❂
Plant Dia
18
T opics in This chapTer
❂By Jay W. Pscheidt, Extension Plant Pathologist, Oregon State University. Edited by Lindy du Toit, Plant Diagnostician, Washington State University, Puyallup, and Warren Copes, Ornamental Plant Pathologist, Washington State University.Adapted by Michele Hébert, Extension Faculty, Agriculture and Horticulture, Cooperative
Extension Service, University of Alaska Fairbanks.
❂Pathogens
❂Conditions necessary for biotic dia ❂The dia cycle ❂Dia diagnosis ❂Plant dia control ❂Fungicides for home gardeners
E
very gardener has put in plants with hopes for
wonderful flowers, fruits or vegetables, only to have tho hopes dashed as the plants get sick and die. The plants are considered diad. Many things can cau plants to become diad, including biotic (living) agents, abiotic (nonliving) factors or a combination of the two. This chapter focus only on living agents — fungi, bacteria, virus, nematodes and parasitic plants. Other chapters discuss abiotic factors such as nutrient deficien -cies, lack of water, temperature stress and combinations of the problems as they relate to specific types of plants. Some plant dias have had tremendous impacts on society. Perhaps foremost among the is Phytophthora late blight, a fungal dia that caud the Irish potato famine in 1845. Approximately 2 million people either starved or left Ireland, many for the United States. Powdery mildew and downy mildew are fungal dias that devastated the French wine industry until Bordeaux mixture was found to control them.
In the United States, the fungal dia chestnut blight was introduced accidentally into New York City in the late 1800s on imported Chine chestnut trees. The Chine trees were resistant to the blight, but American chestnut trees were not. In fewer than 40 years, 30 million acres of chestnut trees had died. Chestnut blight remains a problem in the eastern United States. Dutch elm dia also was in-troduced accidentally. It infects and kills elm trees through-out the nation.
The examples are prominent becau they caud so much damage. Plant dias vary in how much trouble they cau, depending on a variety of conditions, including the susceptibility of the plant, and the organism’s dia cycle. Minimizing their impact requires correct diagnosis of
332 • Plant Dia—Chapter 18
Dia terminology
General terms Dia—Abnormal and harmful physio-logical condition brought about by living
(biotic) agents, such as fungi, bacteria,
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nematodes and virus, or by nonliving
(abiotic) factors, such as nutrient deficien-cies and water stress.
Dodder—A parasitic ed plant lacking chlorophyll; its thread-like yellow body
twines around its host, from which it
withdraws food and water.
Downy mildew—A group of fungal patho-gens who spore production looks like
downy growth on the undersides of
israelleaves.
Exclusion—A process by which certain plants are not allowed in an area to pro-
tect against pests and dias from other areas.
Haustoria—A modified hyphal branch that grows into a plant cell to absorb food and water.
Host—A plant afflicted with a dia. Hypha—A single filament of a fungus. Immunity—A relationship between a plant and pathogen in which the plant does not
become diad.
Incubation—A period of development dur-ing which a pathogen changes to a form
that can penetrate or infect a new host
plant. Some fungi, for instance, grow a
holes是什么意思structure called a penetration peg that can grow through a plant’s cell walls. Infection—The condition reached when a pathogen has invaded plant tissue and es-
tablished a parasitic relationship between itlf and its host. Inoculation—The introduction of a pathogen to a host plant’s tissue.Inoculum—The parts of a pathogen that infect plants.
Powdery mildew—A group of fungal patho-gens who spore production caus
white to gray powdery mycelia on the
outside of their host. Quarantine—A regulation forbidding sale or shipment of plants or plant parts in an
area, usually to prevent dia, inct,
nematode, or weed invasion. Resistance—Qualities in a host plant that make it retard the activities of a pathogen. Sanitation—The process of removing sourc-es of plant pathogens from a growing area (for example, by cleaning up plant debris
and sterilizing tools and growing media). Spore—(1) The reproductive body of fungi and other lower plants, containing one or
more cells. (2) A bacterial cell modified to survive in an adver environment. Stylet—A nematode’s lance-like or hypoder-mic-needle-like mouthpart ud to punc-
ture and feed from plant cells. Tolerant—A crop plant that will produce a normal yield even if diad. Pathogens
Bacterium—A single-celled, microscopic organism having a cell wall but no chlo-
rophyll; reproduces by cell division. Fungus—A plant organism with no chloro-phyll that reproduces via spores and usu-
ally has filamentous growth. Examples
are molds, yeasts and mushrooms. Nematod e—A microscopic roundworm, usu-ally living in the soil; many feed on plant
roots and can be dia pathogens. Parasitic ed plant—A higher plant that lives parasitically on other ed plants.
