Plant Disease Agents

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Fungicides and Nematicides
Stephen J. Toth, Jr. Wayne G. Buhler Department
of Entomology Department of Horticultural
Science North Carolina State University North
Carolina State University
Photograph from Jack Bailey.
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Plant Disease Agents
Brown necrotic lesions on potato foliage caused
by air pollution (ozone)

• Living organisms - including fungi, bacteria,
  viruses and nematodes
• Nonliving agents - including unbalanced soil
  fertility, toxic chemicals, air pollution, frost,
  drought, sunburn, wind and hail

Photograph by Gerald Holmes.
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Identifying Plant Diseases

• Symptom - reaction of the host plant to the
  living organism or nonliving agent (e.g., leaf
  spots, wilting, galls on roots)

Alternaria blotch on apple
Crown galls on peach
Photographs provided by Turner Sutton.
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Identifying Plant Diseases

• Sign - physical evidence of the presence of
  disease agent (e.g., mold or fungal spores,
  bacterial ooze)

Green mold on orange (Penicillium)
Bacterial ooze on crabapple (fire blight)
Photographs provided by Turner Sutton.
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Fungi

• Organisms that lack chlorophyll and obtain their
  food by living on other organisms
• Reproduce by spores (aids in identification)
• Attack crops above and below soil surface
• Spread by wind, rain, insects, birds, soil,
  machinery and contaminated seed

Blue mold (apple) fungal spores and fruiting
structures of cherry powdery mildew. Scanning
electron micro-graphs by Alan Jones.
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Bacteria

• Microscopic, one-celled organisms that reproduce
  by dividing in half
• Identified by plant symptoms or by signs of the
  bacteria
• Spread by infected seed, humans, insects, birds,
  contaminated rainwater, irrigation water and
  equipment

Wildfire bacterium of tobacco (Pseudomonas tabaci)
Photograph provided by NCSU Plant Pathology
Department.
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Viruses

• Too small to be seen with ordinary microscope
• Cannot complete their life cycle independently
• Transmitted by insects, infected plants, fungi,
  nematodes, etc.
• No pesticides available to control viruses
  control by using disease-free or resistant plants
  and cultural methods (e.g., crop rotation)

Scanning electron micrograph of tobacco mosaic
virus
Photograph provided by NCSU Plant Pathology
Department.
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History of Fungicide Use

• Prior to 1882 disease control with elemental
  sulfur and copper
• From 1882 to 1934 disease control based on
  organo-metallics (fixed or organo-copper)
• 1934 modern era of organic fungicides began with
  the dithiocarbamates (i.e., thiram)
• 1943 EBDC fungicides introduced, greatly
  improved fungicidal activity

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History of Fungicide Use

• Before mid-1960s fungicides were protectives,
  used at pounds per acre
• Mid-1960s to 1980s fungicides introduced with
  systemic and/or curative effects, used at pounds
  per acre
• 1980s to 1990s sterol-inhibiting fungicides were
  introduced which are systemic fungicides with
  both protective and curative activities, used at
  ounces per acre

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Types of Fungicides

• Protective (preventative) application prevents
  the establishment of an infection
• Curative application interrupts the development
  of an established infection before visible
  symptoms
• Eradicant application interrupts further
  development of an established infection having
  visible symptoms
• Residual remains on surface of the leaf and
  provides protection
• Systemic movement of fungicide inside the plant
  (locally or throughout the plant)

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Classes of Fungicides Inorganics

• Inorganics are protective (preventative)
  fungicides
• Sulfur one of oldest fungicides used, about 8
  million pounds used in 1990 in U. S. works as a
  general growth inhibitor advantages include
  cheap cost and ease of application (dusts)
  disadvantages include limited spectrum of
  activity (best on mildews), must be applied
  frequently at a high rate and phytotoxic at high
  temperatures
• Copper phytotoxic to plants in elemental form
  (use uncommon)

