Did you know: Fungal diseases are common in Phoenix and there are hundreds that have been identified. Here is a list of the top 25 most common fungal pathogens that we see in the Valley:

Root & Vascular Wilts (Highest Threat) 

These are considered the most severe threats because they attack the vascular system or roots, often leading to tree death. 

1. Phymatotrichopsis omnivora (Texas Root Rot) 

• Threat Level: Critical. It is one of the few fungi that can kill a mature, healthy tree in a matter of weeks during summer. 

• Targets: Rhus lancea (African Sumac), Xylosma, Bottle Trees, Silk Oak, Carob, and many non-native broadleaf trees. 

2. Phytophthora spp. (Root and Crown Rot) 

• Threat Level: High. The primary killer of nursery stock and established plants in poorly drained or overwatered soils. 

• Targets: Citrus, Agave, Periwinkle (Vinca), and nearly all landscape shrubs. 

3. Fusarium oxysporum f. sp. canariensis (Fusarium Wilt of Palm) 

• Threat Level: Severe/Lethal. There is no cure. It spreads via contaminated pruning tools. 

• Targets: Canary Island Date Palms (Phoenix canariensis). 

4. Ganoderma spp. (Butt Rot / Varnish Fungus) 

• Threat Level: High. It eats the wood at the base of the trunk, causing trees to fall over unexpectedly (windthrow). 

• Targets: Mesquite, Acacia, Olive, and Mulberry. 

5. Verticillium dahliae (Verticillium Wilt) 

• Threat Level: Moderate to High. A soil-borne fungus that blocks the xylem, causing chronic dieback. 

• Targets: Olive, Ash, Pistachio, and some stone fruits. 

6. Thielaviopsis paradoxa (Palm Trunk Rot) 

• Threat Level: High. Causes "sudden collapse" or bleeding lesions on palms. 

• Targets: Date Palms and Queen Palms. 

7. Armillaria spp. (Shoestring Root Rot) 

• Threat Level: Moderate. Attacks roots of stressed woody plants, often identifiable by honey-colored mushrooms at the base. 

• Targets: Roses, Oaks, and various ornamentals.

Canker Diseases (Structural Threats) 

These fungi enter through bark wounds (sunburn, pruning cuts) and kill the cambium, leading to limb dieback. 

8. Neofusicoccum mangiferae (formerly Hendersonula toruloidea) (Sooty Canker) 

• Threat Level: High. The most common cause of limb dieback in smooth-barked trees in Phoenix. 

• Targets: Ficus, Mulberry, Citrus, and Ash. 

9. Botryosphaeria spp. (Bot Canker) 

• Threat Level: High. Aggressive on drought-stressed trees. 

• Targets: Juniper, Cypress, and many hardwoods. 

10. Cytospora spp. (Cytospora Canker) 

• Threat Level: Moderate. Often attacks stone fruits and cottonwoods under stress. 

• Targets: Cottonwood, Willow, Peach, and Plum. 

11. Hypoxylon spp. (Hypoxylon Canker) 

• Threat Level: Moderate. A stress-related pathogen that causes bark to slough off, revealing black fungal mats. 

• Targets: Oak, Sycamore, and stressed hardwoods. 

12. Seiridium spp. (Seiridium Canker) 

• Threat Level: High (Specific). The primary reason Italian Cypress fail in the desert. 

• Targets: Italian Cypress and Leyland Cypress. 

13. Gliocladium vermoeseni (Pink Rot) 

• Threat Level: Moderate. Opportunistic on palms, usually affecting the bud or weakened fronds. 

• Targets: Queen Palms, Pygmy Date Palms. 

Foliar Diseases (Aesthetic & Stress Threats) 

These rarely kill plants outright in Arizona but cause significant disfigurement and stress. 

14. Erysiphe / Podosphaera spp. (Powdery Mildew) 

• Threat: Low to Moderate. Causes white dusting on leaves, stunted growth. 

• Targets: Roses, Euonymus, Crape Myrtle, Rosemary. 

15. Entomosporium mespili (Leaf Spot) 

• Threat: Moderate. Causes severe defoliation in wet springs. 

• Targets: Indian Hawthorn (Rhaphiolepis), Photinia. 

16. Apiognomonia veneta (Sycamore Anthracnose) 

• Threat: Low. Causes early leaf drop in Sycamores during wet springs. 

• Targets: Arizona Sycamore, London Plane Tree. 

17. Discula fraxinea (Ash Anthracnose) 

• Threat: Low. Similar to Sycamore Anthracnose but affects Ash trees. 

• Targets: Modesto Ash, Shamel Ash. 

18. Gymnosporangium spp. (Rusts) 

• Threat: Low. Creates galls on junipers and rust spots on rosy-family plants.

• Targets: Junipers (gall host) and Apple/Pear (leaf host). 

19. Alternaria spp. (Leaf Spot / Blight) 

• ○ Threat: Low. A common secondary invader on sun-scalded or weak leaves. ○ Targets: Citrus, Ornamentals. 

Turfgrass Pathogens (Landscape Threats) 

Fungal diseases that specifically destroy lawns (Bermuda/Ryegrass) in Phoenix. 

20. Ophiosphaerella spp. (Spring Dead Spot) 

• Threat: High. The most destructive disease of Bermudagrass in Arizona. 

