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A Dictionary
of Horticultural Terms
Acid - Having a pH below
7. The preferred soil reaction of Azaleas, Blueberries, Camellias, Chinese
Magnolias and Dogwoods.
Aerial Phytophthora - A highly
transmissible fungal disease which will devastate Madagascar Periwinkle
(vinca) if they are planted before mid-May, receive less than full sun
and are given overhead watering.
Aerobic - Requiring available
oxygen to live. For example, composting is performed by aerobic organisms;
but rotting occurs under anaerobic conditions.
Agrobacterium - The causal
agent of Crown Gall. A very common one is Agrobacterium tumefaciens,
which is an aerobic, Gram-negative rod.
Alkaline - Having a pH above
7. The characteristic reaction of soils derived from limestone. In such
soils, elements like iron and zinc are not readily available to plants.
Alternaria - A genus of fungi
containing plant pathogens. Causes the following: Citrus Fruit Black Rot,
Early Blight of Potato or Tomato, Gray Leaf Spot on Cabbage, Poinsettia
Blight, Tomato Stem Canker, etc.
Anaerobic - Requiring the
absence of oxygen to live.
Anthracnose - A disease caused
by the fungus Colletotrichum, not Gloeosporium as stated
by some. it is characterized by sunken, dark lesions or spots in leaves.
May be observed in: Ash, Elm, Hackberry. Mulberry, Red Oak, etc.
Apical Dominance - The chemicals
produced by a growing shoot tip which restrict the growth of lower, dormant
buds, keeping them from growing.
Armillaria - Fungi which
attack living woody plants as well as feeding on dead wood. The Honey
Fungus, Armillaria mellea, is present in this area. I am so thankful
it likes to ravage Hackberries!
Bacteria - Plural of bacterium.
Bacterium - A usually unicellular,
prokaryotic organism without a nucleus. Some cause diseases of plants.
Blight - A fast-spreading
fungal or bacterial disease which causes dead patches on stems or leaves
and can kill the whole plant.
Blossom End Rot - In tomatoes,
not a rot, but a dying back of the distal (farthest from stem) end of the
fruit. Occurs when there is insufficient soluble calcium in the soil and
inconsistent watering.
Botrytis - A genus of fungi
containing plant pathogens. Causes the following: Onion Neck Rot and numerous
leaf spot diseases.
Canker - A sunken lesion
caused by the breakdown of tissues from the phloem (food conducting elements)
and outward.
Cephalosporium - A genus
of fungi containing plant pathogens. Cephalosporium diospyri causes
Oak Decline.
Ceratocystis - Also a genus
of fungi containing plant pathogens. Ceratocystis fagacearum is
the organism which causes Oak Wilt.
Chilling Hours - The cumulative
hours below 45 and above 32 degrees F. a particular site receives annually.
Chilling Requirement - The
chilling hours a particular clone needs before over-wintering buds will
open.
Chlorophyll - The green matter
in plants and the essential molecule for photosynthesis.
Chlorosis - A foliar symptom
of a mineral deficiency, stress or disease; the absence of chlorophyll.
Inter-veinal chlorosis (the veins remain darker than the remainder of the
leaf blade) usually indicates a lack of iron.
Crown Gall - A grotesquely-enlarged,
abnormal body on the stem of a plant near the soil surface. Usually caused
by a bacterial infection.
Damping Off - Death of a
seedling by rot occurring when there is inadequate light and ventilation,
as well as too much water and a pathogen.
Disease Triangle - The three
conditions required for a disease to occur, namely: a susceptible host,
a suitable pathogen and an appropriate environment.
Downy Mildew - A fungal disease
of plants characterized by visible, off-white spore masses on the leaf
underside.
Early Blight - The progressive
yellowing and dying of the leaves from the ground up. In tomato, caused
by Alternaria solani.
Epiphytotic - Epiphytotic
is to plants what epidemic is to humans. The Hill Country does not have
an epidemic of Oak Wilt, it has an epiphytotic of Oak Wilt.
Erwinia - The bacterium,
Erwinia
amylovora, a Gram-negative rod, is the causal agent of Fireblight,
affecting only members of the Rose Family. Historically, it was the first
plant disease identified as being caused by a bacterium.
Etiolation - The abnormal
elongation of shoots under low fight levels.
Fairy Ring - An expanding
circle of fungi.
Fertilizer - A product containing
one, two or all three plant macronutrients (nitrogen, phosphorus and/or
potassium).
Fireblight - The Rose Family
disease caused by Erwinia amylovora, marked by the progressive blackening
and dying of shoot tips and leaves, which can kill the whole plant. In
the classic case, the shoot tip is bent back like a crook.
Fungi - Plural of fungus.
Fungicide - Something which
kills fungi. Most of these products, however, do not kill the spores.
Fungus - A kingdom of eukaryotic
(see prokaryote), non-photosynthetic, organisms. They are not a part of
the plant kingdom. Some cause diseases of plants.
Fusarium - A genus of plant
pathogenic fungi; some are saprophytes. Fusarium oxysporum,
which causes wilts, has numerous forms and affects many economically important
plants.
Graft incompatibility - The
eventual failure of the graft union, due to pathogens or physiologic reasons,
usually causing the death of the scion (part being propagated).
Gram Stain - A means of identifying
bacteria into one of two taxonomic divisions. Bacteria which take the violet
stain are Gram-positive and placed in the division Firmicutes, those
dyed by the pink counterstain are Gram-negative and constitute the division
Gracilicutes.
Haustorium - The organ of
absorption of Mistletoe. It occurs in the cambium (any zone of living and
growing tissue not located in a shoot tip) and conductive tissue zone of
the host tree.
Herbicide - A plant killer.
Herbicide injury on non-targeted plants can mimic certain diseases.
