5.2 Weeds, Plant Diseases & Non-Insect Pests
Key Takeaways
- General insecticides do not kill spider mites and often eliminate their predators, triggering secondary mite outbreaks; dedicated miticides are required.
- Weeds are classified by life cycle as annual, biennial or perennial, and by morphology as broadleaf, grass or sedge.
- The plant disease triangle requires a susceptible host, a virulent pathogen and a favourable environment; removing any one prevents disease.
- A sign is the physical structure of the pathogen itself, such as mycelium or spores; a symptom is the host's reaction, such as chlorosis or wilting.
- The garden symphylan is a soil-dwelling myriapod of western Oregon soils that feeds on root hairs and emerging seedlings.
Last updated: August 2026
Weeds, Plant Diseases & Non-Insect Pests
Why this matters: Weeds and plant pathogens follow different rules from insects, and so do mites, symphylans and other non-insect arthropods that conventional insecticides simply do not kill. This section covers weed classification and life cycles, the plant disease triangle, and the signs-versus-symptoms distinction diagnosticians rely on.
1. Non-Insect Arthropods: Arachnids, Symphylans, and Crustaceans
Applicators frequently encounter plant-damaging organisms that resemble insects but belong to distinct subphyla and classes. Because their internal physiology, respiratory structures, and enzyme systems differ substantially from Class Insecta, conventional insecticides often fail completely against non-insect pests.
┌────────────────────────────────────────────────────────────────────────┐
│ INSECTS VS. ARACHNIDS (Mites & Spiders) │
│ │
│ ANATOMICAL TRAIT CLASS INSECTA CLASS ARACHNIDA │
│ ─────────────────────────────────────────────────────────────────── │
│ Body Tagmata (Regions) 3 (Head, Thorax, 2 (Cephalothorax, │
│ Abdomen) Abdomen; fused in mites)│
│ Adult Leg Pairs 3 Pairs (6 Legs) 4 Pairs (8 Legs)* │
│ Antennae 1 Pair (Present) 0 (None - No antennae)│
│ Wings 1 or 2 Pairs (Adults) 0 (Never have wings) │
│ Mouthparts Mandibles, Maxillae Chelicerae, Pedipalps │
│ Chemical Class Insecticides Acaricides / Miticides│
│ *Note: Mite larvae hatch with 6 legs, adding the 4th pair as nymphs. │
└────────────────────────────────────────────────────────────────────────┘
Two-Spotted Spider Mites (Tetranychus urticae)
Spider mites are among the most destructive pests in Oregon fruit orchards, wine vineyards, caneberries, hops, hazelnuts, and greenhouse floriculture.
- Feeding Biology: Spider mites use needle-like chelicerae to puncture individual plant epidermal cells and extract chlorophyll and cellular fluids, producing a fine, pale-yellow stippling across upper leaf surfaces.
- Webbing: As colonies expand, mites spin fine silken webs over shoots, leaves, and fruit clusters. This webbing protects eggs and nymphs from wind desiccation, dislodgement, and contact pesticide droplets.
- Population Dynamics: Spider mite development is heavily temperature-driven. In hot, dry, dusty summer conditions ($>85^\circ ext{F}$ or $29^\circ ext{C}$), a generation can complete in as few as 5 to 7 days. Females lay up to 200 eggs, leading to exponential population explosions that can defoliate entire crop canopies within two weeks.
- Chemical Control: General insecticides (synthetic pyrethroids, organophosphates) do not kill spider mites and often eliminate their natural predators (e.g., Typhlodromus occidentalis predatory mites), triggering massive secondary pest outbreaks. Applicators must use dedicated miticides/acaricides (e.g., bifenazate, hexythiazox, acequinocyl, spiromesifen) or narrow-range horticultural oils.
Other Important Non-Insect Pests in Oregon
- Eriophyid Mites (Rust Mites and Blister Mites): Microscopic, elongated, torpedo-shaped mites possessing only two pairs of legs (4 legs total) situated at the anterior end. They cause foliar russeting, bud galling, and blister-like leaf deformities in pears, apples, and blueberries.
