Fungicide resistance

Why is rust a special disease?

It requires a living host to continue producing spores (obligate parasite).

What is an obligate parasite

an organism that cannot complete its life cycle or reproduce without exploiting a suitable host, making it entirely dependent on the host for survival and nourishment

Where does all southern rust come from?

OVERWINTERS

ON CORN IN MEXICO,

CARIBBEAN & C. AMERICA

and comes in on infected plants

Why do we not worry about rust developing resistance?

It does not have multiple generation in Ga. It coming in from other states prevents a lineage that can form mutations due to us.

What can overwinter and become a problem from a resistance standpoint?

Grey molds and corn leaf blights.

What type of fungicide has some curative effect?

systemic fungicides

WHat is key difference in disease and insect management?

Diseases have no threshold as you cannot cure a plant.

Non-systemic -

does not penetrate the plant; redistribute through vapor phase and rainfall.

•Protectant -

•

applied preventively; acts on spore germination to early infection (penetration of host tissues), no disease symptoms develop.   Must be used often for best results.

•Systemic

- moves into the plant, mostly redistributes towards plant apex or leaf margins (acropetal movement).

•Eradicant -

stops  host colonization after symptoms develop.

•Curative -

acts on post-infection, pre-symptomatic phase.

Why are single sights more prone to resistance?

They only require one genetic mutation to prevent a pesticide from working. Multi-sites hit at multiple pathways which makes developing resistance much harder.

Cross resistance

- pathogen populations that develop resistance to one fungicide automatically and simultaneously become resistant to other fungicides that are affected by the same gene mutation and the same resistance mechanism

Multiple resistance

- pathogen populations that develop resistance to two or more unrelated fungicides, arising from independent mutations that are selected by exposure to each of the fungicides concerned

How do we confirm resistance?

1. documented control failures in the field when fungicides applied according to label

2. rule out all other possible explanations for poor control

3. confirm reduced sensitivity based on laboratory assays

What three factors determines how susceptible any fungicide is to resistance?

1. Is it Multi or single site?

2. How often is it applied

3. How much is applied

What factors determine a pathogens ability to develop resistance?

1. pathogen reproduction (rate, sexual vs. asexual)

2. dispersal mechanism

3. number of disease cycles per season (monocyclic vs. polycyclic)

4. mutation rate

What determines agronomic risks?

annual vs. perennial crop

disease (host) resistance

crop rotation

availability and effectiveness of non-chemical disease controls

What do pathogens do/change to develop resistance?

modification of binding or target site

- most common mechanism

often due to a single point mutation in pathogen DNA

reduced permeability or expulsion of fungicide from cells

- active efflux "pump", requires energy

detoxification of active form or decreased conversion to toxic

alternative biochemical pathways

What is disruptive selection?

The population slowly becomes resistant as some become immune/

How does directional selection work?

The whole population slowly becomes more and more resistant until you would have to apply too much product.

How should you tank mix?

Always use seperate MOA’s so cross resistance is less of a worry.

why should you lower application amounts in a season?

Unnecessary fungicide applications create an environment for resistance to pop up.

Why should you not erradicant apply?

The disease is likely strongest at this point and will have a greater chance of surviving the chemical.

How do we scientifically determine resistance?

1. Get a basline for another genome of the same species

2. get sample sizes

3. see if they are resistant in petri dishes.

What is most commonly mistaken for resistance?

improper application, weather events washing away chemicals, and wrong species identification.