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habitat. A place with a particular kind of environment suitable for the growth of an organism. (20)

haploid. Having a ploidy of one. (16)

haustorium. A specialized branch of a hypha formed inside a host cell by certain plant-parasitic fungi (especially obligate parasites) in order to obtain nutrients. (Pl. haustoria.) (16)

head. In nematodes: That portion anterior to the base of the stoma or stylet. (14)

helicospore. A cylindric, spiral or convolute spore, usually septate. (17)

helicosporous. Having spiral or at least strongly curved, often septate, spores. (17)

hemizonid. In nematodes: Lens-like structure situated between the cuticle and hypodermal layer on the ventral side of the body just anterior to the excretory pore; generally believed to be associated with the nervous system. (14)

hemizonion. In nematodes: A companion structure to the hemizonid; it is smaller and located posterior to the hemizonid. (14)

heteroecious. Requiring more than one host species to complete a life cycle (e.g., of Uredinales) (15)

heterogamy. Plasmogamy between morphologically different gametes. (15)

heterokaryon. A cell that contains genetically different nuclei or a thallus made up of such cells. (15)

heterokaryosis. The result of forming a heterokaryon of a fungus; the condition of a hypha or cell having two or more genetically distinct haploid nuclei. (8)

heterokaryotic. The condition of being a heterokaryon. (15)

heteroploid. Having a complement of chromosomes differing from that characteristic of the species. (16)

heterothallic. The condition of being self-sterile, requiring a partner for sexual reproduction. (15)

heterothallism. The phenomenon in which sexual reproduction requires the involvement of two different thalli. (16)

heterotopy. Hyperplastic symptom in which an organ develops in a position other than its normal one. (20)

heterotroph. An organism that obtains its food from other organisms, living or dead. (20)

heterotrophic. Requiring organic substrates for growth and development; being incapable of synthesizing required organic materials from inorganic sources. (20)

histopathology. The study of pathology of cells and tissues; the microscopic changes characteristic of disease. (5)

holobasidium. A single-celled basidium. (Pl. holobasidia.) (15)

holoblastic. When both outer and inner walls of the conidiogenous cell contribute to the formation of the blastoconidium. (7)

holocarpic reproduction. In fungi, reproduction in which the entire fungal body is segmented into spores. (20)

holomorph. Any fungus considered in its entirety, i.e., including all latent or expressed (anamorphic or teleomorphic) forms. (16)

holotype. The single specimen designated or indicated as "the type" by the original author at the time of publication of the original description. (14)

homokaryon. A hyphal cell, mycelium, organism, or spore in which all the nuclei are genetically identical. (16)

homokaryotic. Refers to a homokaryon. (16)

homothallic. The condition of being self-fertile, able to reproduce sexually without a partner. (15)

horizontal resistance. In a given cultivar: the existence of similar levels of resistance to each of the races of a given pathogen. (16)

host. A plant that supports the growth and development of the parasite that has infected it. (3)

hyaline. Transparent, translucent, or colorless. (16)

hybrid. The offspring of two individuals differing in one or more heritable characteristics. (2)

hybridization. 1. In molecular biology: The formation of stable duplexes between complementary sequences by way of Watson-Crick base-pairing. (10) 2. Cross-breeding. (20)

hydathode. A specialized leaf structure with one or more openings through which water is discharged from the interior of the leaf to its surface. (2)

hydrosis. Necrotic symptom of disease characterized by water-soaking of tissues. (Pl. hydroses.) (20)

hymenium. A palisade-like layer of asci or basidia, including any sterile cells, such as basidioles, paraphyses, or cystidia. (Pl. hymenia.) (15)

hymenomycete. A member of the Hymenomycetes.

Hymenomycetes. The group of Basidiomycetes possessing an exposed hymenium. (17)

hymenophore. Spore-bearing structure; the part of a basidioma bearing the hymenium. (23)

hyperparasite. A parasite of a parasite. (16)

hyperplasia. The enlargement of an organ or tissue owing to an increase in the number of cells. (16)

hypersensitive. The state of being abnormally sensitive. It often refers to an extreme reaction to a pathogen (e.g., the formation of local lesions by a virus or the necrotic response of a leaf to bacterial infection). (10)

hypersensitivity. The expression of extreme reactivity by a plant in response to a potential parasite or pathogen, the plant's response commonly serving to limit or prevent parasitization/disease. (16)

hypertrophy. Increase in cell size causing an increase in the size of an organ or tissue. (10)

hypha. Filamentous part of a fungus, usually septate and consisting of several cells in linear succession. (Pl. hyphae.) See mycelium. (15)

hyphal fusion. See anastomosis.

hyphal peg. 1. A compound, hyphal, fasciculate projection extending beyond the general level of the hymenium, consisting of two or more parallel or interwoven hyphae, encrusted or gelatinized. 2. A projection from a hypha. (17)

Hyphomycetes. A group of the Deuteromycetes without differentiated pycnidia or acervuli. (17)

hyphopodium. A short branch of one or two cells of the epiphytic mycelium of a black mildew fungus. (Pl. hyphopodia.) (21)

hypodermis. In nematodes: A thin tissue layer beneath the cuticle that thickens to form the dorsal, lateral, and ventral chords, which extend the length of the body. (14)

hypodermal. Pertaining to the hypodermis. (14)

hypogeous. Growing below ground. (21)

hyponasty. More rapid growth of the lower side of an organ than of the upper side. (20)

hypoplasia. Underdevelopment resulting from an abnormal paucity of cells. (20)

hypovirulence. A reduced level of virulence in a strain of pathogen resulting from genetic changes in the pathogen or to the effects of an infectious agent on the pathogen. (16)

hysterothecium. An ascocarp that opens by a slit. (Pl. hysterothecia.) (21)