- Garden Symphylan (Scutigerella immaculata): Small, white, translucent, soil-dwelling centipede-like myriapods possessing 12 pairs of legs and long, active antennae. Symphylans inhabit rich, moist agricultural soils in western Oregon (especially the Willamette Valley), feeding voraciously on root hairs, root tips, and emerging seedlings of vegetable, berry, and mint crops.
- Crustaceans (Sowbugs and Pillbugs): Terrestrial isopods that breathe via gills, requiring constantly moist environments under mulch, leaf litter, and drip lines. They occasionally clip tender seedling roots and low-hanging ripe strawberries.
2. Weed Biology, Morphology & Life Cycles
A weed is defined as any plant growing where it is not wanted, competing directly with cultivated crops, turfgrass, or forest trees for light, soil moisture, mineral nutrients, and physical root space. Weeds also serve as alternative reservoirs for insect vectors, plant viruses, and fungal pathogens.
┌────────────────────────────────────────────────────────────────────────┐
│ WEED MORPHOLOGICAL GROUPS │
│ │
│ 1. GRASSES (Monocotyledoneae): │
│ • Cotyledons: Exactly 1 seed leaf emerges upon germination. │
│ • Foliage: Narrow, upright blades with parallel veins and split │
│ leaf sheaths wrapping the stem. │
│ • Stems: Round or flattened, typically hollow between nodes. │
│ • Growing Point (Basal Meristem): Located at or below the soil │
│ surface, allowing grasses to survive mowing, grazing, and foliar │
│ contact herbicide burns. │
│ • Root System: Fibrous, high-surface-area root network. │
│ • Examples: Crabgrass, barnyardgrass, annual bluegrass, quackgrass.│
│ │
│ 2. BROADLEAVES (Dicotyledoneae): │
│ • Cotyledons: Exactly 2 seed leaves emerge upon germination. │
│ • Foliage: Broad, net-veined (reticulate) leaves with petioles. │
│ • Stems: Solid, branched structures with nodes and internodes. │
│ • Growing Points (Apical Meristems): Positioned exposed at stem │
│ tips and leaf axils, making them highly vulnerable to foliar │
│ contact and systemic broadleaf herbicides. │
│ • Root System: Prominent taproot or branched taproot system. │
│ • Examples: Dandelion, redroot pigweed, Canada thistle, plantain. │
│ │
│ 3. SEDGES (Cyperaceae): │
│ • Morphology: Monocots resembling grasses, but possessing solid, │
│ triangular stems ("sedges have edges") and leaves arranged in │
│ distinct sets of three. │
│ • Habitat: Thrives in wet, saturated, or poorly drained soils. │
│ • Propagation: Spreads via extensive rhizomes and underground │
│ nutlets / tubers that resist standard cultivation. │
│ • Example: Yellow nutsedge (*Cyperus esculentus*). │
└────────────────────────────────────────────────────────────────────────┘
Weed Life Cycles and Control Vulnerabilities
Understanding a weed's life cycle determines the precise chemical mode of action and application timing needed for successful eradication.
