BACK TO THE BASICS

What Are Fungicides?

  • a fungicide is any compound that is toxic to fungi. A fungicide can work to control disease in one or a combination of ways: from inhibiting spore germination, hyphal growth, or limiting development of spores. Fungicides may be fungistatic, meaning they prevent new growth or sporulation, or fungicidal, meaning they kill the fungus outright
  • fungicides can be applied to the seed, fertiliser, soil or foliage. The most common uses for field crops are as seed or foliar treatments
  • any fungicide used in Australia must be registered with the Australian Pesticides and Veterinary Medicines Authority (APVMA). By using only registered agricultural chemicals the user, industry and general public can be assured that, if the chemical is used in accordance with the label, the risks posed by that use in terms of public health, trade and the environment are minimised.

 

Seed, Fertiliser and Foliar Applications of Fungicides

Seed Treatments

Fungicide seed treatments protect seed viability and inhibit diseases like seed rot and seedling blight. Seed treatments protect the seed either by controlling fungi present on the seed surface or carried internally in the seed, or by controlling fungi present in the soil or on crop residue in the soil. Some treatments will protect a young seedling against early leaf disease or root rot infection, but in most cases, seed treatments are no longer effective after seedling emergence. A seed treatment can also reduce the potential for introducing a pathogen into an area where it is not established:

  • in general, seed treatments may have either systemic or contact activity, (see later). A systemic product is required to control fungi carried within the seed’s embryo, cotyledons or seed coat, (i.e. smut in barley, ascochyta in lentil). A systemic seed product is also needed to control early seedling diseases such as scald in barley, stripe rust in wheat, or early season infection of ascochyta in lentil. A contact or protectant product is adequate for surface-borne or soil-borne fungi
  • the use of seed treatments is becoming a common practice because treating seed ensures the crop gets off to a good start and is relatively ‘cheap’ insurance. Keep in mind that seed treatments will not ‘cure’ a poor seed lot that has high proportions of dead, damaged or infested seed.

Fertiliser Treatments

  • the application of fungicides has become increasingly used in cereal crops during recent years. Fungicides applied in these can reduce the severity of the root disease take-all and give long term control of stripe rust. It is important to remember that fertiliser-applied fungicides do not control bunts and smuts so seed applied fungicides are still required.

Foliar Fungicides

Foliar fungicides are an important tool to protect the above ground plant from infection by disease pathogens. The goal of disease management is not necessarily to eradicate the disease, which is impossible, but to reduce it to levels below the economic threshold:

  • routine applications of fungicide are generally not economical. Both the fungicide product and its application are a significant cost to the farmer, so it is important to understand the disease cycles. Use proper scouting techniques to determine disease risk
  • fungicides are most effective if applied in a preventative manner, either at the first sign of disease (for polycyclic diseases) or if disease development is forecasted (for monocyclic diseases)
  • unfortunately, there are limited decision support systems or forecasting tools available to determine when the economic threshold for diseases is reached and when a fungicide application is warranted.

 

Residues

In recent years an increase in the rate of detection of unacceptable fungicide residues has been noted in industry funded residue monitoring programs, eg. National Residue Survey. The increase is strongly associated with lack of decontamination of equipment used to store, transport and handle fungicide treated fertiliser, particularly in relation to flutriafol.

Traceback investigations conducted by DEDJTR have shown that use of silos, augers and bins to store flutriafol treated fertiliser is directly linked to unacceptable residues. As there are no instructions on how to adequately decontaminate such equipment once it has been used to handle flutriafol treated fertiliser, it is critical that growers have separate equipment for grain handling. Whilst this may seem expensive it will avoid the grower selling contaminated produce, which can jeopardise market access for all Australian growers if detected by an export market. It is also an offence under the Agricultural and Veterinary Chemicals (Control of Use) Act 1992 for a grower to sell contaminated produce.

Traceback investigations have also shown a high likelihood of some residues resulting from backloading of grain in trucks used by contractors to transport flutriafol treated fertiliser. Whilst current industry codes of practices may attempt to instruct drivers on how to decontaminate their trucks, there are no instructions on the label of flutriafol products.

