slide show about chemical control of pathogens

1. Department of Plant Pathology

Faculty of Agriculture and Forestry Sciences
“Chemicals in plant disease
management”

2. 2
Chemical control
• Chemicals killing pathogens
• Biocides
• Fungicides (most commonly used)
• Bactericides
• Insecticides
• Nematicides
• Chemicals suppressing pathogens
• …static vs. …cidal
• Eg. Fungistatic vs. fungicidal

3. 3
Worldwide use of fungicides
Crop Disease Pathogen
Apples Scab Venturia inaequalis
Bananas Leaf spot (Sigatoka) Mycosphaerella musicola
Cereals Smuts, seed rot,
seedling blight
Various pathogens
Cocoa Black pod Phytophthora palmivora
Coffee Leaf rust
Berry diseases
Colletotrichum coffeanum
Various pathogens
Cotton Seed- and seedling diseases Various pathogens
Grape Powdery mildew
Downy mildew
Erysiphe necator
Plasmopara viticola
Potato Late blight Phytophthora infestans
Rice Blast Pyricularia oryzae
Tobacco Blue mould Peronospora tabacina

4. 4
Factors that influence the need for
chemical application
• Pathogen
• Mono- or polycyclic
• Virulence, population size (inoculum potential)
• Host
• Resistance of host, population size
• Environment
• Environmental conditions favourable for disease development
Host
Environment
Pathogen

5. 5
Effects of chemicals
• Inhibit pathogen growth before and/or after infection
• Total control is unlikely
• All individuals in pathogen population are not affected in the
same manner
• Chemical deposition is not uniform
• Adequate disease suppression / management
• Depending on crop, disease and environment
• Monocyclic pathogen
• Pathogen population substantially decreased
• Polycyclic pathogen
• Infection rate maintained at a very low level
• More than one application sometimes necessary
• Reduction of residues; degraded or wash-off by rain
• Protection of new growth

6. 6
Effects of chemicals..
Monocyclic pathogens
• Aim is to suppress the amount of initial inoculum
• Single application is sufficient when..
• plant is only susceptible for a short period
• inoculum is only available for a short period
• Example
• Seedling diseases; smuts of wheat
• Several applications are needed when..
• plant is susceptible for extended periods
• inoculum is available for extended periods
• Example
• Phytophthora root rot
• Wilt diseases (Fusarium, Verticillium)

7. 7
Effects of chemicals..
Policyclic pathogens
• Single application
• Delay onset of epidemic development
• Sometimes sufficient to prevent economically important losses
• Example
• 1/2 sprays to wheat before flowering suppress leaf rust
(Puccinia recondita) to economically important levels
• Several applications
• Suppress rate of epidemic development, as well as amount of
initial disease
• Reduce efficacy of secondary inoculum
• Limit disease to levels under economical losses
• Cost of total disease control not economically feasible
• Eg. powdery mildew of grape

8. 8
Side-effects of agro-chemicals
• Human
• Little to no acute toxicity to human
• Absolute safety difficult to prove
• Carcinogenic
• Unsafe chemicals removed from markets
• Re-registration of older chemicals
• Environment
• Kills natural enemies or beneficial organisms
• Toxic to macro- or micro fauna
• Eg. benomyl is toxic to earthworms and predatory mites

9. 9
Side-effects of agro-chemicals…
• Induction of non-target pests/pathogens
• Better control, more foliage, more favourable microclimate =
favor certain diseases
• Eg. Sclerotium rolfsii leaf blight of peanuts was more severe when
leaf spot diseases were treated with a fungicide relatively
ineffective against Sclerotium
• Fungicide effective against certain groups of fungi; less
effective against other groups; shift in the population
• Eg. Phosphonates highly effective against Oomycete (Plasmopara,
Phytophthora, Pythium), but less effective against Cylindrocarpon
• Population shift occurred in grape nurseries after phosphonates where
repeatedly used to control downy mildew
• Fungicide resistance
• [will be discussed later on]