| Life Cycle Category | Growth Duration & Reproductive Strategy | Seasonal Subtypes & Representative Weeds | Optimum Timing & Management Tactics |
|---|---|---|---|
| Annuals | Complete their entire life cycle (seed germination, vegetative growth, flowering, seed set, and plant death) in less than 12 months. Propagate strictly by seed. | • Summer Annuals: Germinate in spring, grow actively in summer, set seed and die with the first fall frost (redroot pigweed, lambsquarters, large crabgrass, barnyardgrass).<br/>• Winter Annuals: Germinate in late summer/fall, overwinter as vegetative rosettes, flower and drop seed in early spring, dying as summer heat arrives (annual bluegrass, common chickweed, shepherd's purse). | • Pre-emergence herbicides applied before seed germination creates a chemical barrier in the upper soil layer.<br/>• Post-emergence herbicides applied to small, actively growing seedlings (2- to 4-leaf stage).<br/>• Mowing or cultivation before seed heads form. |
| Biennials | Require two full growing seasons to complete life cycle. Propagate exclusively by seed. | • Year 1: Seed germinates, producing a low-growing basal rosette and a deep, fleshy taproot that stores carbohydrates; overwinters in vegetative state.<br/>• Year 2: Plant responds to vernalization, "bolts" by sending up a tall flowering stalk, sets thousands of seeds, and dies (bull thistle, tansy ragwort, poison hemlock, wild carrot). | • Year 1 (Rosette Stage): Highly vulnerable to systemic broadleaf post-emergence herbicides (2,4-D, triclopyr, clopyralid).<br/>• Once bolting occurs in Year 2, herbicide translocation to the taproot decreases dramatically, making chemical control largely ineffective. |
| Perennials | Live for three or more years. Can reproduce by seed AND vegetatively via underground structures. | • Simple Perennials: Spread only by seed; possess a thick taproot or crown that regenerates new shoots if severed (dandelion, buckhorn plantain, curly dock).<br/>• Creeping Perennials: Spread by seed AND aggressive vegetative structures: rhizomes (underground stems), stolons (aboveground runners), tubers, bulbs, or creeping roots (Canada thistle, field bindweed, quackgrass, Himalayan blackberry, Japanese knotweed). | • Systemic translocated herbicides (glyphosate, triclopyr) applied in late summer / early fall when the plant naturally moves photosynthates downward to underground storage organs.<br/>• Avoid shallow tillage on creeping perennials; cutting rhizomes into fragments propagates and spreads new independent weed colonies. |
Weed Seed Bank Dynamics and Dormancy
Weed management requires recognizing that the visible weeds in a field represent only a tiny fraction of the total population. The soil seed bank consists of millions of dormant and viable seeds buried in the soil profile.
- Seed Longevity: Seeds of weeds like velvetleaf, redroot pigweed, and common lambsquarters can remain viable in undisturbed soil for 20 to 50+ years.
- Dormancy Mechanisms: Physical seed coat impermeability, physiological embryo dormancy, and light/temperature requirements prevent all seeds from germinating at once, ensuring weed survival across unfavorable seasons.
3. Plant Pathology: The Disease Triangle, Pathogens & Diagnostics
A plant disease is an abnormal, continuous physiological disruption in a plant caused by an infectious biotic agent (pathogen) or persistent abiotic stress that results in altered growth, structural deformation, reduced yield, or death.
┌────────────────────────────────────────────────────────────────────────┐
│ THE PLANT DISEASE TRIANGLE │
│ │
│ SUSCEPTIBLE HOST │
│ ▲ │
│ / \ │
│ / \ │
│ / \ │
│ / ⏰ \ │
│ / TIME \ │
│ / \ │
│ ▼ ▼ │
│ VIRULENT ◄─────────────► CONDUCIVE │
│ PATHOGEN ENVIRONMENT │
│ │
│ 1. SUSCEPTIBLE HOST: Crop/plant species lacking genetic resistance. │
│ 2. VIRULENT PATHOGEN: Active, viable disease-causing biotic organism. │
│ 3. CONDUCIVE ENVIRONMENT: Favorable temperature, humidity, and leaf │
│ wetness duration that permits spore germination and infection. │
│ 4. TIME: Sufficient continuous exposure under favorable conditions to │
│ complete infection, colonization, and symptom manifestation. │
│ │
│ CRITICAL PRINCIPLE: If ANY single component of the triangle is │
│ eliminated or disrupted, infectious disease CANNOT occur. │
└────────────────────────────────────────────────────────────────────────┘
Major Plant Pathogen Groups
- Fungi and Oomycetes: Responsible for more than 80% of all infectious plant diseases. Fungi are eukaryotic, non-photosynthetic organisms composed of microscopic, filamentous threads called hyphae (which aggregate into a mass called mycelium) and reproduce via airborne, waterborne, or soilborne spores. Oomycetes (water molds such as Phytophthora and Pythium) produce swimming, flagellated zoospores in water-saturated soils, causing devastating root rots, crown rots, and sudden oak death. Examples: Powdery mildew, apple scab (Venturia inaequalis), Botrytis gray mold, rusts, Verticillium wilt.
