Factoring in all the variables related to crop production costs isn’t easy, and determining whether the costs of crop loss due to fungal diseases is worth the price of fungicides can be a daunting task.

Approximately 20,000 fungal plant pathogens have been identified worldwide. Understanding these fungal diseases, their potential impact and possible control measures means that decisions on whether to apply fungicides – or not – are made using the facts and weighing the risk factors against the potential benefits.

“The big three of plant diseases are fungi, bacteria and viruses, in that order,” said Dan Egler, Extension plant pathologist, Southwest Purdue Agricultural Center. “Most plant diseases are caused by fungi by a wide margin.”

Some common fungal diseases include: late blight, powdery mildew, anthracnose, gummy stem blight, septoria leaf blight and downy mildew. Fungal disease can be soil borne or foliar. Fungal spores regularly overwinter in crop fields, or in alternate hosts. They can survive for long periods of time and can travel extensive distances. These spores are easily spread by wind, rain and air currents.

Spores form on stems and leaves of plants and when conditions are right, germinate and enter the plant tissue. Fungi are not able to produce their own chlorophyll and must survive by feeding on living or dead organic matter. When a fungus enters a plant cell, it typically weakens it. The fungi spread throughout the plant, utilizing the plant’s nutrients and sometimes spreading toxins.

Some fungal diseases, such as verticillium wilt, can affect a wide variety of crops. Fungi can also be very host-specific, like the fusarium fungi, where one pathogen causes a disease of a specific crop, such as watermelon fusarium wilt. Other fusarium species cause problems in other crops, such as fusarium head blight in grains.

What’s at stake?

“Perhaps the top reason that fungi are such good plant pathogens is there are so many propagules (spores),” Egler said.

“In addition, pathogens such as phytophthora blight also have resistant spores that survive many years. Some fungal pathogens, such as downy mildew of cucurbits, travel many miles on wind.”

A report by CropLife Foundation, “The Value of Fungicides in U.S. Crop Production,” studied 50 crops that growers commonly treat with fungicides to control over 200 fungal diseases. Some of these fungal diseases, such as apple scab, are 100 percent prevalent in some regions of the country. Others are not as widespread, but typically damage most of the crop when infection is present.

Historically, fungi diseases have caused loss of life due to crop failures and starvation, such as the Irish potato famine. Fungal disease have also caused the cultivation of certain crops to be abandoned in a given region – such as pears in the Eastern U.S. and plums in the South – prior to the advent of any chemical fungicides.

The development of fungicides, from the inorganic sulfur, copper and Bourdeaux mix most commonly used pre-1940s and still in use at present, to today’s wide assortment of products, including biofungicides, provide growers with numerous options.

Fungicides are broadly classified as contact or systemic. But within those categories, they may work in a variety of ways to combat the fungal pathogens.

“Twenty to 25 years ago, the selection of fungicides available to vegetable growers was limited to what are known as multisite fungicides. These fungicides are known as multisite fungicides because they affect many parts of the fungal biology,” Egler said. “Today, there is an almost bewildering assortment of fungicides available to the grower. Most of the newer fungicides have a single mode of action and are systemic.”

With this wide assortment of products, a grower should be able to tailor their use of fungicide to best meet the needs of the crop. But complicating matters are the product cost, the efficacy of the product, the risk of fungal disease and the development of pathogen resistance to fungicides.

Weighing the cost

The CropLife Foundation research report estimates the cost of fungicide use in terms of the economic value to growers in increased yields due to the application of fungicides, compared to those anticipated if no fungicides were applied, by using U.S. Department of Agriculture fungicide use data and estimating yield losses if acreage had been left untreated.

The report estimates that nationwide, for every dollar spent on fungicides in 2005, the gain to growers was $14.60. Grape and apple growers were found to benefit the most from fungicide use.

The cost of utilizing fungicides doesn’t only include the cost of the product. Equipment costs, labor, time away from other tasks and the yield reduction if not applied all factor into the equation. Practicing crop rotation, cleaning diseased materials from fields, managing the environment to make it less conducive for fungal growth and planting resistant varieties are concurrent measures to take and may reduce fungicide costs.

Proper application timing, rates and selection of the best material to target a probable threat can keep costs down. Using forecast models can avoid unneeded applications and alert growers when conditions exists.

“The grower will make a decision about whether to use a fungicide based on how much loss the disease may cause, the cost of the fungicide and the possible return of the crop,” Egler said. “A valuable crop may be worth fungicide whereas a common crop may not be.”

Read more: Conquering Realities of Herbicide Resistance