10. 10
Agro-chemicals and the society
• Public opinion
• Safety of fresh or perishable produce (“Food safety”)
• Ignorance of possible negative effects of chemicals
• Negative effects of chemicals on the environment
• Public pressure
• Market accessibility
• Market preference
• Regulating the use of agro-chemicals via law
• Registration of agro-chemicals

11. 11
Registration
• Fertilizers, farm feeds, agricultural remedies
and stock remedies act, 1947 (Act 36 of 1947)
• Production, control, packing and labeling of agricultural
remedies that are going to be marketed and sell, must
comply to …
• the same composition and efficacy as registered product
• the same chemical, physical and other characteristics as
registered
• Law regarding facilities, personnel, material,
maintenance and record keeping during production,
packing, labeling and distribution

12. 12
Registration… Act 36…
• Suitability and efficacy of agricultural remedies
• Results of trials that proves suitability and efficacy
must accompany the application for registration
• Trials must by executed inside the RSA by the applicant or
competent authorities
• Trials open for inspection by the Registrar
• Determination of toxicity and potential hazards
• Agriculture remedy classified according to ..
• LD-50 values (rat [oral and inhalation] and rabbit [dermal])
• Observation of possible systemic intake, chronic poisoning,
carcinogenic effects or teratogenicity in rats or rabbits

13. 13
Registration… Act 36…
• Additional law regarding..
• Containers (physical features)
• Labeling of containers
• Invoices
• Sampling
• Quality control
• Acceptable deviation of contents (active ingredients)
• 2.5 to 15%, depending on total contents of a.i.
• Offences and penalties

14. 14
Guidelines for registration trials
• Determine biological efficacy
• 3 trials over 2 seasons in 2 areas on 2 cultivars
• Treatments
• High volume application
• Low volume if it is going to be registered as such
• Recommended concentration + higher and lower
concentration
• Comparable standard compound (treated control)
• Untreated control
• Minimum infection levels of 15-50% in controls
• Infection levels on all relevant plant parts or times evaluated

15. 15
Guidelines for registration trials
• Determine phytotoxicity
• High volume sprays with expected concentration for
registration, as well as 2x expected concentration
• At least two of the most important and sensitive
commercial cultivars
• Evaluate any deviations in size, shape, colour and
russeting of plant parts

16. 16
Guidelines for registration trials…
• Determine mixability
• Possible incompatibility and/or phytotoxicity if product
is mixed with other agro-chemicals in one tank
• Laboratory tests
• Determine whether coagulation and/or flocculation occurred
• Field trials
• Mix with insecticides, fungicides and/or surfactants as used
normally in the field
• High volume application on one of the most important and
sensitive cultivars
• Determine possible phytotoxicity and/or visual residues

17. 17
Guidelines for registration trials…
• Residue analyses
• Determine what residue is left behind after application
• Recommended, 2x concentration and untreated control with
standard commercial application apparatus
• 3 climatological areas where crop is cultivated
• Pull samples to determine degradation curve over time
• 3 to 5 different times, depending on whether product is fast,
medium of long to degrade
• Information used to determine the maximum residue
levels (MRL) and the withholding periods

18. 18
Guidelines for registration trials…
• Effect on fermentation and wine quality
• 2 trials with healthy grapes of any 2 white cultivars
• Treatments
• Maximum amount of applications until withholding period
• 1X and 1½X recommended dosages
• Untreated control
• 4 replications
• 2x20kg grapes from each replication of each treatment
harvested at the recommended ripening index
• Effect of compounds on fermentation and wine quality
determined by researchers at the ARC Infruitec-
Nietvoorbij

19. 19
Guidelines for registration trials…
• Results statistically analised
• Application of registration to the registrar
• Proposed label
• Residue degrading curve to support withholding period
• Effect on fermentation and wine quality
• Assurance of phytotoxicity
• Results from field trials
• Control
• Standard compound
• Recommended concentration, as well as higher and lower
concentration as recommended, of product for registration

20. 20
Additional guidelines
• Practical training of personnel/workers
• Basic knowledge of agro-chemicals
• Groups and formulations
• Procedures
• Residue
• Environmental risks
• Risks for humans
• Preparation and mixing
• Calibration of application apparatus
• Correct application practices
• Correct disposal of empty containers