• Targets: Hybrid Bermudagrass. 

21. Rhizoctonia solani (Large Patch / Brown Patch) 

• Threat: Moderate. Causes circular brown patches in lawns during humid transitions. 

• Targets: Paspalum, Bermudagrass, St. Augustine. 

22. Pythium spp. (Pythium Blight) 

• Threat: High. "Grease spot" disease that destroys overseeded Ryegrass rapidly in warm, wet fall weather. 

• Targets: Perennial Ryegrass (Overseed). 

23. Sclerotinia homoeocarpa (Dollar Spot) 

• Threat: Low to Moderate. Causes small, silver-dollar-sized spots on neglected turf. 

• Targets: Bermudagrass. 

24. Bipolaris spp. (Leaf Spot / Melting Out) 

• Threat: Moderate. Causes thinning of grass during shoulder seasons. 

• Targets: Bermudagrass and Ryegrass. 

25. Gaeumannomyces graminis (Take-All Root Rot) 

• Threat: Moderate. Causes declining patches in turf, destroying roots. 

• Targets: Bermudagrass and St. Augustine. 

Native & Landscape Cactus/Tree Threats 

These are often the most "famous" and visible bacterial issues in the Phoenix landscape. 

1. Bacterial Necrosis of Saguaro (Erwinia cacticida / Pectobacterium carnegieana) ○

• Target: Saguaro Cactus (Carnegiea gigantea). 

• Symptoms: The classic "black goo" disease. It causes black, foul-smelling liquid to ooze from lesions on the trunk or arms. It rots the internal flesh, often causing arms to fall off or the cactus to collapse. 

• Trigger: Wounds from frost, wind, or physical damage allow the bacteria to enter. 

2. Oleander Leaf Scorch (Xylella fastidiosa) 

• Target: Oleander (Nerium oleander).

• Symptoms: Leaves turn brown and "scorched" at the tips and margins, separated from green tissue by a yellow halo. It eventually kills the entire shrub branch by branch. 

• Vector: Spread by the Glassy-Winged Sharpshooter insect. 

3. Slime Flux / Bacterial Wetwood (Various bacteria including Enterobacter & Klebsiella) 

• Target: Mesquite, Mulberry, Elm, and Cottonwood. 

• Symptoms: Dark, wet, bubbling streaks on the trunk that often smell sour or alcoholic. It attracts flies and beetles. 

• Significance: While rarely lethal on its own, it causes unsightly staining and indicates internal pressure and stress. 

4. Crown Gall (Agrobacterium tumefaciens) 

• Target: Roses, Euonymus, Fruit Trees, and Willows. 

• Symptoms: Large, woody, tumor-like growths (galls) appear at the soil line (crown) or on roots. 

• Significance: Stunts growth and restricts water uptake. It is soil-borne and can persist for years. 

5. Olive Knot (Pseudomonas savastanoi) 

• Target: Olive Trees (Olea europaea) and Oleanders. 

• Symptoms: Rough, corky galls or knots form on twigs and small branches, often causing dieback of the tips. 

• Trigger: Bacteria enter through leaf scars or wounds during wet weather. 

6. Agave Soft Rot (Pectobacterium / Erwinia spp.) 

• Target: Agaves (especially Agave americana and A. weberi). 

• Symptoms: The heart of the agave turns into a mushy, foul-smelling rot, often allowing the central spike to be pulled out easily. 

• Trigger: Often introduced by the Agave Snout Weevil. 

Citrus & Fruit Tree Threats 

Bacterial issues are a major concern for the Phoenix citrus industry and backyard growers. 

7. Citrus Stubborn Disease (Spiroplasma citri) 

• Target: Citrus (especially Navel Oranges). 

• Symptoms: Trees remain stunted and "stubbornly" refuse to grow. Fruit is small, lopsided, or acorn-shaped and stays green (doesn't ripen properly). 

• Vector: Beet leafhopper. 

8. Fire Blight (Erwinia amylovora) 

• Target: Apples, Pears, Pyracantha, and Loquat. 

• Symptoms: Branches suddenly wilt and turn black as if scorched by fire. The tips often curl over into a "shepherd's hook" shape. 

• Trigger: Warm, wet weather during spring bloom. 

9. Citrus Blast (Pseudomonas syringae) 

• Target: Citrus (especially Grapefruit and Oranges).

• Symptoms: Black lesions on leaf petioles (where the leaf meets the stem) and sudden dieback of twigs on the windward side of the tree. 

• Trigger: Occurs after winter storms where wind drives rain into leaf tissues. 

10. Citrus Greening / HLB (Candidatus Liberibacter asiaticus) 

• Target: All Citrus. 

• Status: High Alert / Quarantine. While established in other states, it has been detected in Arizona (Nogales area) and is under strict quarantine. 

• Symptoms: Yellow blotchy leaves, bitter/misshapen fruit, and tree death. Spread by the Asian Citrus Psyllid. 

11. Bacterial Canker of Stone Fruit (Pseudomonas syringae) 

• Target: Peaches, Plums, Apricots, and Almonds. 

• Symptoms: Sunken, gummy cankers on branches and "shot hole" spots on leaves. 

• Trigger: Stress and winter freeze damage.