Glossary

A

activation:

stage of oospore germination during which the endospore wall becomes eroded and the ooplast swells

allozymes:

isozymes whose variant forms are coded at the same locus; assayed by starch-gel electrophoresis

amphigynous:

pertaining to the sexual stage in which the oogonial stalk is surrounded by the antheridium

antheridium (pl. antheridia):

the male gametangium; a multinucleate, swollen hyphal tip affixed firmly to the basal side of the oogonium

aplerotic:

pertaining to a mature oospore that does not fill the oogonium; when the oospore fills the oogonium it is plerotic

axoneme:

main core of a flagellum in which two central tubules are surrounded by nine double microtubules

B

basal plug:

a plug bulging into a sporangium from the cross wall, separating it from the sporangium

C

caducous:

pertaining to sporangia that become dislodged readily and separated from the sporangiophore (e.g., Phytophthora palmivora), in contrast to being persistent (e.g., Phytophthora cinnamomi) (also deciduous)

chemotaxis:

movement of zoospores toward a chemical stimulus

chlamydospore (pl. chlamydospores):

an asexual spore, usually globose but occasionally ovoid, that is delimited from the mycelium by a septum; may be terminal or intercalary with a thickened inner wall; survives for long periods in soil

coenocytic:

pertaining to a multinucleate mycelium in which the nuclei are not separated by cell walls

conidium (pl. conidia):

a nonmotile spore that is usually caducous (deciduous). Occasionally the caducous sporangia of Phytophthora infestans are called conidia because they are capable of being spread by splashing water to adjacent plants and may infect a leaf before zoospores form; however, the term "sporangium" is more appropriate and most frequently used.

D

deciduous:

pertaining to sporangia that become dislodged readily and separated from the sporangiophore (e.g., Phytophthora palmivora), in contrast to being persistent (e.g., Phytophthora cinnamomi) (also caducous)

diclinous:

having the oogonium and its antheridium on different hyphae

DNA:

deoxyribonucleic acid; a constituent of chromosomes in nuclei that acts as a blueprint for the synthesis of proteins and contains the hereditary information for the development of the organism

E

electrophoresis:

chromatography process by which particles (especially proteins and enzymes) migrate at different rates through a starch or acrylamide gel in an electric field. The gel is stained to mark the location of the protein and then photographed.

electrotaxis:

movement of zoospores toward weak electrical fields on plant roots

epidemiology:

study of disease that spreads widely and affects a large population of individuals

epiphytology:

study of the character, ecology, and outbreaks of plant diseases; since the principles are similar to those of epidemiology, the latter term is more frequently used

exit pore:

an opening through which zoospores pass when leaving a sporangium; papillate sporangia and semipapillate sporangia have exit pores 5–7 µm wide, whereas nonpapillate sporangia have exit pores about 12 µm wide

F

flagellum (pl. flagella):

an appendage on the zoospore used in motility. The zoospore has a long whiplash flagellum with a smooth surface and a shorter tinsel flagellum on which mastigonemes (hairlike structures) occur.

G

gene-for-gene:

pertaining to genetic relationships in which a host plant (e.g., soybean) possesses a race-specific resistance gene and the pathogen (e.g., Phytophthora sojae) possesses a corresponding gene for virulence

H

haustorium (pl. haustoria):

branch of the mycelium within an infected plant that does not break the host cell but becomes appressed to it and through which nutrients are absorbed

heterokaryon:

cell containing genetically different nuclei

heterokont:

pertaining to zoospore fungi with different types of flagella; e.g., in Phytophthora species, the zoospore has both a whiplash flagellum and a tinsel flagellum

heterothallic:

pertaining to sexual reproduction in which conjugation is possible only through interaction of different thalli. In heterothallic Phytophthora species, oospores form only when the A1 and A2 mating types are paired; however, several reports indicate that oospores form in single-mating-type cultures following certain chemical or environmental stimuli.

homothallic:

pertaining to sexual reproduction that can take place without the interaction of two different thalli

hypha (pl. hyphae):

one of the threads of mycelium

hyphal swelling:

globular swelling that forms at a branch point or within the mycelium. It is by definition not separated from the coenocytic mycelium by a septum. When separated by a septum and having a thickened inner wall, it is termed a chlamydospore.