An example is mistletoe.
Chapter 18—Plant Dia • 333
Dia terminology, continued Pathogen—A dia-producing organism. Phytoplasma—A microscopic, bacteria-like organism that lacks a cell wall. Virus—An infectious agent too small to e with a compound microscope; it multiplies only in living cells.
Symptoms (external or internal physical dis-ea characteristics expresd by a host plant) Blight—Rapid, extensive discoloration, wilt-ing, and death of plant tissue. Blotch—A blot or spot (usually superficial and irregular in shape and size) on leaves, shoots or fruit.
Canker—A dead place on the bark and cortex of twigs, stems or trunks; often discolored and either raid or sunken. Chlorosis—An abnormal yellowish-white or gray color of plant parts resulting from
incomplete destruction of chlorophyll. Damping-off—Decay of eds in the soil or young edlings shortly after they emerge;
usually caud by Rhizoctonia, Pythium,
or Fusarium fungi. Dieback—Progressive death of shoots, branch-es or roots, generally starting at the tips. Dwarfing—The underdevelopment of any plant organ. Enation—Epidermal outgrowths on le
aves or stems.
Epinasty—An abnormal downward-curving growth or movement of a leaf, leaf part or stem. Etiolation—Development of yellow, long, spindly growth as a result of insufficient
light.
Fasciation—A distortion of a plant that results in thin, flattened and sometimes
curved shoots.
Flagging—Loss of turgor and drooping of plant parts, usually following a water
shortage.Gall—An abnormal, localized swelling on leaf, stem or root tissue. Mosaic—Nonuniform foliage coloration with a more or less distinct intermingling of normal green and light green or yel-
明确英语lowish patches.
Mottle—An irregular pattern of light and dark areas.
Necrosis—Death of plant tissue. Phyllody—A change from normal flower structures to leafy structure
s. Rot—Decomposition and destruction of tissue.
Rugo—Wrinkled. Rust—Yellowish-brown or reddish-brown scar tissue on a fruit’s surface. Scab—A crust-like dia lesion.
Water-soaked—Lesions that appear wet and dark and usually are sunken and/or
translucent.
Wilt—(1) Lack of freshness and turgor and drooping of leaves from lack of water. (2)
A vascular dia that interrupts a plant’s
normal uptake and distribution of water. Witches’ broom—Abnormal brush-like development of many weak shoots. Signs (physical evidence of a dia-causing agent)
Bacterial slime—A gooey or dried mass of bacterial cells that oozes out of plant
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tissues.
Conk—A fungal fruiting structure (e.g., shelf or bracket fungi) formed on rotting woody plants.
Cyst—The swollen, egg-containing female body of certain nematodes; can be en
on the outside of infected roots. Fruiting body—A fungal structure that con-tains or bears spores. Mycelia—Mass of fungal threads (hy-phae), which compo the vegetative
body of a fungus.
334 • Plant Dia—Chapter 18
the problem and an understanding of tech-niques for growing healthy plants.
Pathogens
Dia causing organisms are pathogens. They are microscopic or very difficult to e or recognize without magnification. Fungi, bacteria, virus, nematodes and even plants can be pathogenic on garden plants. Patho-gens generally get nutrients, water and ev-erything they need to reproduce from their host. Such a relationship is called parasitic. Some pathogens can infect veral kinds of plants; others require a specific host. Fungi
The largest group of plant pathogens, fungi come in a wide variety of forms. In general, they are mult
icellular organisms with a thread-like body. The threads, which are called hyphae, have cell walls. When many threads mass together, they form a mycelium. Further growth of a my-celium may produce fruiting bodies, where xual or axual spores are formed. The characteristics of the spores, fruiting bodies and mycelium are ud to identify
and diagno fungal problems.
