Glossary

A

acropetal

Growth or movement directed toward the tip of a shoot or branch. Fire blight spreads acropetally through vascular tissue. The opposite of basipetal.

The pest or disease population level at which management intervention becomes justified. Below the threshold, the cost of treatment outweighs the damage; above it, damage accelerates. IPM programs are built on thresholds rather than calendar spraying.

adjuvant

Any substance added to a spray tank to improve performance. Surfactants, stickers, spreaders, and pH buffers are all adjuvants. Copper fungicide applications in particular benefit from proper adjuvant use.

adventitious

Arising from an unusual position: roots emerging from stems, buds forming where none normally exist. Trees under stress sometimes produce adventitious shoots along the trunk. Mulch volcanoes promote adventitious rooting at the expense of structural root systems.

amendment

Any material mixed into soil to improve its physical or chemical properties. Comite, aged bark, and composted leaf mold are common organic amendments in the Puget Sound lowlands. Amendments improve drainage in clay soils and water retention in sandy soils, but they are not the same as fertilizer.

Used in 9 guides

armored scale

A scale insect (family Diaspididae) covered by a waxy plate that separates from the insect body underneath. Armored scales produce no honeydew, cause direct tissue damage through feeding on plant cells, and typically have one generation per year in the Puget Sound region.

See also: soft scale , crawler , honeydew

Used in 2 guides

ascospore

A sexual spore produced inside a sac-like structure (ascus) by fungi in the phylum Ascomycota. In apple scab, ascospores form over winter inside pseudothecia in fallen leaf litter and are forcibly discharged into the air during spring rains, initiating the primary infection cycle.

B

basipetal

Growth or movement directed toward the base of a shoot or root. Verticillium wilt moves basipetally in the vascular system, which is why symptoms appear in the upper canopy first. The opposite of acropetal.

BBCH

A standardized numerical scale for describing plant growth stages, from seed germination (00) through senescence (97). Each two-digit code identifies a specific phenological event. HortGuide uses BBCH codes in the field observation log to standardize event recording across species.

biofix

A specific, observable biological event used as the starting point for a degree-day accumulation model. For apple scab, the biofix is 50% green tip on McIntosh flower buds (Gadoury & MacHardy 1982). Different pest and disease models use different biofix events.

budbreak

The moment a dormant bud splits open and new growth becomes visible. Budbreak timing is driven by accumulated heat (GDD) after sufficient chill hours have been met. In the Puget Sound lowlands, most deciduous trees break bud between late February and mid-April depending on species.

C

caliper

The diameter of a tree trunk, measured six inches above the soil line for trees under four inches, and twelve inches above for larger trees. Caliper is the nursery industry standard for sizing shade and street trees.

cambium

The thin layer of actively dividing cells between bark and wood that produces new growth each year. All radial trunk growth originates here. Damage to the cambium from construction equipment, weed trimmers, or girdling roots is often fatal because the tree cannot repair the vascular connection.

canker

A localized area of dead bark and cambium on a branch or trunk, often sunken or discolored. Cankers may be caused by fungi, bacteria, or environmental stress. Fire blight bacteria overwinter in cankers, making them both a symptom and a source of reinfection.

Used in 22 guides

chill hours

Hours of exposure to temperatures between 32°F and 45°F during winter dormancy. Many deciduous trees and fruit crops require a species-specific accumulation of chill hours before they can respond to spring warmth. The Puget Sound lowlands typically provide 800 to 1,200 chill hours per winter.

See also: dormancy , budbreak , vernalization

Used in 1 guide

Growing Degree Days: The Number That Tells You What Your Plants Are Actually Doing

chlamydospore

A thick-walled resting spore produced by certain fungi and oomycetes as a survival strategy during unfavorable conditions. Phytophthora produces chlamydospores that persist in soil for years and germinate when soil becomes saturated, initiating new infection cycles.

