Fungicides and NematicidesFungicides and Nematicides

Photograph from Jack Bailey.

Stephen J. Toth, Jr. Wayne G. BuhlerDepartment of Entomology Department of Horticultural ScienceNorth Carolina State University North Carolina State University

Plant Disease Agents

• Living organisms - including fungi, bacteria, viruses and nematodes

• Nonliving agents - including unbalanced soil fertility, toxic chemicals, air pollution, frost, drought, sunburn, wind and hail

Brown necrotic lesions on potato foliage caused by air pollution (ozone)

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)

Bacterial ooze on crabapple (fire blight)

Green mold on orange (Penicillium)

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 micro-graphs by Alan Jones.

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.

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.

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

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• 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• 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.