Farmers generally don’t apply many fungicides to sunflowers, particularly when compared to some other crops, preferring instead to rely on disease resistance bred into the plant.
But there is evidence that you may see a positive financial response to even a modest fungicide programme, particularly in a wet year when fungal diseases can be especially problematic.
“Full soil tillage, minimum of 3 year’s crop rotation, seed treatment combined with foliar fungal curatives and preventives are all recommended tools that farmers have while managing fungal disease in their farms”
Dincer Erin, European Agronomy Manager for Nuseed
This article explores some of the issues around applying fungicides to sunflowers.
But first, some background on plant diseases and fungicides.
Like all living organisms, plants can succumb to disease-causing organisms such as viruses, bacteria, or fungi. Collectively these disease-causing organisms are known as pathogens.
Plant pathogens attack in different ways; some colonise tissue in the plant, others develop on the leaf surface, others may affect specific areas such as the roots, stems, or leaves.
Pathogens can cause visible changes in plant growth or appearance, known as symptoms, including, but not restricted to, a loss of green colour (known as chlorosis), spotting, browning, distorted growth, dead tissue, or even plant death.
Fungal pathogens use a variety of enzymes to penetrate the plant cuticle and cell wall, release the nutrients inside, and gain further access into the plant.
As the amount of green leaf area is reduced by discolouration, leaf rolling, or dieback, the plant’s ability to absorb sunlight and photosynthesis is reduced.
The plant will also have to fight off the toxic effects of the disease and will not be functioning at total capacity.
To maximise plant growth, development and yield, we need to minimise the infection, spread and development of fungal diseases.
There are at least 30 diseases that can affect sunflowers; fortunately, only a few result in economic significant yield losses. Here are some examples.
● Downy mildew (Plasmopara halstedii), widespread, early infection of seedlings produces stunted plants that may die or vigour; severe infestations can result in a 30% yield loss.
● Fusarium root and stem rot (Fusarium sp.), an increasingly significant disease in Russia and possibly surrounding regions.
● Phoma black stem (Phoma macdonaldii) infection during early growth stages can reduce yield by 30%.
● Phomopsis stem canker (Diaporthe helianthin), a significant yield-limiting disease, stems weakens and break, can result in 40% yield loss.
● Rust (Puccinia helianthin), resistant varieties have controlled rust below economic levels; however, there is some indication that the disease is increasing, which can result in 40% yield losses.
● Sclerotinia (Sclerotinia sclerotiorum), a widespread disease, affects all parts of the plant, can lead to 60% crop losses.
Fungicides are a pesticide that can control fungi and keep plants free from disease.
Fungicides are classified in several ways but generally work either as a protectant or curative.
Protectant fungicides provide a barrier that prevents the fungus from entering and damaging plant tissues. They need to be applied before the disease has a chance to infect the plant, and new plant tissue is generally unprotected.
Curative fungicides can stop the disease after the infection has started or after the first symptoms are observed, but application timing is critical. Although they are called curative, symptoms already present, such as spotting or browning, will not be “cured” by the fungicide.
Fungicides can be further classified by their mode of action, such as contact, systemic or translaminar.
Protectant fungicides are contact; they remain outside the plant surface, killing the fungi on contact.
Curative fungicides are systemic; they are absorbed across the leaf cuticle and can be transported around the plant to kill fungal diseases where they have gained access. Some systemic fungicides may only move a short distance inside the plant, while others can move throughout the plant, including the roots.
Translaminar fungicides move from the upper, sprayed leaf surface to the lower, unsprayed leaf surface.
● Protectants provide a barrier that prevents the fungus from entering the plant.
● Curative stop the disease after the infection has started.
Fungicide Modes of Action
● Contact fungicides protect only where the spray is deposited; they are not taken up into the plant.
● Systemic fungicides are taken up and redistributed through the plant via the xylem vessels.
● Translaminar fungicides redistributed from the upper, sprayed leaf surface to the lower, unsprayed surface.
Foliar fungicides on sunflowers
Although farmers generally don’t apply fungicides to sunflowers, most commercially grown seeds will be treated with a fungicide to protect the seedling.
Seed dressings, combined with good inherent disease resistance, as found in Nuseed varieties, will provide sufficient protection through the crop’s lifecycle.
However, in addition to seed dressings and disease resistance, field research continues to take place to see if applying foliar fungicides can provide additional yield improvement and economic benefits.
Why are foliar fungicides not routinely used on sunflowers?
One issue is that, depending on the country, the range of fungicides registered for use on sunflowers and available to growers can be limited.
This is because fungicide manufacturers decide that the market for a fungicide in a particular country isn’t large enough to justify the cost of registration. This is one of the reasons we see pesticides registered for use in one country and not another.
The economic response drives the decision to apply a fungicide; if the economic response in terms of increased yield is more than the cost of treatment, you have a justification for treating a crop.
However, you can only determine the economic response if you have agronomic information, and reliable agronomic information on the economic impact of sunflower diseases is often vague or lacking.
A brief look at the academic research on this subject shows a lot of trial work has been carried out, but it appears that in many cases, this has yet to be distilled and filtered into practical agronomic advice.
Currently, there is limited information on when foliar fungicides can or should be applied to sunflowers.
What does the research tell us?
Sunflower rust severity assessment diagrams. Friskop et al. (2011).
The research essentially tells us what we would expect; that disease reduces yield, and fungicides can reduce infections.
One large trial in North Dakota in the USA, which ran over four years, assessed fungicide efficacy and timing in controlling Rust (Puccinia helianthin).
It showed that for every 1% increase in the severity of rust, yields decreased by 6.6%. (Friskop et al., 2015).
The same study also demonstrated management of rust depended greatly on timing.
A single fungicide application at R5 was more effective at reducing disease severity than any other timing investigated.
This could be because earlier applications may not have protected later emerging leaves, while later applications may have had a more limited impact on disease control because a high severity had already been reached.
As a result of these studies, a threshold of 1% disease severity on the upper four leaves, at or before R5, has now been widely accepted by the US sunflower industry.
Some producers are now routinely applying systemic fungicides (fluxapyroxad + pyraclostrobin) with positive yield responses, sufficient to offset the cost of treatment.
In many cases, seed dressings and robust disease resistance in Nuseed varieties will be more than sufficient in managing diseases.
It would be inappropriate to base agronomic decisions on data from one trial, but work is being done, and the evidence suggests that, in some instances, there is a justification for foliar fungicides in sunflowers.
However, the research needs to be completed locally and to a level that gives confidence in the following agronomic recommendations.
As growers continue to increase yield, then foliar-applied fungicides may have a place in the agronomy toolbox.
|Climate change and plant pathogens
The subject of foliar fungicides in sunflowers might become more of an issue as climate change continues to have an impact.
Alterations in weather conditions may lead to biological changes in pests and diseases and impact plant physiology and structure, increasing plants’ vulnerability towards pests and diseases. (FAO 2021).
We may find plant diseases that are of limited significance today may become economically significant tomorrow.