Postharvest Alternatives to Chemical Fungicides

Dr. Ron Porat

Dept. of Postharvest Science of Fresh Produce

ARO, The Volcani Center, Bet Dagan, Israel

Why do we need to search for alternative replacements for chemical fungicides?

Health concerns regarding fungicide toxicity resulted in public demand for chemical-free products.

New regulations and restricted residue limits.

Development of pathogenic strains resistance to current used fungicides.

Difficulties in registration of new fungicides.

A picture of the show window of a major coffee shop in England

Postharvest alternatives to chemical fungicides

Physical treatments (heat, UV).

Biological control.

Safe chemicals (GRAS) (soda, calcium, oils).

Induction of pathogen resistance.

Combination of various alternative treatments.

Physical treatments

UV irradiation

Several studies showed that low dose UV irradiation can disinfect the commodity and enhance pathogen resistance.

Drawbacks in the use of UV irradiation:

1. Low doses do not eradicate all pathogens.

2. High doses cause injuries and increase decay.

3. Fruit tolerance to UV changes during the season.

4. Consumers do not want “irradiated” produce.

Overall, although many studies, especially during the late 80’s and early 90’s, UV irradiation did not become a popular commercial postharvest treatment!

Heat treatments

Hot air – holding the fruit for 2-4 days at ~38C.

This treatment allows healing (curing) of the wounds, while it arrests pathogen development.

Hot water - dipping or rinsing the fruit with hot water.The hot water treatment disinfects the produce without causing heat damage.

Effects of hot water on P.digitatum spore germination

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Many studies showed that hot water dips (2-3 min at 53ºC) reduces decay development.Recently, a new method combining a short hot water rinsing and brushing treatment (55-60ºC for 20-30 s) was developed to clean and disinfect agricultural products.

Hot water rinsing and brushing of citrus fruit

Advantages

1. HWB cleans the fruit

2. HWB removes and kills surface pathogens

3. HWB smoothens wax surface and seals wounds and cracks

4. HWB induces pathogen and chilling resistance

5. HWB is part of the packinghouse sorting line

‘Star Ruby’ grapefruit before and after hot water brushing

Sterilization of the fruit surface following HWB at increasing temperatures

Effects of HWB on fruit wax surfaceControl HWB

Induction of grapefruit resistance against P. digitatum by HWB treatments

Fruit were inoculated 24 h after the heat treatment

Summary

HWB has many advantages:

it cleans the fruit, disinfects it from surface pathogens and is built as part of the packinghouse sorting line.

HWB has a major disadvantage as a replacement of fungicides:

It doesn’t have any residual effect. Therefore, it can sterilize the fruit surface but can’t protect from later infection during storage and shelf-life.

Biological control

Biological control comprises using natural occurring microorganisms to compete with the pathogens.

Modes of action of biocontrol antagonists

1. Competition on space and nutrients.

2. Production of antibiotics

3. Direct parasitism

4. Induction of host pathogen resistance

Mode of action of biocontrol antagonists

Colonization in wound sites(competition on space)

Attachment to fungal cell walls(direct parasitism)

Commercial biocontrol products availablefor postharvest use

Experimental Biocontrol Products

MicrofloMicrofloShemer - Bayer

Summary

Advantages of biocontrol:

1. Environmental safety.

2. Quite effectively reduces the development of some diseases.

Disadvantages if biocontrol:

1. Not as effective as chemical fungicides.

2. Can not kill the pathogen, and therefore can not protect from already existing infections but only from later infections.

3. Its effectiveness is inconsistent.

Safe chemicals (GRAS)

The idea is to apply safe edible compounds also known as “food additives” like baking soda, calcium salts, ethanol and various oils that persist antifungal activity to inhibit pathogen growth.

Vacuum infiltration of calcium ions strengthens the cell walls and reduces decay in apples.

Application of sodium carbonate (soda ash) and sodium bicarbonate (baking soda) reduces decay in citrus.

Application of ethanol reduced decay in grapes .

The problem is that food additives are not such effective as chemical fungicides in reducing decay development!

Induction of host disease resistance

The idea is instead of applying toxic fungicides to “kill” the pathogens to treat the commodity with various elicitors that increase tolerance against pathogen infection.

For example, it is known that various compounds, such as jasmonic acid and salicylic acid, and environmental conditions like UV irradiation and heat can increase pathogen resistance.

Currently, there are a few commercial compound (“Bion”, “Messenger”) that are used to increase pathogen resistance. However, there effectiveness in postharvest use is yet questionable.

Integrative control of postharvest diseases by combination of alternative treatments

Some alternative treatments have sanitizing effects but do not persist further protective activity (hot water, UV).

Other treatments, have protective activity but do not have curative activity and can not disinfect already existing infections (biocontrol agents).

Some treatments persist both curative and protective activity (soda).

By combining various treatment together, it is possible to achieve both curative and protecting activity, and to improve the efficacy of decay reduction.

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Integrated Control of Postharvest Decay on Red Grapefruit

Curative effect

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Integrated Control of Postharvest Decay on Red Grapefruit

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Summary

By combining two or three different alternative

postharvest treatments with different modes of

action (curative and protective activity), it is

possible to reduce decay development almost

as well as chemical fungicides do!

Thank you for your attention!