I

isokont:

pertaining to zoosporic microorganisms with two flagella of equal length

isozymes:

variant forms of an enzyme that share the same catalytic function; assayed by starch-gel electrophoresis

M

mastigoneme:

short, hairlike, lateral protuberance on tinsel flagella

matric potential:

component of water potential; the force that holds water in soil resulting from the absorption or capillary effects characteristic of various pore sizes in soil

monoclinous:

having the oogonium and antheridium on the same hypha

mycelium (pl. mycelia):

tubular strands that make up the body of the fungal microorganism. In Phytophthora species, mycelium is nonseptate; however, plugs (often called false septa) can be seen in old mycelium.

N

noncaducous:

pertaining to sporangia that are not dislodged or separated from the sporangiophore; persistent

nonpapillate:

A characteristic feature of the distal end of the sporangium is the papilla (Latin for nipple or bud). Nonpapillate sporangia have an inconspicuous papilla. Sporangia with an inconspicuous papilla usually have wide exit pores (10–12 µm).

O

oogonium (pl. oogonia):

the female gametangium, in which the oospore forms after fertilization by the antheridium

oospore (pl. oospores):

zygote or thick-walled spore that forms within the oogonium after fertilization; may be long lived

P

papilla (pl. papillae):

common feature at the apex of a sporangium (Latin for nipple or bud)

papillate:

A characteristic feature of the distal end of the sporangium is the papilla (Latin for nipple or bud). Papillate sporangia have a distinct and conspicuous papilla. Exit pores are usually 5 µm or less.

paragynous:

pertaining to the sexual stage in which the antheridium is attached to the side of the oogonium

pedicel:

after the caducous sporangia are shed, the hypha remaining attached to the sporangium is the pedicel and may be short (<5 µm), medium (5–20 µm), or long (>20 µm).

pellucid body or pellucid spot:

translucent, nucleus-containing body, two or more of which can be seen inside and near the periphery of the mature oospore

phloem:

plant tissue in which carbohydrates are transported

plerotic:

pertaining to an oospore that fills the oogonium; one that does not fills the oogonium is aplerotic

proliferation:

formation of a sporangium within an empty sporangium that has previously emitted zoospores (internal or nested) or after the sporangiophore has emerged from the empty sporangium (external)

R

RFLP:

restriction fragment length polymorphism. After DNA is purified and treated with certain enzymes that react at different specific regions on the DNA molecule, the lengths of the resulting fragments are categorized by use of starch or acrylamide-gel electrophoresis. The relative differences in fragment lengths are characteristic of genetic groups.

RNA:

ribonucleic acid; a molecule similar to DNA that functions in the decoding of genetic information as it is used in the cell

S

semipapillate:

A characteristic feature of the distal end of the sporangium is the papilla (Latin for nipple or bud). Semipapillate sporangia have a slightly flattened, less prominant papilla then do papillate sporangia.

sporangiophore (pl. sporangiophores):

stalk on which the sporangia are formed; may be unbranched or branched to form a simple sympodium or a compound sympodium, respectively

sporangium (pl. sporangia):

sac within which zoospores form, especially when water is cooled to about 10°C below ambient temperature. In solid substrates, sporangia usually germinate by germ tubes.

sympodially:

at the foot of; referring to sporangiophores that emerge externally from the base of the previous sporangia (simple sympodium). When the sporangiophore is branched, it is called a compound sympodium.

sympodium (pl. sympodia):

at the foot of; referring to sporangiophores that emerge externally from the base of the previous sporangia (simple sympodium). When the sporangiophore is branched, it is called a compound sympodium.

T

tapered base:

used to describe both sporangium and oogonial stalk

X

xylem:

plant tissue containing vessels and tracheids through which water is transported

Z

zoospore (pl. zoospores):

spore that forms within the sporangium and exits through the terminal pore, has both a tinsel flagellum and a whiplash flagellum, and is capable of swimming for several hours