- Bacteria: Single-celled microscopic prokaryotic organisms lacking membrane-bound nuclei. Bacteria cannot penetrate intact, healthy plant cuticles on their own; they gain entry strictly through natural openings (stomata, hydathodes, lenticels, nectaries) or mechanical wounds (pruning cuts, insect feeding, hail trauma, wind abrasion). Dissemination occurs through wind-driven rain, irrigation splashing, contaminated pruning tools, and insect vectors. Examples: Fire blight (Erwinia amylovora in apples and pears), bacterial canker (Pseudomonas syringae in sweet cherries), crown gall (Agrobacterium tumefaciens).
- Viruses and Viroids: Submicroscopic obligate intracellular parasites consisting of a nucleic acid core (RNA or DNA) enclosed within a protective protein capsid. Viruses highjack host cellular replication machinery to reproduce. Because they lack independent locomotion, plant viruses depend completely on vectors (aphids, leafhoppers, thrips, whiteflies, root-feeding nematodes) or mechanical transfer via grafting tools and pruning sap. Symptoms include systemic mosaic patterns, chlorotic ringspots, vein clearing, leaf enations, and severe plant stunting. No curative chemical "viricides" exist for field crops; management relies on virus-free certified stock, vector suppression, resistant cultivars, and immediate rogueing (destruction) of infected plants.
- Plant-Parasitic Nematodes: Microscopic, unsegmented, translucent roundworms living in soil, root tissue, or foliar bulbs. Plant-parasitic species possess a hollow, needle-like mouth spear called a stylet used to puncture plant cell walls, inject digestive enzymes, and siphon out liquefied cellular contents. Belowground damage includes root knots/galls (Meloidogyne spp.), lesion necrosis (Pratylenchus spp.), stubby roots, and root rot complexes. Aboveground symptoms mirror severe drought stress and nutrient deficiency.
- Abiotic (Non-Infectious) Disorders: Plant injuries caused by non-living environmental, chemical, or physical factors: freezing temperatures, sunscald, drought, waterlogged/anoxic soils, soil compaction, nutrient deficiencies (e.g., iron chlorosis in high pH soils), salinity, and pesticide drift/carryover phytotoxicity. Abiotic disorders are non-transmissible from plant to plant.
The Diagnostic Rule: Symptoms vs. Signs
Accurate disease diagnosis requires distinguishing between how the host plant responds and the physical presence of the causal pathogen.
| Diagnostic Category | Scientific Definition | Concrete Field Examples |
|---|---|---|
| Symptom | The plant's internal or external physiological and morphological reaction to infection or environmental stress. Symptoms describe the altered state of the host. | • Chlorosis: Yellowing of green foliar tissue due to chlorophyll breakdown.<br/>• Necrosis: Browning and localized death of plant cells/tissues.<br/>• Wilting: Loss of plant turgidity caused by vascular xylem blockage.<br/>• Cankers: Sunken, localized dead lesions on bark, twigs, or woody stems.<br/>• Galls / Tumors: Abnormal swelling or proliferation of host cells.<br/>• Damping-Off: Collapse and death of young seedlings at the soil surface. |
| Sign | The actual physical structures, vegetative tissues, or reproductive bodies of the pathogen itself visible on or within diseased host tissue. | • Fungal Mycelium: White, fuzzy, or thread-like fungal mats on leaves.<br/>• Spore Pustules: Powdery white spores of powdery mildew, bright orange pustules of cedar-apple rust.<br/>• Fruiting Bodies: Black pepper-like pycnidia, perithecia, or shelf conks on wood.<br/>• Bacterial Ooze / Streaming: Viscous milky exudate oozing from cankers or seen streaming from cut stem vascular bundles under water.<br/>• Nematode Cysts: Tiny pearl-like female nematode cysts clinging to roots. |
Test Your Knowledge
A vineyard manager identifies white, powdery fungal growth and spore chains covering grape leaves that exhibit chlorosis and stunted shoot vigor. In plant pathology diagnostics, how are these observations categorized?
A
B
C
D