See also: zoospore , oospore , inoculum , sporulation

Used in 1 guide

Phytophthora Root Rot: The Water Mold That Loves Your Soil

chlorosis

Yellowing of leaf tissue due to insufficient chlorophyll production. Causes include nutrient deficiency (especially iron in high-pH soils), root damage, poor drainage, and disease. Interveinal chlorosis (yellowing between green veins) is a common diagnostic pattern.

conidium

An asexual spore produced by many fungi, typically on the surface of infected tissue. In apple scab, conidia form on leaf lesions 15 to 18 days after infection and splash to new leaves via rain, driving secondary infection cycles throughout the growing season. Plural: conidia.

cornicles

A pair of small tube-like projections on the rear end of an aphid that function as exhaust pipes, releasing defensive compounds. Cornicles are a diagnostic feature distinguishing true aphids from similar-looking insects like whiteflies and psyllids.

Used in 1 guide

Aphids: Why Spraying Is Usually the Wrong First Move

crawler

The mobile, first-instar nymph or larval stage of scale insects. Crawlers are the only exposed, vulnerable life stage of scale, and all contact-based treatments must target crawler emergence. Timing by GDD is critical for effective management.

See also: armored scale , soft scale , instar

Used in 2 guides

cultivar

A plant variety that has been selected and propagated for specific desirable traits: flower color, disease resistance, growth habit, fruit quality. Cultivar names appear in single quotes after the species name: Acer palmatum 'Bloodgood'. Not all named plants are cultivars; some are botanical varieties that occur in the wild.

D

diapause

A period of suspended development in insects, triggered by environmental cues like shortening day length or cooling temperatures. Similar to dormancy in plants but specific to arthropods. Many overwintering pests enter diapause in fall and resume development in spring when GDD accumulation resumes.

dieback

Progressive death of branches starting at the tips and moving toward the trunk. Dieback is a symptom, not a diagnosis: it can result from drought stress, root damage, vascular disease (Verticillium, Phytophthora), canker infections, or winter injury. The pattern and speed of dieback help distinguish causes.

Used in 23 guides

dormancy

A period of minimal metabolic activity and suspended visible growth. In the Puget Sound lowlands, most deciduous trees are dormant from leaf drop (November) through budbreak (February to April). Dormancy is the safest window for pruning disease-susceptible species because pathogens are also inactive.

See also: chill hours , budbreak , diapause

Used in 29 guides

drip line

The imaginary circle on the ground directly below the outermost edge of a tree's canopy, where rain drips off the leaves. Feeder roots concentrate in a zone extending from the trunk to well beyond the drip line. Fertilizer, mulch, and irrigation should target this zone.

Used in 5 guides

E

erineum

An abnormal, hair-like or felt-like growth on a plant leaf caused by eriophyid mite feeding. Erineums appear as velvet patches or blistered areas and are the plant's defensive response to mite irritation. Damage is cosmetic; the plant remains unharmed.

exfoliating

Bark that peels, flakes, or sheds in layers, revealing contrasting inner bark. Paperbark maple (Acer griseum), stewartia, and river birch are valued for their exfoliating bark. The characteristic is ornamental, not a sign of disease.

Used in 2 guides

F

frass

Insect excrement, often visible as fine sawdust-like material at the base of a tree or in bark crevices. Frass is a key diagnostic indicator for boring insects like bronze birch borer and bark beetles. The color and texture of frass can help identify the species responsible.

G

gall

An abnormal swelling or outgrowth on plant tissue caused by insects, mites, fungi, or bacteria. Galls on leaves are usually cosmetic. Galls on stems or roots (like crown gall caused by Agrobacterium tumefaciens) can be structurally significant.

Used in 19 guides

GDD

Growing degree days: a measure of accumulated heat used to predict biological events like pest emergence, bloom timing, and disease risk. Calculated by summing daily heat units above a base temperature. HortGuide uses GDD base 32°F (GDD₃₂) as the site-wide standard; source base temperatures are recorded in profiles for traceability.

See also: phenology , BBCH , chill hours

Used in 1 guide

Growing Degree Days: The Number That Tells You What Your Plants Are Actually Doing

H

honeydew

A sticky, sugary liquid excreted by sap-feeding insects including aphids, soft scales, mealybugs, and whiteflies. Honeydew coats surfaces below the canopy and serves as a substrate for sooty mold. Its presence is often the first visible sign of an infestation above.

host range

The set of plant species a pathogen or pest can infect or feed on. Broad host range pathogens like Verticillium dahliae (hundreds of species) are harder to manage through crop rotation or replacement planting than narrow host range organisms.

hysteranthous

Producing flowers before leaves emerge. Most cherries (Prunus), magnolias, and witch hazels are hysteranthous, blooming on bare wood in late winter or early spring. The opposite of synanthous. A useful field identification cue during spring bloom.

