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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.


The botanical images on this site were produced by The Photon Hunt.

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Copyright at Common Law by Manuel Flores