Corn disease updates

This week there has been a slight uptick in the amount of foliar disease reports in corn, likely as many people are actively scouting prior to making a fungicide application decision.  The most common disease, as you may expect is Grey leaf spot.  This disease is present to varying degrees in most fields.  No big surprise there.

We have seen and received several reports and samples of Diplodia leaf streak.  This disease can be caused by two different species of Diplodia, D. macrospora and D. maydis.  We have thusfar identified those samples where we were able to acquire spores as D. macrospora, but we have many more samples to assess.  Diplodia leaf streak can easily be misdiagnosed as Grey leaf spot, Northern corn leaf blight, or other foliar diseases and disorders.  Characteristic foliar symptoms include oblong, irregular lesions with green/yellow edges.  Occasionally, blocky lesions are observed.  Often “targets” can be seen in the lesions, where the initial infections occurred (Figure 1).   Older lesions contain black pycnidia, which resemble tiny pinheads or dots.  You will not see these in Grey leaf spot or Northern corn leaf blight lesions.

Diplodia infections can appear blocky. Sometimes targets can be observed within the lesions. Photo E. Alinger.

 

The other disease that we have started to look for more intensely is Southern rust.  Everyone should be familiar with this one, as it hammered parts of Illinois in 2016.  Remember, that year the corn was not as far along, and there was a period of persistent heavy rains throughout many parts of the state.  The disease likes it wet and warm, typically 80 F or more, and the earlier it arrives on a plant the more damage is can cause.  Thus, the need for spraying also is related to when it arrives in a given field (Table 1).  Remember, Southern rust does not overwinter here.  It needs to blow in every year from the south.  When stripe rust arrives early, (VT or earlier), and warm, wet weather occurs, severe epidemics can result.  If the disease arrives late (R3 or later) the amount of potential damage and yield impacts are reduced.  Under the correct conditions the Southern rust fungus reproduces rapidly, producing  spores that can infect new tissues or plants (Figure 2).

Table 1.  The amount of damage and yield impact due to stripe rust is related to the time of arrival on the crop. From Crop Protection Network

Through the Southern rust iPiPe map,  we can see where Southern rust is located, preventing it from sneaking up on us without notice.   The only item that is not included in the map is the incidence and severity of the Southern rust reports, which provide additional information for determining the risk of disease.  For example, a single leaf with a few Southern rust pustules  OR a field with severe Southern rust infestations result in the the county being turned red on the iPiPe map (Figure 4).  For this reason, it is important to stay on top of reports and scout when you notice Southern rust is nearby.

Figure 2.  The Southern Rust Life cycle. Photo Crop Protection Network

This year conditions have not been very conducive for Southern rust in the United States.  Until a few days ago, no reports of the disease were present.  Over the last week, more reports have occurred, and extremely low levels of the disease were detected yesterday and today in Franklin and Bond Counties in Illinois.  Most corn in those areas was well past R1 and close to R3 in many areas.  In both cases, the disease was only detected in a single field in each county and was extremely difficult to locate.

Figure 4 The Southern Rust iPiPe map as of 7/19. Red=detected. Green = scouted none detected. Yellow = suspected, not confirmed

 

Signs of Southern rust include pustules, typically somewhat bunched on the foliage, what often have a light brown to orange color.  Pustules are circular, and tend to be found mostly on the upper side of the leaf blade, and may be surrounded by chlorotic halos.  Occasionally you can find the fungus infecting stalks.  When you touch a pustule of Southern rust with your hand, you will notice it having a dusty, rusty appearance (hence the name rust).  Common rust, which is rarely impactful to corn production in Illinois, produces scattered, red to brown pustules.  Pustules are often somewhat elongated, and can be located on both sides of the leaf surface.  It is important to be able to distinguish Southern rust from Common rust due to their different potential impacts on crop yield.

 

Southern rust pustules are often circular, orange, and located on the upperside of the leaf. Note the halos surrounding the lesion. Photo N Kleczewski

 

Although Southern rust currently is not considered to be a significant threat to the corn crop, it is important to continue to scout, as changes in the environment could cause the situation to change.  If you think you may have found Southern rust, email me at nathank@illinois.edu or tag me on the picture on twitter @ILplantdoc with an indication of the severity (amount of pustules on leaves) and incidence (approximate number of plants with symptoms).  Additional information on this disease can be found on the Crop Protection Network.


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!


Black Cutworm: Management Considerations in Corn

With corn planting wrapping up throughout most of Illinois, the time has come to scout for cutworms. While several species of cutworms infest early season corn, black cutworm (Agrotis ipsilon) is the most likely to cause economic damage. We have received a handful of reports of cutting in southern Illinois, with more expected in the coming weeks as heat units begin to accumulate. If you are not doing so already, follow Kelly Estes’s reports on the Bulletin and through Twitter (@ILPestSurvey) for up to date information on black cutworm moth flights and degree-day accumulations. These updates provide excellent guidance on when to expect damage in your part of the state (remember that the predicted dates are a forecast, and are subject to change based on actual temperature accumulations).