I

inoculum

The pathogen material (spores, bacterial cells, infected tissue) capable of initiating new infection. Fallen leaves, mummified fruit, and overwintering cankers are common inoculum sources. Sanitation practices aim to reduce the inoculum load before the next infection cycle begins.

instar

A developmental stage of an insect larva between molts. Many pest management decisions depend on targeting a specific instar: early instars are usually more vulnerable to insecticides than later ones. Bronze birch borer larvae, for example, cause the most cambial damage in their later instars.

IPM

Integrated pest management: a decision-making framework that uses monitoring, action thresholds, cultural practices, biological controls, and targeted chemical applications to manage pests with minimal ecological disruption. IPM does not mean "no spraying"; it means spraying only when monitoring data justifies it.

K

kickback activity

The ability of a systemic fungicide to arrest an infection that has already started, within a narrow window after spore germination. For apple scab, myclobutanil and propiconazole provide 36 to 96 hours of kickback activity after a rain event. Once the window closes, the infection is established and no fungicide will stop it.

L

leader

The dominant upright stem of a tree that establishes the central axis of growth. Most conifers and many deciduous shade trees develop a single central leader. Competing leaders (codominant stems) create weak branch attachments that are prone to failure. Structural pruning addresses leader selection early in a tree's life.

leatherjacket

The larval form of a crane fly (family Tipulidae). Leatherjackets are gray-brown, wrinkled, legless grubs about 1.5 inches long when fully grown. They feed on grass roots and crowns through winter and early spring, causing the brown patches visible in February and March lawns.

M

microsclerotia

Tiny, dark resting structures produced by Verticillium dahliae fungus that persist in soil for years, sometimes indefinitely. Microsclerotia germinate in response to root signals and initiate infection. Once established in soil, microsclerotia cannot be eliminated through cultural practice; they define permanent site risk.

Mills table

A decision-support chart relating air temperature to the hours of leaf wetness required for apple scab infection. Originally published by Mills in 1944 and revised by MacHardy and Gadoury in 1989, who found ascospores need about three fewer hours than Mills calculated. The table remains the foundational tool for timing fungicide applications against apple scab.

mine

A feeding tunnel created by insect larvae between the upper and lower surfaces of a leaf. Leafminers of various species produce characteristic mine shapes (serpentine, blotch, tentiform) that aid identification. Most leafminer damage is cosmetic on established plants.

Used in 19 guides

N

necrosis

Death of plant tissue, typically appearing as brown or black areas on leaves, stems, or roots. Necrotic tissue does not recover. The pattern of necrosis (marginal, interveinal, spotty, or zonal) is a primary diagnostic tool for distinguishing diseases, nutrient deficiencies, and environmental stress.

O

oomycete

An organism that behaves like a fungus but is more closely related to algae. Oomycetes include water molds like Phytophthora. Unlike fungi, oomycetes depend absolutely on free water for spore dispersal and infection. They are not controlled by fungicides targeting fungal cell walls.

oospore

A thick-walled sexual spore produced by oomycetes through fusion of male and female gametes. Oospores are particularly resistant to desiccation and can persist in soil for years. Phytophthora oospores germinate when soil becomes saturated, producing zoospores that swim through water toward plant roots.

See also: zoospore , chlamydospore , sporulation , inoculum

Used in 1 guide

Phytophthora Root Rot: The Water Mold That Loves Your Soil

P

parthenogenesis

Asexual reproduction in which females produce viable offspring without mating. Many aphid populations and some root weevil species reproduce by parthenogenesis, allowing females to clone themselves and produce numerous generations rapidly. This reproductive strategy enables pest populations to explode from a single colonizing individual.

Used in 1 guide

Root Weevil: The Night Shift Pest in Every Western Washington Garden

pathogen

An organism that causes disease in a host plant. In the Puget Sound lowlands, the major pathogen groups are fungi (powdery mildew, anthracnose, Phytophthora), bacteria (fire blight, Pseudomonas), and oomycetes (often grouped with fungi but biologically distinct). Viruses are rarely significant in ornamental landscapes here.

