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What’s ailing this corn? (And why should I care?)

An experiment was planted on May 22 at the University of Minnesota’s Northwest Research & Outreach Center (NWROC) in Crookston. Three different corn hybrids were planted for a foliar fungicide experiment and an additional four hybrids were planted to act as ‘sentinels’ to be monitored throughout the growing season for new or emerging disease and pest issues.

Various sized lesions began appearing over the last couple of weeks on leaves of some of the hybrids. These lesions could be observed from outside of the plots (Figures 1 and 2).
Figure 1. Diseased leaves can be observed from the edge of the field.

Figure 2. A closer look at diseased plants.

Large sections of entire leaves were tannish-brown and crispy (aka: dead, Figure 3) and there were indications that additional leaf area would soon die as lesions are continuing to expand (Figure 4).
Figure 3. Leaf lesions could cover up to or more than half of the leaf area. This leaf area is not contributing to plant growth and development.
Figure 4. The leading edge of leaf lesions feature pale green tissue that becomes water-soaked and dies over time as the disease progresses.

Some of the lesions revealed clues that help in disease diagnosis including greyish-green flecks and shiny ooze (Figure 5) characteristic of Goss’s bacterial blight.
Figure 5. Greyish-green flecks within lesions (white arrow) and shiny, dry bacterial ooze (blue arrow) can help in diagnosing Goss's bacterial blight.

Why do we have these symptoms in 2019?

It is compulsory among plant pathologists to think and talk about the three aspects that must all come together in order for disease caused by a pathogen to occur: a virulent pathogen, a susceptible host plant and environmental conditions that favor their interaction. 

Susceptible host plant. According to Malvick et al. (2018) in addition to corn there are other hosts of the pathogen that may also serve as a source of inoculum including grain sorghum, annual ryegrass, sudangrass, several foxtail species, johnsongrass, crabgrass, shattercane, big and little blue stem and wooly cupgrass.  

If genetically controlled disease tolerance or resistance is available seed companies provide ratings to alert customers about a hybrid’s relative resistance/susceptibility.  While there is no industry-wide standard when assigning ratings, ratings scores are developed based on a combination of field-based disease data and knowledge regarding the relative susceptibility of the parents used to develop hybrid seed. 

Goss’s wilt ratings for the seven hybrids included in NWROC field trials supplied by seed companies ranged from a score of 5 to a score of 7 on a 1 through 9 scale where 1 is excellent and 9 is poor (Table).  Three hybrids assigned the average Goss’s wilt rating of 5 had an average Goss’ blight incidence ranging between 0 and 1 percent, three hybrids assigned a assigned a rating of 6 had an average Goss’s blight incidence ranging from 1 to 6.6 percent and the hybrid assigned a rating of 7 had an average Goss’s blight incidence of 19.2 percent.  The varied Goss’s blight incidence among hybrids assigned the same rating is likely a function of a company having fewer or more field data when the hybrid was brought to market. 

Table. Hybrid relative maturity in days, Goss’s wilt ratings provided by seed companies and average Goss’s blight incidence at the NWROC in 2019
*Companies rated hybrids on a 1 through 9 scale, with a score of 1 being excellent and a score of 9 being poor.

Virulent pathogen. Goss’s leaf blight and wilt symptoms are caused by a bacterium called Clavibacter nebraskensis that survives in residue from previously infected corn or grassy weed hosts.  Practices that increase the longevity of this inoculum source, such as conservation tillage, multiple consecutive years of corn production and poor management of weedy hosts can increase disease risk.

Favorable environmental conditions. While the bacterium may not require a wound to cause an infection, wounded leaf tissue does provide a point of entry for the pathogen (Malvick et al. 2018). Consequently violent weather events such as high winds, hail or sand blasting often precede symptoms. 

The weather station at the NWROC collects and compiles weather data each day including daily maximum, minimum and average wind speeds.  This growing season there were several days on which wind speeds could have favored leaf injury. Between June 1 and July 15 there were 9 days with maximum wind speeds higher than 30 mph, four days with maximum wind speeds higher than 35 mph and a storm on July 9 brought 50.9 mph wind (Figure 6).
Figure 6. Maximum wind speed measured each day at the NWROC in Crookston from June 1 through July 15, 2019.

Why should I care?

Leaf area that is no longer alive cannot contribute to plant growth and development and subsequent yield.  While there are no management strategies that can mitigate additional leaf area losses to Goss’s blight this year, knowing that this disease is present is valuable information for several reasons, including: one is less likely to over-estimate potential yield - important knowledge for forward contracting; arrangements can be made for timely harvest and drying as leaf area lost to Goss’s wilt can result in poor stalk strength as plants tend to cannibalize stalk tissue to help fill out kernels; arrangements can be made to reduce inoculum loads before the next corn crop by managing crop residue, extending the rotation away from corn and managing weed hosts; and producers can compare their real-world experiences with Goss’s blight to seed catalogue ratings and have crucial conversations with their seed salespeople to select Goss’s blight tolerant hybrids.

Additional Resources
Malvick, D., Jackson-Ziems, T. and Robertson, A. 2018.  Corn Disease Management: Goss's Bacterial Wilt and Blight. Crop Protection Network. CPN-2010-W

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