Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Corn in many parts of the state is at approaching tasseling through silk. Now is a good time to scout fields to see if there are any disease issues that may need attention. With recent wet weather, diseases such as Grey leaf spot (GLS) have started to pop up in some places. GLS lesions are often small, 1.5 inches or less in length, and blocky in appearance. These symptoms differ from the large, cigar shaped lesions produced by Northern Corn Leaf Blight under cooler conditions.
Symptoms of GLS may be more pronounced in areas such as tree lines or low lying parts of the field, where high humidity levels may persist for longer periods of time. Hybrid selection plays a key role in managing GLS as does residue management and crop rotation. Understanding how these factors reduce risk is important when deciding if a fungicide application might be beneficial for managing GLS, which is our most common foliar corn disease in Delaware.
Figure 1. Left: small, blocky lesions caused by the Grey Leaf Spot pathogen. Right: large, cigar shaped lesions caused by Northern Corn Leaf blight.
Think of your risk for yield limiting disease as a set of stairs, with each factor that contributes to risk moving you another level higher on the “riskcase.”
(Use the handrail on the riskcase at all times.)
The first step (and maybe even 2) is hybrid selection. Hybrids with good to excellent ratings for GLS resistance can do two things, all the time, regardless of weather, year, or other factors: 1) they reduce the ability of the GLS pathogen to infect and grow on foliage and 2) they can reduce amount of spores produced by lesions, thereby reducing the amount of disease pressure experienced over time. Disease pressure concerns us when it reaches an economic threshold, meaning that it will significantly reduce yields and profitability (Figure 2). This means that it needs to reach some crucial point before the crop is made. In corn with GLS (and Northern corn leaf blight) research has shown that number tends to be around 5% of the flag leaf affected by R5 (Figure 3).
Figure 2. A graph showing the relationship between a hybrid with a poor rating for Grey leaf spot (blue line) and a hybrid with a good rating for Grey leaf spot. The ability for the hybrid to slow the disease reduces the total amount of damage caused by the time of harvest.
Figure 3. A diagram showing what a section of a corn ear leaf would look like with 5% GLS-affected leaf area.
The second step that impacts disease is cropping history. Is the field continuous corn? GLS grows and reproduces on corn residue. Continuous production of corn in the same field means that there is a consistent source of food available for the GLS pathogen to survive from season to season, reproduce, grow, and sporulate. Rotation allows for breakdown of corn residue via decomposition, limiting the amount of pathogen present in the field at the start of the growing season. If we again look at Figure 2, consider the blue line to be the disease caused in a corn-corn field and the red line to be a field with a corn-soy rotated field. If you do not rotate your fields, take another step on the riskcase.
The third step is residue. Do you practice no till farming or minimal till farming? More residue on the soil surface means an increased source of food for the GLS pathogen, and improved ability for spores to move into the crop canopy. Is this your situation? Take a step up. Hopefully your legs aren’t burning too badly.
The forth step is disease history in the field. Do you have records of what you saw in the field in years past? GLS (and NCLB) can persist in residue for several seasons. If you had these diseases at significant levels in the past, chances are they are present in the residue. Take one more step if you have a history of these common diseases.
The fifth step is environment and in particular, water. Has it been an excessively wet season? Are you irrigating heavily? GLS needs water for spores to germinate and grow into tissues. Wet years increase disease development.
There you have it, the five big steps. There are some additional factors, such as fertility and planting density that can also play a role, but by and large, the steps above are going to give you an idea of what level of risk you may have in a particular field. That being said, what about additional management this season? Do you need to apply a fungicide? First, scout your fields just before tassel. Do you see GLS at significant levels (5-10% or more) on the 2 leaves below the ear leaf? Are you standing high on the riskcase? If yes, then research indicates that a foliar fungicide at VT-R3 could be beneficial. The greatest probability for a fungicide to pay will be when you are standing high on the riskcase and you see disease on the 2 leaves below the ear leaf by VT. Are you low on the riskcase and you don’t see disease? Perhaps a fungicide isn’t needed.
With all that climbing, it’s time for a burger and fries.