What Is a Fungicide? That Is the Question!

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I love the saying that Spring comes in like a lion and leaves like a lamb. It says we have no earthly idea what Spring holds for us. But by the end of Spring, all will be okay unless summer shows up too quickly. We can have cool, wet periods followed by warm, humid periods or combinations that I could write about forever! Pathogens of turfgrasses are opportunistic and are waiting for the right time to infect and colonize, which could produce symptoms. Wait, that is the basic definition of a plant disease. Yet, this article is titled ‘What is a Fungicide,’ so where are we going with this post? Plant disease is simply the plant’s reaction to pathogen infection and colonization. However, many think the plant should improve immediately if a fungicide is applied. Here comes the definition of a fungicide – it means fungus killer. Now, fungicides do kill fungal cells, and they can kill many fungal cells, but when a fungicide application is made, it does not eradicate the fungus! Essentially, the fungicide puts the fungus on pause, and once the fungicide dissipates and environmental conditions are suitable for the fungus to grow, it does.

I know, I know, you bought the latest and greatest fungicide for a boatload of money. It should work immediately, right? Well, it likely does, but measuring fungal growth inside the plant is no trivial thing. Okay, why doesn’t the plant recover if it stops the fungus? Remember the statement above about how volatile springs can be? Well, imagine how plants respond to environmental stimuli that are all over the place! Not well at all. These fungicides work well, and many work rapidly, but the fungicide does not dictate recovery. Plants require light, air, food, water, and specific temperature requirements to grow optimally. If those do not occur when the fungicide is applied, then the plant will not recover from the symptoms induced by the pathogen. Hence, the disease is the PLANT’s reaction to pathogen infection and colonization. The pathogen, and in this particular case, a fungus, is hindered due to the application of fungicide; how long does it take for the plant to recover? The answer to that question depends on the time of year and many other conditions the turfgrass manager provides.

When samples are submitted to the Turfgrass Diagnostic Lab, it is a common question. Therefore, we decided to examine this question in our program here at NC State. The first experiment we established was to see how long creeping bentgrass would take to recover from dollar spot with single applications of some of the finest fungicides. The bottom line is that the plants in our trial took two weeks to overcome the disease.

The chart titled "Curative Suppression of Dollar Spot with Fungicides" from NC State University displays the percentage of dollar spot per plot over time, from May 13 to May 31. It compares six treatments: Secure, Maxima, Posterity, Daconil Ultrex, Chipco 26GT, and Control. All treatments show a decline in dollar spot initially, but the Control treatment rises sharply towards the end of May, indicating higher disease severity without fungicides. Secure and Daconil were re-applied on May 31. A smaller inset graph shows the Dollar Spot Risk Percentage over the same period.

We have presented these data a few times, and a common critique, which is valid, is that we did not apply additional fertility to promote regrowth. We have added urea to another experiment and a commonly used plant growth regulator, trinexapac-ethyl. Neither product aided or hindered recovery from dollar spot in our trials compared to applying a fungicide alone. In two separate experiments replicated twice each, it is remarkable that in each experiment, it took two weeks for creeping bentgrass to recover. Thus, if a foliar disease develops on your turf this time of year, please only expect rapid recovery once conditions are suitable for plant growth!

The chart titled "Recovery From Dollar Spot with Fungicides, Fertility and PGRs" from NC State University tracks the percentage of dollar spot from May 10 to June 16. It compares treatments including Posterity, Primo MAXX, Urea, and their combinations against a Control. The chart shows that the combination treatments generally reduce dollar spot more effectively over time compared to individual treatments and the Control.

You might wonder what happens when the pathogen affects the roots, stolons, or rhizomes. Since the pathogen likely caused the damage when the environment was conducive for growth at a different time from when the symptoms occurred, curative control may not be possible. All pathogens have a defined incubation period, the time from infection to the expression of symptoms. Many foliar pathogens have incubation periods of a few days to even a week, but many root pathogens may take 14 to 28 days to colonize enough root tissue to cause damage. For example, based on work conducted in our lab, the pathogens that cause take-all root rot of bermudagrass take about 21 days to cause symptoms. Imagine a warm, wet October followed by a cold November and December. This would allow the pathogens to infect and cause root damage in October, but since the cold developed in November, bermudagrass would likely start dormancy, and symptoms may not develop. Since the plants are not growing and developing new roots, the damage has already been done, and symptoms develop in the spring as bermudagrass wakes up from dormancy. This is probably utterly frustrating because the symptoms are present, yet applying a fungicide is useless because the pathogen is not active during this time. Another critical point with fungicide applications is the fungus MUST be actively growing for the fungicide to be effective, as the fungus absorbs fungicides while it grows. That is one scenario, but what about if the disease develops when the pathogen is active?

We have examined this with Pythium root rot in creeping bentgrass. We allowed Pythium root rot to develop in a research area where, on average, about 30% of the disease was present in each plot. We applied excellent Pythium root rot products and did not see industry-acceptable curative control at all in two years of conducting the trial! We observed curative control as disease severity in the non-treated control plots continued to increase throughout the study. Yet, disease severity decreased dramatically compared to the non-treated controls in plots treated with fungicides. At the end of both trials, the treated plots still had over 20% disease severity, which would not be acceptable.

The chart titled "Curative Suppression of Pythium Root Rot in 2019" from NC State University shows the percentage of Pythium root rot per plot from June 25 to July 26. It compares various treatments: Segway (0.45 fl oz, 0.6 fl oz, 0.9 fl oz), Union (2.9 fl oz, 5.6 fl oz), and a Control. The Control plot shows an increase in root rot, while the treated plots show varying degrees of suppression, with Union 5.6 fl oz and Segway 0.6 fl oz being particularly effective in reducing the disease severity.

In summary, fungicides work, and they work well! Many display strong curative activity when the appropriate time is allowed to recover. Understandably, the turfgrass industry’s clientele wants, even demands, results immediately that are only feasible in some situations. This is why so many turfgrass pathologists focus on preventative control for disease management. If a disease develops at your facility or property, get the disease diagnosed, take preventive measures, and educate the clientele on the reality of the situation. Eventually, the symptoms will subside as long as growth is not hindered too much (maybe another future rant ) and preventive measures can be employed in the future.