See also: inoculum , vector , host range

Used in 27 guides

phenology

The study of recurring biological events and their relationship to climate. In practice, phenology means tracking when plants bloom, when pests emerge, and when diseases activate, then using those observations to time management actions. GDD is the quantitative backbone of phenological tracking.

phloem

The vascular tissue that transports sugars and other organic compounds from leaves downward to roots and other growing tissues. Phloem sits just inside the bark, outside the cambium. Girdling damage (from string trimmers, rodents, or circling roots) severs the phloem and starves the root system.

post-emergent

A herbicide applied to weeds that have already germinated and are actively growing. Post-emergent products work through foliar contact or systemic uptake. They are the fallback when pre-emergent timing was missed.

pre-emergent

A herbicide applied to soil before weed seeds germinate, creating a chemical barrier that prevents root development in germinating seedlings. Timing is critical: the product must be in place before soil temperatures reach the germination threshold for the target weed. In the Puget Sound lowlands, forsythia bloom is the traditional phenological indicator for crabgrass pre-emergent timing.

protectant fungicide

A fungicide that must be present on the plant surface before a pathogen arrives. Protectants prevent spore germination on contact but have no activity against infections that have already started. Once rain washes the product off or new unprotected growth emerges, coverage is gone. Examples: captan, sulfur, chlorothalonil, copper.

Used in 1 guide

Dormant Season Spraying: Your Winter Window for Pest and Disease Control

pseudothecium

A flask-shaped fruiting body produced by certain Ascomycete fungi. In apple scab, pseudothecia develop over winter inside dead leaf litter on the ground and produce ascospores the following spring. Reducing leaf litter through fall sanitation directly reduces the number of pseudothecia and, therefore, the primary inoculum load. Plural: pseudothecia.

R

resistance

A plant's genetic ability to limit or prevent infection by a specific pathogen or damage by a specific pest. Resistance is not immunity: resistant cultivars may still show some symptoms under heavy pressure. In HortGuide profiles, resistance ratings range from highly resistant to highly susceptible.

See also: susceptibility , tolerance , cultivar

Used in 15 guides

root flare

The widening base of a tree trunk where it transitions into the root system. The root flare should be visible at the soil surface after planting. Buried root flares (from deep planting or mulch volcanoes) promote bark decay, adventitious rooting, and structural instability.

rootstock

The root system and lower trunk portion of a grafted plant, selected for vigor, soil adaptation, disease resistance, or size control. In ornamental landscapes, Japanese maples and flowering cherries are commonly grafted onto species rootstocks. Rootstock suckers should be removed promptly.

S

scab

A plant disease caused by fungi in the genus Venturia, characterized by raised, rough, corky lesions on fruit and olive-green to black spots on leaves. Apple scab (Venturia inaequalis) is the most common form in the Puget Sound lowlands, affecting all Malus species. The name refers to the scab-like texture of fruit lesions where the skin ruptures over the infection site.

See also: ascospore , pseudothecium , Mills table

scaffold branch

A primary structural limb that forms the permanent framework of a tree's canopy. Good scaffold branch selection during structural pruning establishes wide branch angles, adequate vertical spacing, and radial distribution around the trunk.

scion

The upper portion of a grafted plant, selected for its desirable ornamental or fruiting characteristics. The scion is grafted onto the rootstock. Cultivar identity comes from the scion; vigor and adaptability come from the rootstock.

scouting

Systematic monitoring of plants for pest and disease activity. Scouting means walking a route on a regular schedule, checking specific plant parts for specific symptoms at specific times of year. It is the data-collection step that makes IPM possible.

senescence

The programmed decline and death of plant tissue at the end of its functional life. Leaf senescence in autumn involves nutrient reabsorption before leaf drop. Premature senescence (leaves dropping in July) indicates stress, not normal aging.

sessile

Fixed in place; unable to move independently. Scale insects are sessile once they settle into their feeding site, cemented to the plant by secreted material. Once settled, they lose mobility and must be managed differently than mobile pest stages.

soft scale

A scale insect (family Coccidae) whose outer covering is the insect's own body wall rather than a separate waxy plate. Soft scales excrete honeydew, which coats foliage and serves as a substrate for sooty mold. Soft scales are larger and more conspicuous than armored scales and typically have one generation per year in Western Washington.

