, often after a wet spring. Needle blight does the same to conifers, turning needles a reddish brown. These are rarely fatal to an otherwise healthy tree, but a heavy infection year after year wears it down. Raking up and disposing of fallen leaves, proper pruning for airflow, and a fungicide on valued specimens keep it in check.
Powdery Mildew
Powdery mildew shows up as a white to gray dusty coating on leaves and new shoots. It is most common where airflow is poor and humidity collects. It rarely kills a tree on its own but can stunt growth and disfigure the canopy. Thinning the crown to improve air movement is the most effective long-term control.
Sooty Mold
Sooty mold coats leaves and branches in a black, soot-like film. The mold itself does not infect the tree, but it is a red flag: it grows on honeydew, the sticky secretion left behind by aphids, scale, and other sap-sucking insects. In other words, sooty mold usually means you have a pest problem feeding above it. Control the insects with an insecticidal treatment and the mold clears up on its own.
Cankers and Black Knot
Canker disease attacks a tree’s vascular system, usually entering through an open wound or torn bark. It shows up as sunken, brownish-gray, mottled areas on the trunk or branches and can slowly girdle and kill a limb. Black knot is a related fungal disease that erupts in large black galls along branches and tends to come back every year if it is not removed. Both call for clean, professional pruning of the infected wood, and tending to bark injuries promptly is the best way to prevent them from getting a foothold.
Vascular Wilts: Dutch Elm and Fire Blight
Some pathogens choke a tree’s plumbing from the inside. Dutch elm disease is a fungus that blocks water transport, and the first sign is yellowing, wilting leaves on otherwise healthy elms. Fire blight is a bacterium that gives branches a scorched, blackened look, as if they were burned. Caught early, both can sometimes be managed by pruning out infected wood well below the damage and, in some cases, fungicide or antibacterial treatment. Left alone, they are frequently fatal.
Mushrooms and Conks at the Base of a Tree
Finding mushrooms growing at the base of a tree or shelf-like conks on the trunk is one of the most important warning signs to understand. Not every mushroom is a death sentence. Many of the white to cream colored mushrooms that pop up in a damp lawn are harmless and simply feeding on dead material in the soil. The color and pattern of the growth matter.
The one you never want to see is honey fungus. It comes up in clusters of thin, white to honey-colored button mushrooms near the base and attacks the near-surface roots, killing the tree from the bottom up. Just as serious are hard, woody conks growing directly out of the trunk or major roots. A conk is the fruiting body of a fungus that is already digesting the wood inside, which means there is decay you cannot see. When we find conks on a mature tree near a house or driveway, we take the structural risk seriously and inspect for internal hollowing.
Algae and Lichen: Usually Harmless
A green, powdery film or crusty gray-green patches on bark often scare homeowners, but algae and lichen are almost always harmless to the tree. They are simple organisms that live on the surface of the bark and do not feed on the tree itself. They prefer moist, shaded spots, so a heavy coating is really a clue about the environment: low sunlight, poor airflow, or ground that stays too wet. Clearing them is purely cosmetic. The growth itself is not the problem, but the damp, shady conditions that encourage it can also invite the fungi that do cause harm, so it is worth checking drainage. If a tree with algae also shows drooping leaves, brittle branches, or stalled growth, that is your cue to look closer.
Common Tree Pests in the Central Valley
California’s warm climate is hospitable to a long list of insects that damage trees. Some chew the leaves, some bore into the wood, and some drain the sap. Here are the ones Fresno property owners run into most.
Borers, Including the Emerald Ash Borer
Wood-boring insects are among the most destructive pests because they tunnel under the bark and cut off the tree’s flow of water and nutrients. The emerald ash borer is the headline threat: a small, metallic-green beetle native to Asia that has spread across North America and kills ash trees. Its larvae feed in winding tunnels beneath the bark, and the adults leave distinctive D-shaped exit holes. Warning signs are canopy dieback starting at the top, new sprouts low on the trunk, and bark splitting. Bark beetles attack drought-stressed pines the same way, which is why heat-stressed Valley trees are especially vulnerable. Borer infestations are hard to reverse once established, so early professional assessment is critical, and a heavily infested tree usually has to be removed before it becomes a hazard or spreads to neighbors.
Bagworms
Bagworms are the larvae of a moth and are named for the silky, cone-shaped sacks they build and hang from branches, most often on evergreens like spruce, juniper, arborvitae, and pine. A single bag can hold up to 1,000 eggs that hatch in early June, and the larvae then strip needles and foliage. In a bad infestation they can eat 80 percent or more of a tree and kill it outright. The most reliable fix is to pick the bags off by hand before they hatch, drop them in soapy water, and seal and trash them. For larger infestations, a properly timed treatment after the eggs hatch is far more effective than spraying the bags themselves.
Aphids, Scale, and Mites
These sap-sucking pests weaken a tree slowly and create that sticky honeydew that leads to sooty mold. Aphids cluster on new growth and tender leaves. Scale insects look like small, immovable bumps stuck to twigs and branches. Spider mites are tiny but show up as fine stippling and webbing on stressed leaves, and they love hot, dusty Fresno summers. Light infestations on a healthy tree are mostly cosmetic, but heavy or repeated pressure stunts growth and invites secondary problems. Insecticidal treatments and keeping the tree well watered and healthy go a long way.