A few management points to consider:

  • Infestations are more likely in later planted corn, as delayed planting means larger cutworm larvae are present at earlier stages of corn development.
  • Black cutworm moths prefer to lay their eggs on grasses, not bare ground. Therefore, fields with grassy weeds present at or shortly before planting are more likely to experience damaging populations. Similarly, monitor fields closely if a grass cover crop (e.g., cereal rye) is terminated while corn is susceptible to cutworm damage (emergence to ~V5).
  • The economic threshold for black cutworm is 3% of plants cut with black cutworms still present in the field. Look for plants that look like they have been cut roughly with scissors close to the base (Fig. 1); plants with intact roots (Fig. 2) were most likely dug up by birds and do not represent cutworm damage. Remember, larvae (Fig. 1) do their feeding at night and hide in residue or just below the soil surface during the day, so you will have to do a little bit of digging near the base of the plant to find them.

    Black cutworm larvae and damage

    Fig. 1. Black cutworm larvae uncovered at the base of a cut plant. Photo: Robert Bellm, Crop Advisor

    Bird damage to seedling corn

    Fig. 2. Seedling corn plant uprooted by a bird feeding on the germinating seed. Note that plant has been pulled from the ground with roots intact. Photo: Glenn Studebaker, University of Arkansas

  • Several Bt corn trait packages offer suppression of black cutworm, but these might be less effective under heavy infestations or against later stage larvae. Most pyrethroid insecticides labeled for use in corn will do an excellent job of controlling larvae as a rescue treatment; just remember that they only pay off when an economic threshold has been reached.

Correspondence:

Nick Seiter: nseiter@illinois.edu – Research Assistant Professor, Field Crop Entomologist, University of Illinois Department of Crop Sciences


Slug Management in Illinois Field Crops

Authors: Nick Seiter, Talon Becker, and Nathan Johanning

Slugs can be a difficult pest to manage when conditions are favorable for them, which has been the case often (particularly in southern Illinois) over the last couple of years. These mollusks can damage both corn and soybean early in the season, along with a variety of other crops; however, they have the potential to be especially problematic in soybean, where they can kill the cotyledons and ultimately reduce stands. There are a few management points to consider for slugs in field crops:

  • Monitor slugs before planting to estimate the severity of the problem. Slugs can be monitored by inspecting residue, or by creating artificial shelters (made from shingles or other flat materials placed in the field to create a dark, damp environment) and inspecting them periodically before planting and during early stand establishment.1

    A slug found under a shingle trap placed in a field prior to planting in southern Illinois. Photo: Talon Becker.

  • Cool, wet weather during stand establishment results in greater slug problems. Slugs require a moist environment to survive, and they perform best when conditions are wet. Cooler temperatures extend soil drying time and delay plant development, leaving seedlings vulnerable to slug feeding damage for a longer period of time. Discussions with several CCAs in southern Illinois highlighted the fact that, while slug damage is a fairly normal occurrence on a small scale in most years, particularly in no-till fields, the mild winter of 2017 followed by wet and cool conditions in the spring after many acres had already been planted likely contributed to the greater incidence of slug damage last season. It appears that soybeans were most affected last season in southern Illinois, with several thousand acres of replanting reported.
  • Reduced tillage and/or certain cover crop systems can lead to larger slug populations. Higher levels of residue retain water and provide harborage for slugs, resulting in an increased probability of slugs reaching damaging levels. Reports from southern Illinois indicate that most problem fields last spring had a cereal rye cover crop that had not been terminated before producing excessive growth, creating a favorable environment for slugs. It is important to manage residue in cover cropped fields, particularly if they are no-till. If you had a problem with slugs last year, or have found concerning levels under your “shingle traps” in the field, make it a priority to terminate the cover crop before too much above ground biomass has accumulated (generally less than 1 ft. of growth). Cover croppers may also consider decreasing their seeding rate or planting a cover crop mix which includes species that winter-kill along with their favorite over-wintering species.
  • Avoid open seed furrows. When planter closing wheels fail to seal the furrow, the resulting trench provides an ideal environment for slugs and allows them to consume developing cotyledons as the seed germinates.
  • Chemical control options are limited. Slugs are not insects, and insecticides do not provide effective control. There are slug-specific baits available, but they tend to be expensive. Note that several formulations of the active ingredient metaldehyde (e.g., Deadline®) are labeled for use in corn, but this molluscicide is not currently labeled for soybean in Illinois.

Ultimately, the most reliable management tactic for slugs is to plant into a warm, dry seed bed, which is not always an option. However, by understanding conditions which are likely to lead to slug problems, you can be better prepared to address them when and where they occur.

Correspondence:

Nick Seiter: nseiter@illinois.edu – Research Assistant Professor, Field Crop Entomologist, University of Illinois Department of Crop Sciences

Talon Becker: tbecker2@illinois.edu – Extension Educator, Agriculture and Natural Resources, Illinois Extension

Nathan Johanning: njohann@illinois.edu – Extension Educator, Agriculture and Natural Resources, Illinois Extension

1 Douglas, M. R. and Tooker, J. F. 2012. Journal of Integrated Pest Management 3(1) DOI: http://dx.doi.org/10.1603/IPM11023