Tar Spot Severity. (Image: Crop Protection Network)
- Fungicides are effective when applied at the onset of infection
- Fungicides have a limited window of effectiveness
- Phyllachora maydis has a latent period where infection is not evident
There have been numerous questions pertaining to tar spot and fungicides this season. Probably the most common question that has been asked is, “If I applied a fungicide, why am I seeing tar spot in my corn?” Below are some points to keep in mind if this question arises.
1.) Fungicides have a limited activity window. Fungicides work by encountering plant tissues and moving into the cuticle. Then, the active ingredients are moved from the cuticle into the intercellular spaces that contain water. They then move with the water across the leaf from the upper to lower sides of the leaf and moving upwards in the plant within the water conducting vessels.
Curative activity is the ability of a fungicide active ingredient to kill a pathogen that has already infected a plant tissue. Residual activity is the ability of a fungicide active ingredient to protect a tissue over time from any new inoculum (spores) that arrive and attempt to germinate or puncture the tissue surface. A good rule of thumb is that fungicides have a 3-week window of protection from the time of application. Curative activity only is observed for infections that took place a few days (3-4) prior to application. Residual activity has to do with how long the active ingredient stays in the cuticle, as well as how well the active ingredient resists degradation. Over time the active ingredients are degraded by the plant and ultraviolet rays and diluted as the plant grows.
a. Points
i. If a spore infected the plant more than 4 days before an application, it is unlikely a fungicide application will stop the fungal hyphae from growing within a plant tissue. This is because the fungus may infect a large area of tissue and the amount of active ingredient that it encounters it may not be sufficient to kill the amount of fungal tissue already established within the tissue. It may slow development. For illustration, let’s say there are 100 active sites in a pretend fungus that need to be inactivated to kill the fungus, and we only have 50 molecules of fungicide active ingredient. We still have 50 active sites that function, allowing the fungus to continue to grow and develop. This is an oversimplification of course but serves to illustrate why we might a fungal pathogen continue to grow and develop after a fungicide application.
ii. If a fungal pathogen produces spores and they land on tissues more than 3 weeks after a fungicide application and the conditions are conducive for disease, the fungicide will not protect the tissues. This is because the active ingredients have all been removed from the cuticle and/or degraded by the plant and the environment.
2.) Phyllachora maydis has a latent period. This is the same for many other diseases such as grey leaf spot. A latent period is the period after infection where we do not see any indication of disease. In the case of tar spot, this latent period is around 14-21 days. Therefore, you may go out on the first of the month, take a fishing trip, return a week later, and suddenly there are a lot of black stroma on the leaves. But it’s been hot out…. What is going on?
a. Points
i. Tar spot needs cool and wet conditions to develop. If it has been hot for the last week or two and you see stroma pop up, that is likely because those infections took place earlier when temperatures were cooler. The pathogen was not showing symptoms of disease yet.
ii. You could have sprayed a fungicide and the leaves looked clean, but the fungus had already been established for many days. The curative activity of a fungicide will not be sufficient to kill these preexisting infections but may potentially slow pathogen growth somewhat. It is unlikely that you will see this effect in a highly disease conducive environment as there will simply be too much fungal tissue in the plant for the fungicide to effectively kill it all.
3.) Phyllachora maydis can travel at least ¾ of a mile on air currents.
a. Points
i. Even if you reduced your infected surface residue to 0% through aggressive tillage, there is a chance that the pathogen could move from nearby fields and residue and infect your field. Reducing residue may slow the onset of disease but will not prevent infections from occurring in an epidemic year.
4.) Time of disease onset and severity during grain fill is important.
a. Points
i. P. maydis is an obligate pathogen and needs green tissues to grow and reproduce. Therefore, it is common to see tar spot develop on foliage after black layer if temperatures cool and it is wet. Yield has been made by this point.
ii. As a guide, roughly 10% severity or more on the ear leaf by R5.5 is needed to see significant yield losses. The image above shows what 10% severity looks like. Often infections look worse than they are and as humans, we tend to overestimate disease severity.
Hopefully these points will help you in future discussions about fungicides and tar spot, although they are also relevant to other foliar diseases in field crops. In corn we shoot for the VT-R2 window as we are looking to protect that ear leaf and above from foliar diseases during the most sensitive stages of pollination and grain fill. Approximately 90% of the carbohydrates needed for grain fill come from the ear leaf and above. Applications within that window will protect the plant from infection by any fungal spores that arrive in that 3-week window (plus a couple days prior) if the environment is conducive for disease. They will not protect the plant much or at all before or after this window.
This season, some applications may have been pushed back to the point that there were a fair number of infections before the applications went on. In other cases, the applications went on but ran out of gas just as some cool and rainy weather pushed into a region. Therefore, it’s not that the fungicide didn’t work, it’s that the timing was not optimal relative to the onset of tar spot. However, remember that there was/is a lot more than tar spot out there. Southern rust was just as important in many areas through the central Midwest that received rains, and grey leaf spot and northern corn leaf blight developed to a significant degree on susceptible hybrids in many areas. Your fungicide application likely reduced these other diseases all which can reduce yields alone or in combination but may not be as evident on dead/senesced tissues as tar spot.