This growing season, delayed planting, combined with continued cool and wet conditions have taken their toll on crops that were planted in the brief periods of optimum weather. Given that there is already delayed emergence, and the wet and cool weather patterns are expected to continue, we could very well see continued delayed growth and development of crops.
This means that we can also expect reaching grain fill later in the growing season, which creates a window of opportunity for plant diseases to impact total yield. The earlier a plant disease occurs during grain fill, the more likely it is to negatively impact yield.
This article will discuss the potential for higher incidence of some diseases, and how you can go about managing their impacts to the best of your abilities.
The Disease Triangle
When evaluating plant disease risk, it’s always best to refer to what plant pathologists call “The Disease Triangle.” The triangle is a visual representation of the three things that must happen in order for disease to occur.
Darcy Telenko, field crop pathologist with Purdue University, provided a great info-graphic that helps illustrate how manipulating the three points (pathogen, host and environment) affects disease presence and pressure. Telenko’s graphic uses Tar Spot as an example. Being able to manipulate all three variables can help reduce severity and disease impact.
The Disease Triangle: Environment
Many corn and soybean diseases thrive in cool wet environments. When identifying the possibility of disease occurrence, it’s best to evaluate future weather forecasts, as well as keep track of the growth stages of crops during the particular time period, as the conditions maybe conducive to disease development.
According to the USDA Midwest Ag-Focus Climate Outlook (5/16/2019), wet conditions will continue as summer progresses, making development and disease a “likely a problem for corn.” Such conditions will leave corn development (or grainfill) in need of growing degree days this season, and forecasts offer no relief on the horizon for the Midwest.
The Disease Triangle: Pathogen
Given the potential that we’ll have a suitable growing environment for various pathogens, it’s best to determine whether or not your field has pathogen inoculum that will be able to thrive in this environment.
Plant pathogens overwinter on diseased plant tissue. While it is always important to perform quality and regular crop scouting in your fields, years like this highlight just how beneficial keeping accurate records of annual field conditions can be. With a knowledge of what you saw in your fields in prior years, you can be more equipped to handle potential diseases in future years.
Regular scouting this year will be very important so that we can discover diseases early, and have the best chance at managing them. It will also allow us to assess future risk next season, and help us to better manage those risks.
According to preliminary testing results from the University of Wisconsin-Madison, on average, tar spot has a 20 percent spore survival rate throughout the winter. This means that inoculum will be present in fields in Eastern Iowa, which had confirmed cases of tar spot in 2018, and the forecasted cool, wet conditions may favor disease development. Consequently, these fields and adjacent fields should be scouted regularly for tar spot so that management practices, such as fungicide applications, can be implemented quickly if necessary.
The Disease Triangle: Host
The final piece of the triangle is the host, and here we need to think about genetics and growth stage of the host when infection occurs. A pathogen may infect a host with resistance but disease either fails to develop, or develops very slowly. A pathogen may also infect a host at a growth stage that is not detrimental to yield.
In soybeans, one such disease to be mindful of this growing season is white mold, which is most yield limiting when it starts during flowering. Given the cool wet conditions slowing down soybean growth, it’s important to scout for this disease, especially due to the fact that the senescing flowers may coincide more with the release of ascospores (spores of the white mold-causing pathogen released by apothecia).
The Sporecaster app, developed by the University of Wisconsin-Madison, is a great tool to consult for risk of white mold development and to determine if spraying a foliar fungicide might be necessary. Should warmer, drier and sunnier weather return in the summer, it is possible that the period of senescing flowers and ascospore release will not coincide, or the flowering periods will be reduced before podding.
As shown in the Figure 1, growing a resistant hybrid or cultivar, could greatly reduce how much disease occurs in your crops. Be sure to consult your seed dealer for more information about your varieties.
Keep updated on surrounding states
Even if your fields did not have tar spot, white mold or other diseases in 2018, paying attention to what diseases are occurring in other states will be important in 2019. Remember, pathogens can travel, predominantly via wind or storms. If a disease is prevalent in a neighboring state the amount of inoculum there will be high and could be a threat to Iowa depending on prevailing winds, and if the other two points of the triangle (host and environment) are conducive for disease development.