Growers are advised to avoid backloading grain into trucks that have been used to transport flutriafol treated fertiliser, regardless of the drivers best efforts and assurances. In this situation the grower may still be held responsible for selling contaminated produce...

Mode of Action

The manner in which the fungicide affects the fungus is called the mode of action. Fungicides are divided into chemical groups based on their mode of action.

Most of the fungicides used in pulse and cereal crops in Victoria fall into five main chemical groups based on their modes of action, as follows: (Refer to Table 12.1 for additional groups).

Benzimidazoles (Group 1)

These products provide protective and curative control and show apoplastic mobility. Mode of action is by inhibiting mitosis, more specifically, by binding the tubulin needed for cell and nuclear division thus resulting in death of the fungus. Examples include the active ingredients carbendazim and thiabendazole, which are used as pulse fungicides. Carbendazim is an S7 Dangerous Poison and can only be used on pulse crops in strict accordance with the product label.

Dicarboximides (Group 2)

Many of the pulse fungicides fall within this group, active ingredients iprodione and procymidone. Iprodione is a contact fungicide with both preventive and curative activity. Procymidone is an S7 Dangerous Poison and can only be used on cereal crops in strict accordance with the product label. It is a systemic fungicide with both preventative and eradicant activity. These two chemicals act on the fungi by inhibiting spore germination and affecting cell division. The benefits of these products are that they control a unique group of fungi including botrytis and sclerotinia.

DMI - Triazoles (Group 3)

All of the fungicides registered for cereal crops are in this group; with the exception of products which also contain a strobilurin (Group 11) (i.e. cyproconazole and azoxystrobin). Triazoles are de-methylation inhibitors (DMI) which prevent ergosterol biosynthesis. Triazoles have systemic activity and provide protective, curative and eradicant control. Examples include the active ingredients propiconazole, tebuconazole, triadimenol, triadimefon, triticonazole, and epoxyconazole.

SDHI (Succinate Dehydrogenase Inhibitors) (Group 7)

SDHIs are locally systemic, movement is translaminar and upward. In Australia, SDHIs are mainly used as seed treatments either on their own or in combinations with other chemical groups. Examples include carboxin (Group 7), penflufen (Group 7), fluxapyroxad (Group 7) or seed treatments combining difenoconazole (Group 3), metalaxyl-m (Group 4) and sedaxane (Group 7). Foliar fungicide formulations have been released overseas, but resistance has built up quickly.QOLs contain the strobilurins which are preventative and locally systemic (translaminar).

QoI (Quinone Outside Inhibitor) (Group 11)

Some Qols, like azoxystrobin can move in the xylem. They are generally mixed with triazoles for agricultural use in Australia to reduce the chance of resistance.

Licencing and Use of Chemicals

Under the Agricultural and Veterinary Chemicals (Control of Use) Act 1992 any person that uses restricted use chemicals such as S7 Dangerous Poisons must hold appropriate authorisation, such as an Agricultural Chemical Users Permit issued by DEDJTR. As noted, this applies to some commonly used fungicides such as carbendazim and procymidone.

All restricted use chemicals must be used in strict accordance with the label, and the user must keep the required records of use of the chemicals.

It is illegal to use unregistered chemicals, and it is also illegal for a grower to sell agricultural produce that contains unacceptable chemical residues.

Off label use of restricted use chemicals in Victoria is not permitted for restricted use chemicals unless a permit has been obtained from DEDJTR.

Off label use of other chemicals may be permitted as long the chemical is not used:

  • at a rate greater than the maximum label rate
  • at a frequency greater than the label frequency
  • contrary to any relevant prohibitive label statements (‘DO NOT statements’).

A permit is required from the Australian Pesticide and Veterinary Medicines Authority (APVMA) to contravene any of these restrictions.

Any person considering off label use must address any risks posed in terms of residues (particularly in terms of export market requirements), occupational health and safety and environment.

