What impact will late planting have on crop diseases?

Most Illinois producers are behind in getting corn and soybeans into the ground this year as a result of persistent rains and cool temperatures.  Some people are wondering what this might mean for some of the diseases we encounter in our field crops.

Keep in mind, disease occurs when you have the correct host, plant pathogen, and environment together.  The longer those three factors are together, the more disease will occur.  Although we cannot speculate much on the long term environmental conditions we will face this year and how that will impact diseases, we can make some educated guesses on how late planting could potentially impact some diseases.  Let’s take a look at our magical plant disease crystal ball, shall we?

  1. Soybean cyst nematodes in soybean and other nematodes in soybean and corn.  These organisms grow and reproduce in/on crop roots, and will continue to reproduce and damage plants over time.  One potential impact of late planting is that these organisms will have less time to damage plants prior to harvest, and therefore their overall impacts may be reduced compared to other seasons.
  2. Sudden death syndrome (SDS).  The fungal pathogen that causes this disease is favored by cool, wet soils.  These conditions also reduce soybean germination, growth and development early in plant development.  Planting later in the season hedges your bets of encountering warmer soils, which are not as favorable for the SDS pathogen.
  3. Pythium species on corn and soybean.  Similar to SDS, Pythium diseases tend to be favored by cool wet conditions.  However, unlike SDS, Pythium seedling diseases are caused by a complex of fungi.  We now know that a single field can host many different pathogenic species of Pythium, which may differ in terms of their optimal temperatures to cause disease.  Consequently, if wet weather is encountered soon after planting, regardless of temperature, issues with these pathogens may still occur.
  4. Residue borne foliar diseases and stem diseases.  There are numerous foliar diseases that can impact soybean and corn, and many stem diseases that may cause issues in soybeans.  Unlike some of the other diseases I mentioned earlier, many of these diseases will be affected by the environmental conditions during the growing season and the when favorable conditions occur relative to plant growth and development.
  5. Rusts.  The rusts that can impact corn and soybean blow in from warmer regions to our south.  Many rusts arrive later in the season, typically after yield has been made, or after in season management decisions have been made/fungicides applied.  A good example of this is common rust in corn.  However, as many of you experienced with southern rust a few years ago, when these diseases arrive earlier in plant development, and conducive conditions persist, they are more likely to result in yield losses.  Consequently, late planting could potentially result in plants being exposed to rusts earlier in growth and development. There has not been much activity with rusts in the south this season, which hopefully translates to less potential issues with these pathogens during the season.  Of course, similar to the residue borne foliar diseases,within season weather is essential for disease development during the growing season.

 


What effect will cold temperatures have on pests and pathogens?

Nathan Kleczewski Research assistant Professor and Extension Field Crop Pathologist

Nick Seiter- Research Assistant Professor and Extension Field Crop Entomologist

 

Many in the Illinois agricultural community are wondering what effects the recent extreme cold might have on pests and pathogens. While it would be nice if the cold temperatures we are experiencing could help to reduce our potential for pest damage, past experience tells us that the most serious pests we deal with are unlikely to be impacted much by these conditions.

Many of the pathogens and insect pests that commonly affect field crops in Illinois are well adapted to survive our winter conditions.  In many cases, pathogens produce recalcitrant survival structures (e.g. cysts in soybean cyst nematode, oospores in Phytophthora, sclerotia in white mold).  These structures allow the pathogen to survive extreme conditions including cold, drought, and flooding. Different species of insects overwinter in different life stages, including eggs (for example, western corn rootworm), larvae (Japanese beetles), pupae (corn earworm, though they do not survive the winter in most of Illinois), or adults (stink bugs). The overwintering stage has characteristics that help these insects to survive the winter, either by adjusting its physiology to better survive the cold, seeking out an overwintering site that protects it (such as soil, tree bark, or leaf litter), or both. The overwintering sites that insects find mean that they are not experiencing the same temperatures that we are when we venture outside. Wind chill has little effect for this reason (even though it has a major, unpleasant effect on us).

Extreme cold temperatures can impact some insects and plant pathogens, particularly those that may not overwinter as well (e.g. powdery mildew).  When cold weather pushes into the Southern regions of the country it can push certain diseases, such as rusts, further south, delaying disease onset in Illinois and other regions further north. The same is true of migratory insects, such as black cutworm and fall armyworm, which do not usually overwinter in Illinois; colder temperatures during winter often delay the arrival of these insects, and may ultimately lead to lower numbers. The opposite is also true – warmer than normal temperatures during the winter can allow these migratory insects to become a problem earlier in the season.

Although cold temperatures may not impact most of the diseases we encounter in Illinois field crops, fluctuation between conditions of cold and warm may have a negative impact on some diseases.  Dormancy by fungi can be broken by environmental conditions such as higher temperatures.  This is similar to what occurs in plants, where warm weather may result in trees flushing out buds and flowers.  Consequently, the wide swings in temperature that we have experienced during the 2018/19 winter may negatively impact some diseases. While some insects (such as stink bugs) can also break dormancy during brief warm periods, many of our most serious pests will stay “hunkered down” until the spring and avoid these fluctuations. Unfortunately, insects and plant diseases are unlikely to suffer as much from the recent cold as we have. The best way to reduce the impact of insects and pathogens on those cold days is to stay inside, grab a hot cup of coffee, and curl up to the latest UI Extension recommendations or UI applied research results guide.


Diagnosing disease related issues in the field

Well, it is that time of year where we start to see issues developing in the field.  Questions such as, “What happened?”  and  “Why me?” will become more common.  The key to managing diseases is proper diagnosis, and this starts in the field.  In my recent post on the Field Crop Disease Blog, I provide several tips for diagnosing issues in the field, and distinguishing disease related problems from abiotic issues.  Check out the post, and sign up for updates!