Hypoxylon - A genus of fungi
which contains plant pathogens. Hypoxylon Canker kills oaks and can be
spread by saws and other pruning tools.
Immune - Absolutely not susceptible
to a disease.
Insect - The largest group
of arthropods. They can damage or kill plants directly or indirectly, as
disease vectors.
Insecticide - Something which
kills insects.
Lesion - Generally, a localized
site of damaged or diseased plant tissue. Can be caused by bacteria, fungi,
viruses or be environmental in origin (freeze injury, mechanical abrasion,
sun scald, etc.)
Mildew - A visible fungal
growth on the exterior of a plant.
Mite - A small arachnid;
usually found in large numbers sucking sap on leaf undersides. Mite infestations
are sometimes confused with diseases.
Miticide - Something which
kills mites.
Mosaic - Well-defined, generally
angular, discolored zones on leaves. One symptom of a viral infection.
Nematicide - Something which
kills nematodes.
Nematode - Tiny, wormlike
animals which can penetrate roots and cause galls (abnormal swelling of
plant tissue) and other malformations. Roots can be damaged or killed.
Not all are bad, there are beneficial nematodes.
Nutrient - One of the elements
required by a plant for its growth. Macro-nutrients are required
in larger amounts than the micro-nutrients.
Parasite - An organism on
or in another one, deriving some or all of its nourishment from its host.
Pathogen - A disease-causing
organism.
Phomopsis - A fungal group
containing plant pathogens. Includes the causal agents of some Stem Cankers
and Twig Blights. Phomopsis juniperivora causes Twig Blight on our
native junipers.
Photosynthate - The organic
products of photosynthesis.
Photosynthesis - The light-mediated
reaction in green plants where water and carbon dioxide become carbohydrates
and oxygen.
pH - The "potency of Hydrogen".
A number from 0 to 14, the negative logarithm of the Hydrogen ion concentration
of a solution. A pH of 7 is neutral. A reading below 7 is acidic,
one higher than 7 is alkaline.
Phymatotrichopsis - A fungal
group containing plant pathogens. Phymatotrichopsis omnivora causes
Cotton Root Rot, the most important fungal disease in our area. Over two
thousand species of plants are susceptible.
Phytophthora - A very large
and quite variable fungal genus containing plant pathogens. Phytophthora
cactorum affects cacti and members of the Rose Family, P. capsici
can kill Peppers, P. fragariea causes Red Stele Rot in Strawberry
plants, and P. infestans (the most studied of all plant pathogens)
causes Potato Blight and Late Blight.
Pierce's Disease - Also called
Pierce's Grapevine Leaf Scald. A disease caused by Xylella fastidiosa,
a very small bacterium. It affects vinifera (French) grapes over
much of Texas, forcing us to grow the resistant, Munson Hybrid grapes instead.
Prokaryote - A usually single-celled
organism (bacterium, etc.) lacking a nuclear membrane. Eukaryotes, having
a nuclear membrane, include plants, animals and fungi.
Pseudomonas - The most important
member of the group, Pseudomonas syringae, is a Gram-negative rod.
It causes Bacterial Stem Canker in stone fruits and dozens of other diseases.
It is also an important ice nucleating bacterium in our atmosphere.
Pythium - A genus of fungi,
closely related to Phytophthora, which contains plant pathogens.
Causes blights, rots and damping off.
Resistant - Able to suppress
a parasite, pathogen or pest by chemical or mechanical means.
Rhizoctonia - A fungal group
containing plant pathogens. Rhizoctonia solani is the causal agent
of Brown Patch in 'Raleigh' St. Augustinegrass.
Rosette Disease - A zinc
deficiency of pecans growing in alkaline soils. Shoot tips do not elongate
normally and the leaves are crowded together, almost like a rosette.
Rust - A plant disease caused
by a fungus in the order Uredinales. So-called because of the orange,
orange-red to rusty color of the spores.
Saprophyte - A plant or fungus
which draws nutrients from dead organic matter.
Scab - A lesion characterized
by a thickening and malformation of the outermost layers of plant tissue.
Smut - A plant disease caused
by a fungus (in the order Ustilaginales), visible as a body of very
dark spores.
Spore - A small, hard reproductive
or resting phase of a fungus, bacterium, fern, etc.
St. Augustine Decline - A
disease caused by Panicum Mosaic Virus. It appears as a minute, stippled
discoloration of the blades, along with a gradual wasting away of the plant.
There is no cure.
Thatch - A layer of undecomposed
stems and roots near the soil surface which can
harbor pathogens and block
the flow of water into the root zone.
Tolerant - Able to live in
the presence of a disease or pest and not be affected.
Trace Elements - Elements
required by plants for their mineral nutrition in smaller quantities than
either the macronutrients or micronutrients.
Vector - An organism which
transmits an infectious agent.
Veinal Necrosis - A symptom
of Oak Wilt in Live Oaks. The interior portions of the leaf are discolored,
yellow to reddish-brown, while the margins remain green.
Verticillium - A genus of
fungi containing plant pathogens. They generally invade vascular tissues
and cause wilt diseases.
Virus - Nucleic acid surrounded
by a protein envelope. It can reproduce only within a host, usually with
disastrous consequences.
Wilt Disease - A loss of
turgor (water pressure) in shoot tips or leaves caused by a pathogen interfering
with vascular tissues.
Witch's Broom - A crowded
mass of abnormal branching and rebranching; usually pathological, sometimes
propagated as a novelty.
Xylella - A genus of bacteria
consisting of aerobic, Gram-negative rods which contains plant pathogens.
Xylella
fastidiosa causes Pierce's Disease in some grapes. It is also causes
Periwinkle Wilt in Madagascar Periwinkle (vinca) and Oleander Scorch in
Oleanders.
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