Making the case for fungicides

Every farmer deals with weeds and most are ready for, if not actively managing pests in one form or another. However, practices become much less uniform and opinions begin to vary when it comes to fungicides. In one camp are farmers who swear fungicides boosted their yields tremendously. In the other camp are farmers who are skeptical that fungicides will deliver as promised.

Despite some skepticism, farmer adoption of fungicide programs seems to be on the rise according to Craig Jorgensen, an agronomist and AGCO employee. “There were very few farmers in general five years ago that sprayed a fungicide,” says Jorgensen. “I’d bet you it’s probably less than ten percent. And now that number has grown pretty substantially because of the yield response they’re getting from spraying a fungicide.”

The Food and Agricultural Organization (FAO) has estimated that 25% of the world’s crops are affected by mycotoxins (toxic substances produced by fungus) each year. They estimate that around 1 billion metric tons of foods and food products are lost as a result of mycotoxins each year.

The reason some fungicides fail is because there has been an inaccurate disease diagnosis. Before you apply a fungicide, you should ensure the problem is not caused by insects, chemical injury, bacterial diseases, nematodes, etc. Fungicides do not protect against any of these issues, only fungal infection. Even if it is a fungal infection, you as a grower have to properly diagnose the issue as many fungicides are not a “cure-all.”

Considerations for Fungicide Application

  • What did I plant previously? For instance, if planting continuous corn, you probably want to consider fungicide because pathogens can survive in corn residue.
  • Weather – rain and humidity are key contributors for disease.
  • History of field disease – if you are planting into a field that has had disease before, consider fungicides or if the field is in a low area.
  • Is the crop susceptible, moderately susceptible or resistant to the disease?

Classification: Fungicides are classified in six different ways

Plant Disease Triangle

Plant Disease Triangle

Mobility in the plant

The “mobility in the plant” refers to whether or not the fungicide is a contact or systemic variety. Contact fungicides remain on the surface of the plant where it is applied but does not go into the plant. It has no after-infection activity. Using contact fungicides means you have to reapply to protect new growth. Systemic fungicides however are absorbed into the plant tissue and can offer some after-infection protection. However, very few fungicides move the fungicide throughout the entire plant. Many only move it upward (upwardly systemic) and others move into treated leaves and only go a short distance (locally systemic).

Role of protection of the plant

Preventative, early-infection or anti-sporulant activity; preventative fungicides form a protective barrier before the pathogen arrives or begins to develop and helps prevent infection. Early infection activity means it penetrates the plant and stops the pathogen in the plant tissue, but is only really effective 24-72 hours into the infection. Typically if a fungicide is early infection, it is also preventative. Also, an anti-sporulant has the ability to prevent spores from being produced, meaning it will stop the spread within the plant and to neighboring plants.

Breadth of activity

Single site vs. multi-site. Will the fungicide only work in one metabolic pathway, enzyme or protein (single site), or will it affect numerous metabolic sites (multi-site) in the fungus?

Mode of action

How exactly the fungicide kills or suppresses the targeted fungus (what biochemical process it targets).

Chemical group

Name given to the group of chemicals that share the common biochemical mode of action, even if they have different chemical structures. Examples include triazoles and strobilurons.

Fungicide Resistance Action Committee (FRAC) group

A code of numbers and letters that are used based on the mode of action.

How are they applied? Fungicides can be applied foliarly, directly to the seed, in the soil or through an irrigation system.

  • Foliar: Need to be applied before fungal spores germinate. Need to keep application (droplet size, nozzle type, pressure, etc.) in mind for optimal use. Use of a wetting agent (surfactant) will help with coverage.
  • Seed: Applied before the seed is planted and protects the seed from soil-borne or seed-borne fungi that cause rotting or seedling blight. This type can help withstand establishment.
  • Irrigation (Fungigaton): Applied through sprinkler irrigation.
  • Soil (In-Furrow): Applied in rows of target crop, typically during planting.
Grey Leaf Spot

Grey Leaf Spot

Just like herbicides and insecticides, fungicides contain an active ingredient and inert materials that improve the performance of a product. Most of the time, they are mixed with water and applied with a sprayer. Also, like weeds, pathogens can become resistant. This typically occurs when a farmer tries to apply once there is already a problem and uses it as a cure instead of a preventative. The fungi can fight it off, making them less sensitive to the action of fungicide.