See also: armored scale , honeydew , crawler

sporulation

The production and release of spores by a fungal pathogen. Environmental conditions (temperature, humidity, leaf wetness duration) control sporulation timing. In the Puget Sound lowlands, the wet springs that favor sporulation are why fungal diseases dominate our disease pressure calendar.

spur

A short, stubby side branch on fruit trees and some ornamentals that produces flower buds year after year. Spur-bearing apple and pear cultivars concentrate their production on these structures. Fire blight frequently enters through infected blossom spurs.

Used in 2 guides

stippling

Fine pale dots scattered across a leaf surface caused by mite or insect feeding that punctures individual leaf cells. Stippling appears as if the leaf was dusted with powder and often precedes more serious damage like defoliation. Mite stippling is a common sign of summer stress and heat damage.

Used in 6 guides

sucker

A shoot arising from the root system or from below the graft union on a grafted plant. Suckers from below the graft are genetically different from the desired cultivar and should be removed at the base. Root suckers from species like black locust and sweetgum can be invasive.

surfactant

A substance that reduces the surface tension of water, allowing spray solutions to spread evenly across waxy leaf surfaces instead of beading up and rolling off. Copper fungicide and horticultural oil applications are more effective with proper surfactant use.

susceptibility

A plant's relative vulnerability to a specific pathogen or pest. High susceptibility means the plant is readily infected and shows significant damage. In HortGuide profiles, susceptibility is tracked in a matrix linking each plant to its known threats with severity ratings.

synanthous

Producing flowers and leaves at the same time. Callery pear (Pyrus calleryana) and hawthorn (Crataegus) are synanthous; most cherries (Prunus) are hysteranthous (flowers before leaves). A useful field identification cue during spring bloom.

systemic

In pesticide use: a product that is absorbed into the plant's vascular system and distributed throughout its tissues, providing internal protection. In pathology: an infection that has spread throughout the plant's vascular system rather than remaining localized. Context determines meaning.

Used in 4 guides

systemic fungicide

A fungicide absorbed into plant tissue that moves within the plant to protect areas beyond the application site. Systemic fungicides can arrest infections already in progress (kickback activity) but only within a narrow post-infection window. Examples for apple scab include myclobutanil and propiconazole.

Used in 4 guides

T

tolerance

A plant's ability to sustain pest or disease damage without significant impact on health, growth, or appearance. Tolerance differs from resistance: a tolerant plant still gets infected but handles it well. Mature trees often tolerate defoliation that would stress a young transplant.

V

variety

In botany, a naturally occurring subdivision of a species with consistent distinguishing characteristics: Cornus florida var. rubra (pink flowering dogwood). Distinct from cultivar, which is a human-selected form. In casual use the terms are often conflated, but in HortGuide profiles the distinction matters for taxonomy.

vector

An organism that transmits a pathogen from one host to another. Insects are the most common vectors in landscape settings: ants, flies, and wasps spread fire blight bacteria between flowers; bark beetles vector Dutch elm disease fungus. Wind and rain splash are abiotic vectors.

vernalization

The requirement for a prolonged period of cold exposure before a plant can flower or resume normal growth. Unlike chill hours (which apply broadly to dormancy release), vernalization specifically refers to the cold requirement for flowering in certain species, particularly biennials and some perennials.

voltinism

The number of generations an insect completes per year. Univoltine insects have one generation; bivoltine have two; multivoltine have three or more. Voltinism determines how many treatment windows exist per season and how quickly populations can build.

W

watersprout

A vigorous, upright shoot arising from latent buds on a branch or trunk, typically in response to stress, heavy pruning, or topping. Watersprouts have weak attachments and poor branch structure. Their proliferation after topping is one reason topping is destructive.

X

xylem

The vascular tissue that transports water and dissolved minerals from roots upward to the canopy. Xylem forms the wood of the tree. Verticillium wilt and Dutch elm disease block xylem vessels, causing the characteristic wilting and branch death pattern.

Z

zoospore

A mobile, flagellated spore produced by oomycetes that requires free water to swim. Zoospores are attracted to chemicals released by plant roots and navigate through soil water to find hosts. Unlike fungal spores, zoospores cannot survive desiccation or be carried on wind. The absolute water requirement makes Phytophthora a water-dependent pathogen.

See also: oomycete , oospore , chlamydospore , sporulation

Used in 1 guide

Phytophthora Root Rot: The Water Mold That Loves Your Soil