Leaf-Eating Caterpillars and Citrus Pests
Oak worms, the larvae of tussock moths, feed on oak leaves and can defoliate large areas, leaving chewed, browning foliage. On citrus, which thrives in Fresno yards, the Asian citrus psyllid is a serious concern: it spreads a disease that causes pale blotched leaves, twig dieback, and small misshapen fruit, and infested trees often decline within a few years. Catching either early and treating on the right schedule protects the rest of the canopy.
Tree Decay and Heart Rot
Decay is what happens when fungi break down the wood itself, usually starting at an injury that lets the pathogen reach the heartwood. As decay spreads, the cellulose and lignin that give wood its strength break down, the affected areas go soft and spongy, and the tree loses structural integrity from the inside out. Arborists generally describe three forms:
- White rot breaks down all the wood components and leaves a pale, bleached, sometimes stringy or spongy texture. It is common in many hardwoods and conifers.
- Brown rot attacks the cellulose and leaves the wood brown, dry, and crumbling into cube-like blocks, which is especially dangerous because it can fail suddenly.
- Soft rot works more slowly on the surface layers in wet conditions.
The danger with decay and heart rot is that the outside of the tree can look fine while the core is hollow. A trunk with significant internal decay can drop a limb or fail entirely in a Valley windstorm. This is exactly the kind of hidden risk that needs a trained eye to assess.
Treatment, and When a Tree Has to Come Out
The right treatment always starts with the right diagnosis. Once we know what we are dealing with, options generally fall into a few buckets:
- Pruning out the problem. Removing infected limbs, cankered wood, or bagworm sacks stops many diseases and pests from spreading. Clean cuts also improve airflow, which discourages fungus. Our Fresno tree trimming and pruning crews do this work correctly and sanitize between cuts so they do not move disease around the tree.
- Targeted treatments. Fungicides, insecticidal sprays, and soil or trunk injections can knock back many infections and infestations when applied to the right problem at the right time.
- Cultural fixes. Correcting overwatering and drainage, mulching properly, and keeping a tree healthy and unstressed is the cheapest and most effective long-term defense, especially against the heat and drought stress that make Central Valley trees vulnerable.
Sometimes, though, a tree is too far gone. When root rot or heart rot has compromised the structure, when a borer infestation has hollowed the wood, or when a disease is killing the tree and threatening to spread to others, the responsible call is removal. A dead or structurally failing tree near a house, driveway, or power line is a genuine safety hazard during Fresno’s windstorms. In those cases our licensed crews handle Fresno tree removal safely, and we will always tell you honestly whether a tree can be saved or needs to go.
Central Valley Conditions That Stress Trees
Most tree disease and pest problems take hold because the tree was already stressed. In Fresno and the wider Central Valley, the big stressors are extreme summer heat, long dry spells, and watering mistakes in both directions. Drought-stressed trees are far more attractive to bark beetles and borers, while overwatered, poorly drained soil is an open invitation to root rot and other fungi. The single best thing you can do to keep your trees disease-resistant is to water deeply but not too often, mulch to hold moisture and protect the roots, avoid wounding the trunk, and have a professional look over your larger trees periodically. A healthy tree fights off most of these problems on its own.
Frequently Asked Questions
How can I tell if my tree is dying or just stressed?
Try the scratch test on several branches. If the wood under the bark is green and moist, that part is alive and the tree may just be stressed and recoverable. If most branches scrape brown and dry, and the canopy is bare with soft spots or conks on the trunk, the tree is likely dying and should be inspected by a professional.
Are mushrooms growing on my tree a serious problem?
It depends on the type and location. Harmless mushrooms often grow in the surrounding lawn from decaying material in the soil. But clustered honey fungus at the base, or hard woody conks coming out of the trunk, point to root or internal wood decay, which is a structural concern worth a professional inspection.
Should I treat a diseased tree myself or call an arborist?
Minor pest issues like hand-picking bagworms are fine to handle yourself. For anything affecting the trunk, roots, or overall health, get a professional diagnosis first. Many diseases look alike, and the wrong treatment wastes money and can make things worse. An arborist identifies the real cause and matches the treatment to it.
Can a tree infected with disease spread it to my other trees?
Yes. Many fungal diseases and pests, including the emerald ash borer, canker fungi, and root rot, spread from tree to tree through the air, soil, contaminated tools, or shared root systems. Prompt pruning, proper sanitation, and sometimes removing the source tree are how we stop an infection from taking out a whole yard.
Worried About a Tree in Fresno? Call Cut Rite Tree Service
Tree diseases, pests, and decay are far easier to deal with when you catch them early. If something about one of your trees looks off, whether it is fungus, a suspicious mushroom, chewed leaves, dieback, or just a tree that is not thriving, the team at Cut Rite Tree Service can identify the problem and give you a straight answer on how to fix it or whether it needs to be removed.
Call Cut Rite Tree Service at (559) 320-7723 for a free estimate, and let our licensed Fresno arborists protect the trees and the people on your property.