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Classes of Fungicides Copper

• Copper is bound to organic and inorganic
  molecules in fixed-type coppers, less toxic to
  plants
• Broad spectrum poison useful as fungicides and
  bactericides protective (preventative)
  fungicides
• Bordeaux mixture (copper sulfate and hydrated
  lime), copper sulfate, copper hydroxide and other
  copper compounds

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Classes of Fungicides Organics

• Organics are protective (preventative) fungicides
• Broad spectrum control, multi-site activity
• Represent 60-70 of fungicides used
• Dithiocarbamates thiram
• Ethylenebisdithiocarbamates (EBDCs) manozeb,
  maneb and zineb
• Captan one of the most widely used fungicides
  worldwide, broad spectrum control
• Chlorothalonil (Bravo, Daconil 2787) widely
  used, ornamentals and turf

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Classes of Fungicides Systemics

• Systemic and/or curative activities
• Benomyl (Benlate) broad spectrum, widely used
• Thiophanate-methyl (Topsin-M) broad spectrum,
  turf and fruit
• Iprodione (Chipco 26019, Rovral) broad spectrum,
  turf and ornamentals
• Metalaxyl seed treatments (Apron), field and
  vegetable crops (Ridomil), and turf and
  ornamentals (Subdue) narrow spectrum of
  activity, effective against certain soil-borne
  diseases

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Classes of Fungicides Systemics

• Sterol inhibitors large group of fungicides,
  widely used, broad spectrum of activity, has both
  protective and curative activity include
  imazalil (Fungaflor), triforine (Funginex),
  fenarimol (Rubigan), mycobutanil (Nova),
  propiconazole (Tilt) and triadimefon (Bayleton)

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Classes of Fungicides Fumigants

• Highly volatile chemicals that have fungicidal
  activity include methyl bromide (controls fungi,
  nematodes, insects and weeds) and chloropicrin

Photograph from USDA/ARS.
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Classes of Fungicides Antibiotics

• Antibiotics are substances produced by
  microorganisms which inhibit growth of plant
  diseases in very dilute concentrations
• Streptomycin (Agri-Mycin) used as dust, spray
  and seed treatment, mostly for bacterial diseases

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Nematodes

• Small, usually microscopic, roundworms
• Nematodes parasitic to plants have a stylet
  (hollow feeding spear)
• Feed on plant roots, stems, leaves and flowers
• Above-ground symptoms include stunting,
  yellowing, loss of vigor and general decline of
  plants

Nematodes under light microscope. Photograph
provided by Tom Melton.
Damage to peanuts by sting nematodes. Photograph
from NCSU Plant Pathology Dept.
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Nematodes
Root knot nematode damage on okra
Photographs from NCSU Plant Disease and Insect
Clinic.
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Classes of Nematicides Fumigants

• Exert toxic action as a gas
• Methyl bromide used since 1941 potent biocide
  soil-fumigant that controls nematodes, fungi,
  insects and weeds
• Chloropicrin used at the end of World War I now
  used as warning agent (2) with methyl bromide
  (98) mixed with 1,3-dichloropropene (Telone
  C-17)
• Others 1,3-dichloropropene (Telone) and vapam
  (Busan)

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Classes of Nematicides Non-fumigants

• Less phytotoxic than fumigants
• Extremely toxic to humans
• Most are granular formulations, easier to apply
• Organophosphates inhibit acetylcholinesterase,
  paralyze and kill nematodes include disulfoton
  (Disyston), ethoprop (Mocap) and fenamiphos
  (Nemacur)
• Carbamates inhibit acetylcholinesterase,
  paralyze and kill nematodes include aldicarb
  (Temik), carbofuran (Furadan) and oxamyl (Vydate)

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Reference

• Ware, G. W. 1994. The Pesticide Book. 4th
  edition. Thomson Publications, Fresno,
  California. pp. 79-82, 139-153.