Fungicide Resistance

  • the fungicides within each chemical group affect the same target sites. Over time, strains of the fungus can develop which are immune to the effects of the fungicide. This is known as fungicide resistance. Hence, it is important to rotate fungicides from different chemical groups (mode of actions) so that resistance will not be selected for
  • if there is resistance to one fungicide within a chemical group it is likely that other fungicides within that group could also be present. This is known as cross resistance
  • the loss of a fungicide due to resistance would be costly to farmers because of increased disease levels and the loss of a product when no alternatives may be present. Therefore, we are all responsible to ensure judicious use of fungicides and to rotate between chemical groups or use a fungicide that contains active ingredients from more than one mode of action group.

 

Registration of Fungicides

Chemicals are always being reviewed for safety and efficacy. As new reduced-risk ones come on board some of the older products are deregistered. The federal body involved in the registration and review of pesticides is the Australian Pesticides & Veterinary Medicine Authority www.apvma.gov.au. The process of registering a product is thorough and requires

information on environmental toxicity, safety to users, safety to consumers, fate of chemical breakdown and efficacy.

Types of Fungicide Activity (Against the Fungal Target)

Fungicides may have one or more of the following types of activity:

Protective / Preventative Activity

Inhibits spore germination or kills germinating spores on the plant surface. Protectant fungicides must be applied to the plant prior to the arrival of the pathogen, or at least before it has a chance to germinate and enter its host. Coverage is very important because the fungicide must be in contact with the spore to be effective; hence new tissues from actively growing plants will not be protected. The majority of fungicides used in field crops in Victoria are in this category, (refer to Table 12.1 for examples).

Curative Activity

Inhibits fungal growth prior to symptoms appearing but can be effective on fungi once they’ve infected the host. Curative activity is limited to the early part of the incubation period, likely only for the first 24 to 36 hours after spore germination and infection. Curative products do not repair tissue that has already been damaged or killed by the fungus. Many fungicides fall within this category, (see Table 12.1 for examples).

Eradicant Activity

These fungicides have the ability to limit growth and spore production even after symptoms are visible. Few fungicides fall within this category. Examples include those active ingredients in the triazole family, e.g. propiconazole, tebuconazole, and triadimefon.

REMEMBER: The goal for any fungicide is to protect plants from infection rather than cure them. Even if a fungicide does have some curative or eradicant properties, they are most effective if applied in a preventative manner.

Types of Movement Within the Plant

In addition to the broad categories of activity listed above, fungicides can also be defined by their movement within the plant:

Contact Fungicides

Contact fungicides provide a barrier on the plant surface to prevent spores from germinating or entering. Minimal redistribution of the product occurs through the vapour phase or by rain, but there is no uptake of the chemical into the plant. Contact fungicides have protective / preventative activity only, therefore good coverage is essential. Examples include the pulse foliar fungicides mancozeb, chlorothalonil and the various copper-based products. Also, many of pulse seed treatments, e.g. the active ingredient thiram.

Systemic Fungicides

Systemic fungicides show some movement from the plant surface, into the plant and within the plant. Hence systemic fungicides can have preventative, curative and eradicant ability. Most are only locally systemic and not all are created equal:

  1. some systemics have what is called apoplastic movement, meaning there is movement within the free space around cells, in cell walls and xylem (water-conducting) elements
  2. some systemics have acropetal movement, meaning movement towards the leaf tip and margins
  3. no systemic fungicides are currently registered that have true systemic activity, also known as symplastic activity, which includes movement into the phloem, (i.e. as for systemic herbicides).

Refer to Figure 12.1, which shows a diagrammatic view of the infection process of ascochyta blight in chickpea. Contact fungicides are effective only at Stage #1; preventing the spore from germinating on the leaf surface, (i.e. chlorothalonil, mancozeb). Once inside the plant, the fungus is protected from dry conditions and the effects of most fungicides (Stage #2 and #3). Those fungicides with systemic or curative activity may have some activity against the fungus at this stage, as long as it is applied within 24 to 36 hours after infection (e.g. boscalid, azoystrobin, pyraclostrobin).

 

Figure 12.1 Diagram of the ascochyta blight infection process in chickpea (leaf in cross- section).

THE DISEASE TRIANGLE!

THE TOP 10 CONSIDERATIONS WHEN DETERMINING A FUNGICIDE APPLICATION