Fungicides are an important part of disease control in modern agriculture and are an integral part of many integrated disease management strategies. In field crops fungicides can be applied to the seed, fertiliser, soil or foliage.
Fungicides 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.
Based on the way they move in the plant we can distinguish between contact and systemic fungicides. Contact fungicides do not move from the point where the spray is deposited and act as a surface barrier that prevents spore germination mainly. However, contact fungicides will not be effective at controlling an existing infection. Systemic fungicides act on the surface but are also absorbed into the plant tissue and transported upwards by the xylem of the plant. Systemic fungicides can be used for the control of latent infections (have curative activity) and symptomatic disease (eradicant activity).
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 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
Fungicides can be applied to a crop in different ways (e.g. to seed, fertiliser or foliage) and at different times during the season. When and how a fungicide is applied to a crop will depend on many factors including the target disease, cost, seasonal conditions and on-farm logistical considerations.
Seed Treatments
There are a large range of disease control options provided by fungicidal seed treatments. For example, in cereals the most basic treatments provide control of bunts and smuts only, while there are others that also provide many weeks of suppression of a range of foliar and/or root diseases. Typically, as the range of diseases controlled and/or length of protection provided increases so does the cost. Seed treatments protect the seed either by controlling fungi present on the seed surface or carried internally in the seed, or by preventing infection from 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. 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.
Fertiliser/In Furrow Treatments
The application of fungicides to fertiliser has become increasingly used in cereal crops in Victoria. Fungicides applied in this way can reduce the severity of the root disease take-all and give long term control of stripe rust. In furrow liquid banding application has also been shown to reduce the severity of rhizoctonia in cereals.
The application of systemic fungicides to fertiliser may reduce the need for a foliar fungicide application later in the season for stripe rust control, thus reducing the in-season disease control requirements at a busy time of the year. However, this prevents the ability to monitor seasonal conditions and disease development and avoid the fungicide cost if the season is not conducive for disease.
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. Foliar fungicides provide growers with the ability to monitor seasonal conditions and disease development to only apply when necessary and then also match the active and rate to the actual disease present. Through scouting of crops for disease, in many cases un-necessary fungicide applications can be avoided.
It is important to note that foliar fungicides are most effective if applied in a preventative manner, either at the first sign of disease or before if disease is present in the region and conducive weather conditions are forecast.
Unfortunately, for many diseases there are a lack of decision support systems or forecasting tools available to determine when the economic threshold for diseases is reached and when a fungicide application is warranted.
