damage

a measurable reduction in host utility (yield, quality or aesthetic value) resulting from pest activity.

damage boundary

the lowest injury level at which damage to the host can be measured.

Darcy's Equation

(from Agron 502, module 8 Overview, Water Storage in Soil)

To characterize soil water flow rates, we determine hydraulic conductivity (K) values for a given soil and soil condition. The hydraulic conductivity value is included in the equation for the flux density:

$LaTeX: \dfrac{\mathrm{V}}{\mathrm{ta}}=\mathrm{K}\ ( \dfrac{\Delta\psi}{\Delta x})$

where:

V = water volume
t = time
a = the cross-sectional area of the soil column.

The rate of water passing through the soil, V/(ta), is called the water flux (Q) and is denoted by Darcy’s equation:

$LaTeX: \mathrm{Q}=\mathrm{K}\ ( \dfrac{\Delta\psi}{\Delta x}) \quad \textbf{Darcy's Equation}$

Dimensions of Q and K are in length per unit time; dimensions of Ψ and x are in length. Using the symbols shown in the figure above, Q = K ( h / L).

daughter nuclei

the two identical nuclei arising from mitotic division of a parental nucleus. Both daughter nuclei have the same number of chromosomes and contain exactly the same genetic information, as the original nucleus.

deficiency symptom

Discernable evidence of abnormal growth resulting from a nutrient shortage is a deficiency symptom. The most common plant nutrient deficiency symptoms are stunted growth and chlorosis.

defoliation

(1) application of a chemical to cause leaf fall or desiccation usually to facilitate harvest. (2) removal of top growth by cutting or grazing.

degree day

An accumulation of heat units above some threshold temperature for a 24-hour period.

degrees of freedom

The number of degrees of freedom associated with a set of sample means is the number of samples minus 1.

delta

Deltas are somewhat like alluvial fans but form where a stream enters a lake or sea. The material being transported is then dumped in a roughly triangular area named after the Greek letter D (delta). The deltas of some large rivers are hundreds of miles across. Deltas have high water tables unless the level of the lake or sea drops relative to the land surface.

denaturation

loss of biological activity or function as result of changes in enzyme, protein, or DNA structure, usually induced by heat or pH change

denitrification

Denitrification is a volatilization process causing nitrogen to be lost from soils that are saturated with water part of the time or in part of their volume. The soil either has enough aeration to produce nitrates by normal nitrification processes or receives nitrate fertilizer. Anaerobic bacteria reduce the nitrates to volatile N₂ or gaseous oxides of nitrogen when the soil is water-logged.

deodorant

A material added to a pesticide formulation for masking unpleasant odors, e.g., cedar oil, pine oil.

deoxyribonucleic acid (DNA)

A double-stranded, helical, structure comprised of base pairs (ATCG) that carry genetic information in the nucleotide sequence. It is normally found, packaged or condensed, within the chromosomes, within the cell's nucleus. Single-stranded DNA is unstable and rarely found in nature.

depolarization

The reduction of the electrical potential of a neuron's membrane.

deposition

In plant pathology, the process whereby a pathogenic agent or pathogen propagule is placed in juxtaposition with plant tissue. Wind, rain splash, and insects are examples of forces that aid in the deposition of pathogenic agents.

descriptive ET

an economic threshold which is determined using knowledge of the population growth rate of the pest.

design control

application of restrictions on randomization in the design of an experiment for the purpose of better controlling recognizable sources of variation; for example, blocking to control soil variation in a randomized complete block experiment.

designed experiments

Designed experiments differ from observational experiments in that data are collected from units that have been manipulated by the researcher in some way before the data is collected. Compare observational experiments.

detassled

Removal of a maize tassel before anthesis as a method to control pollination.

determinant growth habit

a plant completes or nearly completes all of its vegetative growth before it flowers; e.g., the stem ends in a terminal flower part, such as a tassel

determinate

Characterizing the termination of vegetative growth by morphological or physiological causes, or an organism within which that occurs. In a precise sense, determinate growth is terminated morphologically, i.e., by apical flowering. In a broader sense, vegetative growth also can be terminated or arrested by competitive sink demand (e.g., fruit loading) and other physiological stresses. Compare indeterminate.

development

the progressive qualitative changes that an organism undergoes during its life cycle. Compare to growth.

developmental stage

specific discrete and identifiable stages of a life cycle, for example, first leaf, second leaf, first flower, full bloom, physiological maturity, etc. Stage of development indices have been defined for many crops so that scientists may more clearly communicate the timing of specific events to their colleagues. These are often mistakenly called "growth stages". Growth, however, refers to increase in size, weight or height. Development and growth usually proceed coincidentally, but not necessarily in parallel.

diagnostic horizon

Diagnostic horizons are used to define classes in various categorical levels of Soil Taxonomy. Their characteristics are carefully specified and are designed to make Soil Taxonomy more precise than the older systems. Diagnostic horizons that necessarily occur at or near the soil surface are called epipedons.

diapause

A physiological state of arrested metabolism, growth, and development that occurs at one stage in the life cycle.