Some fungi can survive and
grow without a living host. Others
die if they are not in clo asso-
ciation with a host.
Fungi cau plant dis-
eas by making toxins
so do ithat kill plant cells, by
growing within and
plugging up a plant’s
vascular system, by
rotting roots or by
nding root-like struc-
tures into plant cells.Bacteria
Bacteria are single-celled organisms that are much smaller and less complex than plant cells. Many are about the size of a plant chloroplast. Bacteria can build up to high numbers and ooze out of plant tissues. Some bacteria produce slime that may attract incts that spread the bacteria to healthy plants. Bacteria can survive unfavorable con-ditions in plant debris or even in eds. Bacteria cau plant dias by forming toxins or by producing enzymes that break down plant cell walls. Crown gall bacteria actually genetically engineer their host plant to make galls and amino acids, thus giving the bacteria a better place to live and the chemicals they need to grow and reproduce. Virus
Virus particles are compod of a few strands of DNA and are even smaller than bacteria. Electron microscopes reveal them to have many shapes, including long strands, short rods and multisided balls. Virus utilize a host plant’s cell organ-elles to produce more virus. The result can be strange plant colors, forms or struc-tures. Some viral infections, however, don’t result in any visible plant problems. Touching virus-infected plant material and then touching healthy plants can trans-mit some virus. For example, a smoker can transmit tobacco mosaic virus from a cigarette to a plant. In Alaska, some virus are transmitted by incts such as aphids, scales, leaf hoppers and whiteflies. Fungi, mites, nematodes and even parasitic plants also can transmit virus. Some virus also may infect a host plant’s eds and thus be transmitted to the next generation. Potato virus X can be transferred from one garden potato to another by a contaminated garden tool or pant leg (anything that moves sap).
Chapter 18—Plant Dia • 335 Nematodes
Nematodes are multicellular roundworms
that may be no larger than the letter “I” in
the word DIME on a U.S. coin. Becau they
are clear and live in the soil, they are im-
possible to e without magnification.
All plant pathogenic nematodes have a
mouthpart called a stylet. The stylet is like a
lance or hypodermic needle that the nema-四六级报名官网
unforgetabletode us to puncture plant cells and feed
from them.
Some nematodes move from root to root,
while others t up one feeding site in a
single root. Feeding may cau root lesions
or galls that restrict water and nutrient flow
to the host plant. Other nematodes weaken
the plant through mass feeding. A few foliar
nematodes attack above-ground plant parts.
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Movement of soil or infected plant parts can transmit nematode dias.
Parasitic plants
Many Alaskans notice moss and lichen growing in trees; this vegetation is not parasitic, it just us the tree as a platform. Some plants are truly parasitic to other plants. Dodder, for example, produces flow-ers and eds, but has no chlorophyll. Thus, it cannot manufacture its own food. It has a thread-like yellow body that twines around its host. Root-like haustoria penetrate the host plant and withdraw food and water. Some parasitic plants, such as mistletoe, manufacture chlorophyll but have no real roots and depend on their host (on hemlock in Southeast Alaksa) for water and nutri-ents. Seeds of parasitic plants are spread by birds or contaminated soil, or they may be shot out of plant structures like little bombs. Conditions necessary for biotic dias
In order for a biotic (pathogenic) plant dia to occur, three conditions must be met:
• The host plant must be susceptible.• An active, living pathogen must be pres-ent.
• The environment must be suitable or favorable for dia development.
All three of the factors must occur simultaneously. If one or more is abnt or unfavorable, dia does not occur. The relationship among a plant, a pathogen and the environment is shown in Figure 1. The more the circles overlap, the more vere the dia. This is referred to as dia triangle.
A host plant’s genetic makeup determines its susceptibility to dia. This suscepti-bility depends on various physical and bio-chemical factors within the plant. Stature, growth habit, cuticle thickness and stomatal Figure 1.—Dia is a result of simultaneous interactions between the environment, host and pathogen.
Environment
2014上海中考分数线Host Pathogen
Dia
