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Southern rust of corn is a foliar fungal disease that has a repeating lifecycle. Disease can build over time if the environment is conducive for disease development
- Foliar fungal diseases can reduce photosynthetic leaf area and movement of carbohydrates into developing grain.
- Disease only develops when the correct pathogen, a susceptible host, and correct environmental conditions for the pathogen cooccur.
- Foliar diseases develop over time and the disease cycle repeats. The number of repeating cycles plays a large role in the ultimate impact of the disease on yield.
- Scouting and assessing overall risk help to maximize return on investment with fungicide applications in field crops.
During the growing season, field crops can succumb to diseases by several fungal foliar diseases. Examples include, grey leaf spot, northern corn leaf blight, southern rust, and tar spot in corn, and frogeye leaf spot, Septoria leaf spot, and target spot in soybeans. In order for disease to occur, a pathogenic organism must come into contact with a susceptible plant host, while environmental conditions are favorable for pathogen growth and development. If any of these conditions are not met, disease does not occur. This interaction is represented by the disease triangle, which is a required slide in almost every plant disease management publication or educational session. For diseases that only have a single disease cycle or window to cause disease in a season, we have an idea of the likelihood that a fungicide application may be useful during the season. This is because we know that specific environmental requirements for the pathogen need to be met for a short period of time. Great examples of this are Fusarium head blight in small grains and white mold in soybeans. These diseases have a very limited window for infection, so as long as we know the pathogen is present in the field and the crop is near that critical period of growth, we can predict disease risk with decent accuracy using weather forecasts and environmental data.
Contrast this with foliar fungal diseases. These diseases infect foliage, which starts to develop early in the season, and persists throughout crop maturity. The window that the crop is in a susceptible period is very wide. In addition, weather varies throughout the season. In seasons where we have prolonged periods of humidity/rain, such as 2018 and 2021, foliar diseases develop to significant levels. This is because the environment allows the pathogens to continuously grow, cause symptoms on foliage, and produce spores. These spores can be redistributed to healthy tissues and the cycle repeats itself as long as the environment remains favorable for disease.
Often, we have periods where diseases may develop for short periods of time and then stop because environmental conditions are not conducive for disease. A great example of this is common rust of corn. The common rust pathogen needs cool and wet conditions to continue to develop. As a result, we seldom observe this disease during critical phases of corn growth and development during the middle of the summer, as temperatures exceed that which allows the pathogen to grow and develop. Southern rust, on the other hand, does best under hot conditions, and can develop during hot, reletively dry periods during critical periods of kernel establishment and grain fill.
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Southern rust of corn can reproduce rapidly when it is hot, and has a small leaf wetness requirement. These are reasons why the disease can develop and spread rapidly.
These facts are reasons why thresholds for the majority foliar fungal diseases on field crops are not reliable. There simply are too many moving pieces that can cause a disease to develop rapidly or stop development across a relatively large potential window where disease can occur and reduce yields. This differs from insects, where we know that temperature drives development, and we know how much damage a given individual insect can have on the crop over time. For this reason, it is important to understand the factors that increase risk for these diseases and to scout during the season, to assess individual field situations. A good way of evaluating risk can be found by clicking this link .
If you easily observe foliar diseases at a level above 3-5% and you are approaching R3 in soybeans or R1 in corn, then a fungicide application may be a good investment at the standard R3 (soybean) or VT/R1 (corn) timings. If diseases start to develop later, the window for these foliar pathogens to develop and cause losses is reduced. In some instances, if we enter a prolonged period of wet weather later in the season (such as 2018 and 2021) then applications made a little later in the application window may be more efficacious as residual control will last longer into the later phases of grain fill.
Figure 1. A hypothetical example of a foliar fungal pathogen with a repeating lifecycle. In this case, temperature and canopy humidity / leaf wetness are the major drivers of disease progress. In this example, a prolonged period of disease caused the disease to develop through a significant portion of the middle of the season.
Figure 2. A hypothetical example of a foliar fungal pathogen with a repeating lifcycle. In this case, temperature and canopy humidity / leaf wetness are the major drivers of disease progress. In this example, environmental conditions only favored disease development for a short period of time.
Figure 3. A hypothetical example of a foliar fungal pathogen with a repeating lifecycle. In this case, temperature and canopy humidity / leaf wetness are the major drivers of disease progress. Here, disease develops early in the season, but stops for a period mid-season. When conditions are conducive, disease continues to develop later in the season.