Pan evaporimeter: an aboveground circular metal trough about 1.5 m diameter and 30 cm deep and filled with water. The number of mm of water evaporated from the pan indicates approximately how much water the crop is losing, provided there is no water stress. It can be a guide to how much irrigation water needs to be applied to replace transpiration losses.

Pathogen: a microorganism that causes disease.

Perennial weeds: plants that live for several years; the most difficult perennial weeds to control are those that propagate both by seeds and by vegetative means.

Phloem: specialized plant tissue mainly for conducting organic substances.

Photosynthesis: process used by plants to make sugars (carbohydrates) for growth. Essential ingredients in the process are solar radiation to provide energy, the green chlorophyll of the leaves and shoots to trap and convert that energy, carbon dioxide from the air to provide the carbon in the carbohydrate, and water.

Primary inoculum: spores or fragments of a mycelium capable of initiating a disease.

Pustule: a spore mass developing below the epidermis, usually breaking through at maturity.

Rachis: the long axis of the ear or spike to which the spikelets are attached.

Reductase: an enzyme that catalyzes reduction.

Resistance: inherent capacity of a host plant to prevent or retard the development of an infectious disease.

Sclerotium (pl. sclerotia): a dense, compact mycelial mass capable of remaining dormant for extended periods.

Senescence: phase of plant growth that extends from full maturity to death; the normal death of leaves.

Solar radiation: the energy from the sun that reaches the ground, remaining after passage through the atmosphere and clouds. After half that energy is visible light. Plants can use about half for photosynthesis.

Spike: an alternate name for the ear or head.

Spikelet: plant structure bearing the grains in a cereal head. The spike (or ear) is divided into 15 to 30 spikelets; each spikelet is made up of 2 to 6 florets where the grains form, and surrounding structures of glumes, lemmas, and paleas.

Spore: a minute reproductive unit in fungi and lower plant forms.

Sporulation: period of active spore production.

Stem node: where a leaf arises on the stem; can be felt as bumps or joints when the finger and thumb are run up the stem. Internodes are the regions between the bumps.

Stephenson Screen: a white wooden box with angled slatted walls allowing through ventilation but no direct light, mounted about 1.5 m above the ground in which meteorological instruments are kept. Instruments are usually for measuring humidity and maximum and minimum temperatures.

Sterile glumes: two modified leaf structures at the base of each spikelet.

Stomata: the active pores on both surfaces of leaves in wheat, that, by changing their apertures, control the rate at which carbon dioxide enters and water vapor exits the plant.

Striate: displaying narrow parallel streaks or bands.

Susceptible: being subject to infection or injury by a pathogen; non-immune.

Symptom: a visible response of a host plant to a pathogenic organism.

Telium (pl. telia): pustule containing teliospores.

Teliospore: thick-walled resting spore produced by rust and smut fungi.

Terminal (top) spikelet: the final spikelet to be formed; it is produced at 90º to the plane of all other spikelets. When it appears, potential fertile spikelet number for the spike has been determined and can be counted under a microscope.

Tiller: shoot or branch that grows from the axil (inside base) of a leaf, the point where the leaf joins its own shoot.

Tolerant: the ability of a host plant to develop and reproduce fairly efficiently while sustaining disease.

Transmission: the spread of a disease agent among individual hosts.

Transpiration: leaves are covered in stomatal pores; when these pores are open, water vapor leaks through them from the inside of the leaf to the air, a process called transpiration.

Urediospore: an asexual spore of the rust fungi.

Vector: organism capable of transmitting inoculum.

Virulence: the relative ability of a microorganism to overcome the resistance of a host.

Water soaked: appear wet, darkened, and partially transparent.

Water use efficiency (WUE): the amount of material produced by the crop for each unit of water used. It may also be expressed as the amount of water required to produce biomass or grain.

Withertip: death of the leaf beginning at the tip, usually in young leaves.

Xylem: specialized plant tissue for conducting water and inorganic salts from roots to leaves.

Zero tillage: also called direct drill. In one pass the machinery cultivates a row slot, drills the seed, and places the fertilizer in the slot and covers the seed. Relatively little of the previous crop residues are disturbed or incorporated.