When mixing fungicide, remember overly acidic or alkaline water can reduce fungicide activity, especially if pH is over 8.0. You should also spray the pesticide fairly quickly after mixing; fungicide loses its effectiveness if left to sit more than 12 hours. This time frame is even lower if the water quality is low.

So Why Use Fungicides? – Economic Impact

Are fungicides worth the cost? It really depends on the price of the crop, treatment and application compared with the potential yield loss, etc. Fungicides are more common in higher value crops such as fruits, vegetables and specialty crops.

In traditional field crops, using fungicides with soybeans has shown to raise yields between 2.6-5.3 bu/acre and 7.9 bu/acre in corn. Disease pressure has a lot to do with yield response. Agronomists at Pioneer Hybrids tested 10 research locations and had yield response anywhere from 0.6 to 22.6 bu/acre.

Fungicide Resistance

Northern Corn Leaf Blight

Northern Corn Leaf Blight

Fungicide resistance follows all the same principles as herbicide resistance – a magic chemical will only continue to do its magic if it’s managed.

That’s the case with the strobilurin group of fungicides, which became popular over the past decade for controlling soybean rust. Strobilurin fungicides (Quadris, Headline) work against several different fungi and are translaminar, meaning they can move through treated leaves and protect both sides. They were the first synthetic, site-specific chemicals to control three major groups of fungi, and farmers demanded that protection 10 years ago when soybean rust first appeared in the U.S.

“A lot of things came together at once,” said Carl Bradley, a plant pathologist at the University of Kentucky. Higher commodity prices gave farmers the incentive to treat, the government labeled strobilurins for use in numerous crops and the chemical companies began to advertise the benefits of treatment.

The chemical works well, but resistant strains of fungus showed up much more quickly than most experts predicted. Probably fungus with the most destructive potential is frog-eye leaf spot, which has become almost completely immune to the fungicides in some regions.

“We need to get away from the silver-bullet mentality. There are other classes of fungicide that have effectiveness,” Bradley said.

Manage resistance to strobilurins by:

  • Limiting their use and by using them as a component of an integrated program with other fungicides. Check the mode of action to make sure different products aren’t all strobilurins.
  • Choosing varieties with resistance to frog-eye leaf spot (or other diseases of concern).
  • Rotating crops that are susceptible to fungus.
  • Including multi-site contact fungicides that have a low risk of resistance. No more than half of the applications in a season should include strobilurins.

While treating with fungicide isn’t especially costly, treating without having a resistance management program can be disaster. Resistant strains can become immune to fungicides. In trials where there is fungicide resistance, the plants with resistant disease strains treated with strobilurins suffered more yield loss than plants that weren’t treated at all.

White Mold

White Mold

“It’s a problem when we use fungicides that aren’t necessary,” Bradley said. When farmers expect a yield bump, but don’t have disease pressure that calls for fungicide, they unnecessarily spend input dollars but also risk losing in the long-term.

In trials with varieties susceptible to frog-eye leaf spot, Bradley has seen 40% yield loss from the disease.

Since fungus should be treated early (sometimes before obvious visible signs of the disease), Bradley recommends that farmers consider variety susceptibility, disease pressure in their region, their own rotation pattern and weather when deciding whether to treat.

For some farmers, a well-planned fungicide program has made all the difference in their yields. For others, questions still remain as to efficacy and ROI. Like much of farming, it may take nothing short of on-farm experience to determine the value for some. “When a grower sees that he can spend some money but get a bigger return back, they’re willing to do that,” says Jorgensen. “They’ve got to, there’s fewer farmers feeding more people so they’ve got to get more bushels off every acre.”