Control of plant diseases

biosecurity control method

preventing pathogens from entering the territory of the farm

quarantines and inspections

eradication after pathogen incursion

production of disease free planting material

agronomy control method

actions applied within farm, before and during growing season

genetics control method

breeding and sowing of crop that possesses resistance to pathogens

crop protection product control method

externally applied products like fungicides, insecticides, biocontrol agents, etc

avoidance / evasion exclusion

part of biosecurity

plant grown in areas where environmental conditions are not conclusive to pathogen vectors

9 agronomical methods

1. disease eradication from planting material

2. disease eradication from growing medium and soil

3. elimination of pathogen hosts

4. elimination of crop debris

5. crop rotation

6. intercropping and companion cropping

7. sowing timing

8. row spacing and orientation

9. fertilizer interactions

sources of genetic resistance

1. cultivars adapted to local conditions, obtained from breeder’s own stocks

2. other lines of same crop, global collections maintained by authorities

3. wild relatives of crop species (subject to Cartagena Protocol Convention on Biological Diversity)

4. any above can be subject to mutagen, genome edited, or transgenically modified

landrace

local cultivar improved by traditional agricultural methods

how to maximize the lifespan of disease resistant cultivars

1. use strong resistance genes

2. use recessive resistance genes

3. stack resistance genes

4. use different cultivars with different resistance profiles in same farm or in successive years

5. monitor for breakdown of resistance genes

6. limit pathogen population using fungicides and crop hygiene

what genes have been introduced into plants to help improve resistance?

genes from plant viruses (protein coat genes confer resistance)

antimicrobial proteins (proteins that exhibit antifungal properties like PR protein family)

antimicrobial metabolites (biosynthetic genes involved in metabolites that promote disease resistance)

host-induced gene silencing

uses plant-expressed RNA interference taken up by invading pathogen, silencing it

transgenic plants express dsRNAs

spray-induced gene silencing

dsRNAs and siRNAs are sprayed onto plant surfaces and uptaken by pathogens

8 classes of crop ortection products (CPPs)

1. fungicides (most used)

2. insecticides

3. acaricides (spiders and mites)

4. nematicides

5. antibiotics

6. herbicides (second most used)

7. plant growth regulators

8. rodenticides

basic substances

simple chemicals that have been used for many decades, generally regarded as safe

botanicals

single molecules and mixtures derived from plants, not subjected to deliberate chemical modification

pesticide regulation

regulation started in 1960s

registration costs roughly 200 million

what is written on fungicide labels?

crop and disease type to be used on

upper and lower limits on dose

limit on spray number throughout growing season

what products it can be mixed with

withholding period (minimum period between final spray and harvest date)

mode of action

biochemical process that CPP inhibits to reduce pathogen growth

spectrum

range of host-disease combos controlled by CPP

resistance

evolved and heritable decrease in pathogen sensitivity to CPP

preventative

CPP must be applied prior to pathogen attack

curative

CPPs applied after infection, cure existing disease

eradicant

CPP that kills established pathogen

contact

CPPs must come into direct contact with pathogen

systemic

CPP travels within plant and controls diseases remotely from point of application

types of basic substances

copper and sulfur compounds

copper salts, lime sulfur, calcium hydroxide

sulfur fungicide MOA

depends on release of hydrogen sulfide (H2S), damages fungal and oomycete cell membranes, inhibits mitochondrial function

phytotoxicity has been observed, not very specific

copper fungicide MOA

absorbed by hyphae and forms complexes with sulfhydryl, hydroxyl, amino, and carboxyl groups, inhibiting enzymes and causing nonspecific damage

what is FRAC code for?

fungicide resistance action committee

fungicides with same FRAC code share same MOA, if pathogen starts showing resistance to one in a certain FRAC code, stop using and avoid all other pesticides in that FRAC code

dithiocarbamate fungicides

first synthetic fungicides

mancozeb

MOA: reacts with -SH group in pathogen proteins

methyl benzimidazol carbamates (MBC)

first systemic fungicides

curative and preventative

MOA: inhibition of mitosis by binding to microtubules

MIC

minimum inhibitory concentration

lowest concentration of a fungicide that inhibits growth in an isolated artificial media

EC50

concentration of fungicide that inhibits pathogen growth rate by 50%

resistance factor (RF)

ratio of EC50 between resistant isolate and sensitive one

size reflects degree of resistance in affected pathogen

RF 1-10: not noticable in field situations but a stepping stone towards serious resistance

RF 10-40: noticable in field, combatted by increasing dose or frequency of CPP application

RF > 40: fungicide is not providing useful control

cross resistance

isolates showing resistance to one specific compound found to be resistant to most compounds in a given group

reason for classification into MOA groups and FRAC codes

encourages cooperation between pesticide companies

sterol biosynthesis inhibitor-C14 demethylases (DMIs)

systemic fungicide

also called demethylation inhibitors or triazoles

MOA: inhibits biosynthesis of sterols in pathogen

broad range of action (but not oomycetes)

foliar sprays, seed treatment or post harvest

prefix tria- or suffix -conazole

quinone outside inhibitor (QoI)

systemic fungicide

MOA: bind to quinol site of complex 3 in mitochondrial ETC

very safe, large spectrum

control all fungal and oomycete pathogens, low phytotoxicity

mostly preventative

succinate dehydrogenase inhibitors (SDHIs)

systemic fungicide

carboxins, boscalid, bixafen

carboxins used to control basidiomycete seed-borne diseases (smuts and bunts)

second gen shows action also against ascomycetes but not oomycetes

MOA: inbitits complex II (succinate dehydrogenase)

fungicides against oomycetes

oomycides/oomyticides

sensitive to QoIs, phenylamides (PA) and carboxylic acid amines (CAA)

phenylamides, metalaxyl and benalaxyl are protectant and curative

biological control agents (BCAs)

living organisms that control pests and pathogens

works through competition, antibiosis, stimulation of plant defense mechanisms, and hyperparasitism

most on the market now work by antibiosis

can potentially provide protection that extends longer than one growing season

antibiotics

control bacterial diseases

few currently in use because they are very prone to resistance

streptomycin

organophosphate nematicides

many originally developed as insecticides, but are taken up systemically and are effective against nematodes

isothiocyanates

metam, dazomet

active against nematodes, weeds, soil insects, soil fungi

put in soil at least 2 weeks before planting

carbamates

active against nematodes, soil insects, some foliage insects

what types of pathogens do chemicals applied as sprays or dusts target?

fungal and oomycete diseases

seed treatment

chemicals applied to control pathogens carried on seeds to prevent decay after planting or damping off of seedlings

precautions must be taken that viability is not lowered

when planted chemicals on seed diffuse into surrounding soil

3 main strategies on when to apply CPPs

1. when crop reaches a certain growth stage

2. await a certain threshold of disease

3. use a disease prediction model

importance of weather forecasting in crop disease management

different pathogens require specific environmental conditions

accurate weather forecasts help farmers determine what pathogens are likely to infect their crops at a given time, know what to use

resistance management

1. use minimum fungicide dose that gives adequate disease control

2. ensure that pathogen is exposed to fungicide for at least minimum time

3. use fungicides with different MOAs as mixtures or successive applications

4. monitor resistance so you can detect it early

5. use integrated pest management

integrated pest management

use of all available and useful methods to control pests

also called integrated disease management

1. eliminate/reduce initial inoculum

2. reduce effectiveness of initial inoculum

3. increase host resistance

4. delay onset of disease

5. slow secondary cycles

management methods from least to most toxic

prevention

cultural/sanitation

physical/mechanical

biological

chemical