dicot

Angiosperms that have two cotyledons within the seed; dicots include most broad-leaved plants.

differentiating characteristic

The defined properties of a soil designated by a name in a classification system are termed differentiating characteristics. Additional accessory characteristics are also associated with most members of the named group, but the soil name would apply to a soil without these accessory characteristics as long as its differentiating characteristics fit the definition. Still other accidental characteristics show no consistency in relation to the soil name.

differentiation

the process of development of specialized organs starting with morphologically and physiologically similar cells. A leaf palisade cell and a xylem vessel cell started life the same but, through differentiation, developed unique properties and specialized functions.

diffuse solar radiation

Radiation reaching the surface of the earth after having been scattered from the direct solar beam by molecules of suspensoids in the atmosphere.

diffusion

Diffusion tends to equalize the concentrations of ions and molecules in a body of liquid or gas as a result of the random motion of ions and molecules. Of the three processes that facilitate nutrient uptake by bringing ions and roots together, diffusion is an equalizer that delivers nutrients only when needed. A concentration gradient is established when plants absorb a nutrient like K⁺ or NO₃^- faster than it is delivered by root interception and mass flow. Diffusion then functions to alleviate the nutrient depletion that has occurred in the zone next to the root.

dihybrid cross

A cross of parents homozygous for two traits, each controlled by a single gene, both genes with alleles that express complete dominance, with the assumption that these genes are independent of each other

diluent

A substance used as a carrier for concentrated pesticides; can be liquid or solid, e.g., refined oil, organic flour.

diminishing returns

A German scientist named Eilhardt Mitscherlich (1794-1863) concluded that plant growth response to a limiting element is not proportional, as Liebig's law of the minimum indicates, but rather follows a law of diminishing returns. Mitscherlich developed an equation to express this law mathematically: dy/dx = k(A-y). This equation defines a curve that indicates a progressively smaller return for each successive unit of nutrient supplied.

dioecious

Staminate and pistillate flowers are on different plants of the same species.

diploid

Having two sets (genomes) of chromosomes; chromosome number of 2n, as in a zygote.

direct injury

injury to the part of the host which forms the marketable commodity

direct solar radiation

In actinometry, that portion of the radiant energy received at the instrument "direct" from the sun, as opposed to diffuse sky radiation(link to it), effective terrestrial radiation, or radiation from any other source.

disease

the state or condition brought about by the pathogen, meaning literally "lack of ease". Disease cannot be transmitted, only the pathogen which causes the disease.

disintegration

Physical processes are dominant during the formation of soil parent material by disintegration of rock. The crystals of various minerals contained in the original rock (primary minerals) are separated from one another by several processes. Ice formation, root penetration and expansion, and physical movements fracture rocks. The swelling and shrinking effects of wetting and drying and expansion and contraction caused by temperature changes also tend to disintegrate rock.

dispersion

The separation of soil aggregates into individual particles is called dispersion. A sodium compound is added to disperse the soil colloids before a mechanical analysis is done.

divalent

having a chemical valence of two; also, bonded to two other atoms or groups.

diversion

Diversions are a form of terrace designed to carry water away from an area where it might cause damage. Diversions typically carry more water and, therefore, need larger channels than most terraces.

dominance

>--Intraallelic interaction at a heterozygous locus when one allele partially or completely masks the expression of another allele.

dormancy

A seasonally recurring period in the life cycle when growth, development, and reproduction are suppressed.

dormant oil

Petroleum oil applied to trees only when foliage is not present.

dorsal

referring to the upper side. Compare to ventral.

dorsal vessel

Primary organ of hemocoel circulation; extends from the abdomen to the head and consists of a posterior heart and anterior aorta.

dough stage

a stage of seed development, usually applied to cereals, when the endosperm is soft. Occurs between the milk stage and the hard stage of maturity.

drainage system

Drainage systems may be described as random, regular, or interceptor, and as surface or subsurface. Random systems are used where there are scattered wet spots and the drainage system goes in whatever direction is expedient for carrying the water to an outlet. Regular systems use several parallel ditches or tile lines to drain large, wet areas. Interceptor drains catch water from adjoining areas and keep it from running onto or seeping into the area being drained. Surface systems carry runoff water from the soil surface, and subsurface systems remove water from beneath the soil.

dry matter

plant material remaining after oven drying to constant weight. Dry matter is the amount of dry matter in the feed. It is the percentage of feed that is not water.

dry weight

the weight of a substance, plant or soil, after oven drying to constant weight. Compare to wet weight.

duripan

Duripans are hard layers cemented by silica (SiO2) and other materials precipitated when water evaporates into dry substrata. Duripans are especially common in arid regions where a porous material underlies the solum. Iron oxides, calcium carbonate, and other salts are often deposited along with the silica in arid regions. Duripans are hard whether wet or dry (in contrast to fragipans which soften when wet). Some duripans are hard enough to be mistaken for bedrock; all are hard enough to prevent the penetration of plant roots.

dust

A dry pesticide formulation prepared by milling the pesticidal compound into a fine powder that is then diluted with a dry material like organic flour.