Disease requires the presence of a pathogen, a conducive environment and a susceptible host. By eliminating any of these, growers can reduce disease risks. Fungicides to protect from pathogens are only cost-effective if they target the correct pathogen, the pathogen is actually present, the disease can affect the crop in concern and the environment is right for diseases to develop.
Cost of the fungicide, versus its effectiveness and impact on the crop, has to do with proper use of the product. Many variables will impact how effective any given fungicide application will be. Some things to consider are:
1. Mode of action
Because fungicides often become ineffective as fungal pathogens develop resistance, they need to be used only as warranted. Growers also need to alternate products between those with different modes of action. The Fungicide Resistance Action Committee (FRAC) assigns each mode of action a classification. The incorporation of biological fungicides is also suggested for best long-term results.
“The disadvantage of the single mode of action fungicide is that fungi may become resistant to these types of fungicides. One mutation of the fungus may allow it to overcome the fungicide, making the fungus resistant to it,” Egler said. “Growers have learned to alternate fungicides with different modes of action sequentially.”
Contact fungicides coat the plant and have multiple modes of action. Leaves formed after product application are not afforded production, nor will any surface uncoated during application have protection. With systemic modes of action, new leaves may be protected, depending on whether the product moves in a translinear or acropetal manner within the plant.
Different modes of action will be effective at different times during fungal development within the plant. Using a fungicide at the wrong time for its mode of action is ineffective. Fungicides may have to be reapplied at specified intervals to maintain effective disease prevention.
While all fungicides are best used preventatively, applying them when conditions aren’t conducive to disease isn’t cost effective. Knowing when fungal pressures are high and taking precautions to reduce their prevalence as much as possible, can help decrease fungicide application costs.
Read more: Make Your Fungicides Last
“Most fungal diseases require leaf wetness for infection to take place,” Egler said. “The amount of time the leaf needs to be wet varies per disease. For some diseases, programs have been developed that allows growers to time fungicide applications based on the weather conditions.”
MELCAST, developed by Dr. Rick Latin, Purdue University, is one such program. Instead of applying fungicides at a certain calendar date, growers using MELCAST will only apply it when the conditions are primed for fungal growth, often saving several applications each season, Egler said.
This forecasting of conditions conducive to specific fungal disease pressures allows growers to save on fungicide costs, while still allowing them to apply fungicide preventatively to control disease risk. But the specific environmental requirements of many fungal pathogens are not yet known. However, by timing fungicide applications to coincide with conditions likely to enhance disease concerns, crops can be protected without undue chemical applications.
“In most years, downy mildew of cucurbits doesn’t arrive in the Midwest until late August. In some years, cucurbit downy mildew does not show up in the Midwest at all. However, when this disease does arrive, it can be devastating,” he said.
In 2014, cucurbit downy mildew arrived in Indiana in July, requiring growers to apply fungicides for the disease. The cucurbit downy mildew forecasting program helped growers to avoid unneeded fungicide applications in years when pressures are low, but allowed them to apply an application at a time not otherwise normally known for fungal disease pressure when necessary. Other forecasting tools available include TOMCAST, for fungal foliar diseases of tomatoes. Michigan State researchers have adapted the TOMCAST program for asparagus and carrots in that state.
“Growers who are not using a disease-forecasting program can learn to apply more fungicide applications during wet periods. Rainy weather tends to promote foliar disease. Overhead irrigation also may increase foliar disease severity,” Egler said.
Phytophthora with pythium and downy mildews, are actually oocytes, not true fungi, which can spread rapidly from plant to plant in a field if conditions are right. They thrive in wet soils and are soil-borne pathogens. Utilizing an effective fungicide for these pathogens on valuable crops can slow the spread of disease. But materials effective on oocytes are not always effective on true fungi and vice versa, so due diligence when making product selections requires an understanding of pathogen pressures and crop susceptibility.
Wet soils and poor drainage can make conditions conducive for many soil-borne fungal pathogens. Avoid planting in soils with known drainage issues.
5. Crop value
When dealing with a low-value crop, growers might opt to forego fungicide applications. When disease pressures are low, opting to hold off on fungicide use may be practical. If a disease could cause widespread damage to a primary crop, a high-value crop or to many crops, fungicide application costs may be justified, whereas they may not be if the disease is very crop specific, or the crop is not of high value to the grower.
6. Crop Resistance
Cultivar tolerance and resistance to fungal diseases can also help to reduce fungicide costs. Fungal diseases can overcome host resistance in time, but planting resistant varieties can be one way of eliminating the need for certain fungicide applications. Many growers, however, may have a variety that is in high demand, but is not resistant and they must decide whether to treat that variety, or not grow it, if fungal problems are a concern.
7. Yield impact
One other consideration when deciding whether a fungicide is needed is the probable impact of the disease on crop yield. For example, downy mildew late in a crop cycle will impact leaves, but much of the crop can be harvested prior to any disease concerns developing in the fruit. If the fruit is still young, fungicide makes more sense. Phythophora is more likely to impact fruits directly, so more aggressive treatment may be warranted when conditions exist for disease to occur, or if early disease has been spotted in the fields.
Some foliar diseases are tolerated by the crop as long as a certain percentage of the crop canopy is adequately covered by fungicide application. Applying fungicides at the correct application rates, using properly calibrated equipment, prevents overuse or insufficient spray applications.
Read more: Harness the Strength of Your Fungicides