Transcript Plant Disease Agents

Fungicides and Nematicides
Stephen J. Toth, Jr.
Department of Entomology
North Carolina State University
Wayne G. Buhler
Department of Horticultural Science
North Carolina State University
Photograph from Jack Bailey.
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.
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.
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.
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 micrographs by Alan Jones.
Bacteria
Wildfire bacterium of tobacco
(Pseudomonas tabaci)
Photograph provided by NCSU
Plant Pathology Department.
• 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
Viruses
• Too small to be seen with ordinary
microscope
• Cannot complete their life cycle
independently
• Transmitted by insects, infected
Scanning electron micrograph
plants, fungi, nematodes, etc.
of tobacco mosaic virus
• No pesticides available to control
viruses; control by using diseasefree or resistant plants and cultural
Photograph provided by NCSU
Plant Pathology Department.
methods (e.g., crop rotation)
History of Fungicide Use
• Prior to 1882: disease control with elemental sulfur
and copper
• From 1882 to 1934: disease control based on organometallics (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
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
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)
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)
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
Classes of Fungicides: Organics
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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
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
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)
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.
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
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.
Nematodes
Root knot nematode damage on okra
Photographs from NCSU Plant Disease and Insect Clinic.
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)
Classes of Nematicides: Non-fumigants
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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)
Reference
• Ware, G. W. 1994. The Pesticide Book. 4th
edition. Thomson Publications, Fresno, California.
pp. 79-82, 139-153.