- Wet weather has delayed planting of corn and soybeans in the southern part of our territory
- Delaying planting may impact the impact of pests and pathogens
- Following reports and scouting will be essential to account for an atypical season
This season wet weather has delayed planting in the southern part of our territory. Although the main concern with delayed planting revolves around discussions of sufficient time to canopy and accumulated GDU’s, we also need to be aware of the potential impacts it may have on pests and pathogens. Pests and pathogens have evolved with plant hosts in such a way that their lifecycles coincide with the lifecycle of the plant. When the planting of fields in an area are delayed, four things can happen: 1) Fields that were planted on time become major targets for pests of that crop, as it is the only game in town; 2) The pest or pathogen “timing” is misaligned, leading to fewer issues in the delayed crop; 3) Late planted fields may be infected earlier in development than typical; 4) Nothing different occurs. This article will discuss several major pests and pathogens and the effect of delayed planting on their crop impact.
Insect Pests: Insects develop according to GDU accumulation, which is specific to each insect. When fields are planted late, it is possible that the timing of insect emergence or egg hatch and crop growth may not coincide.
Stinkbugs: Populations typically are low during corn emergence in most middle to upper latitudes, and plants are often well into vegetative growth or later by the time populations start to be observed to any degree in fields. Early feeding of corn by stinkbugs can cause deformed banana ears which result in reduced yields. Threshold per 100 plants: V1-V6 >13; V14-VT >10; R1-R2 >28.
Corn rootworm: Corn rootworm requires 684–767-degree days to achieve 50% egg hatch. The larvae hatching from eggs need to find corn roots within a few hours or they will die. As of 5/28 in Champaign County, 691 growing degree days have accumulated. This means egg hatch has already reached 50% by the end of this week in this latitude. Less CRW damage may be observed in late planted fields that have not emerged by this point in time. Follow local extension updates or websites (such as https://data.prairie.illinois.edu/warm_pdd/ in Illinois) to track degree days for this and other important insect pests. Assessment for CRW damage to hybrids typically should be conducted around VT.
Black Cutworms: Typically, our corn is far enough along that by the time black cutworms lay eggs and those hatch and grow into caterpillars, the chance for cutting is low. However, we are seeing black cutworms show up on time, meaning that if those caterpillars are active and plants are younger than V5, we may see some fields at risk for cutting. Scout early in the day or at dawn, and/or check under residue near stalks. Threshold: VE-V5-2 or more caterpillars per 100 plants. Late day or early morning insecticide sprays may be more efficacious than those made during mid-day. Corn with aboveground Bt traits should be protected to a degree.
Japanese Beetles: You may observe more of these in fields planted on time. You also may see more foliar damage earlier in soybean growth and development in late planted fields. Threshold: 30% defoliation before R1, 20% between R1 and R3 in soybeans. In corn, if there are >3 beetles per ear with silks less than ½ inch in corn where pollination less than 50% complete.
Diseases: Diseases such as most nematodes and soilborne diseases are unlikely to be severely affected by delayed planting, as chemical signals from host roots typically play a critical role in breaking dormancy of the overwintering structure. However, soils may be warmer when planting is delayed, meaning that seedlings will germinate and grow more quickly. This results in less damage by seedling pathogens and enhanced compensation capacity by the crop.
Residue borne foliar fungal diseases: (Grey leaf spot, tar spot, and northern corn leaf blight in corn, frogeye leaf spot in soybean). These will be normal in fields planted on time. However, it is possible that delayed planting may result in more disease earlier in the growth and development of the crop if we enter a significant period of wet weather. In many cases, delayed planting will result in vegetative crop growth occurring when temperatures are elevated, which may actually reduce early season disease, as these diseases need a closed canopy to retain humidity and facilitate infection by spores. Your risk increases in delayed fields as you add on the following: 1) continuous soybean or corn; 2) no till; 3) irrigation. Ensure that fields at elevated risk are thoroughly scouted weekly. Thresholds do not work like they do with insects due to complexities of disease cycles, coinfection with numerous other diseases, and type and degree of resistance of the host. A general rule of thumb is if you are seeing >5% disease severity on the leaf below the ear leaf before VT you are more likely to see a return on investment of a fungicide application.
Southern rust: Southern rust blows into the Midwest and eastern parts of the US from southern areas each year. Often this pathogen does not enter most parts of the Midwest until the crop is near black later or afterwards. Delayed planting increases the likelihood that the disease will enter these fields at earlier points in development and therefore cause more issues. The tough thing about Southern rust is that it does very well when it is hot out and only needs 4 hours of leaf wetness to infect leaves. That is typically what we may see when we have dew formation on leaves as day transitions to night. If southern rust is observed before R3 and temperatures are between 75-93°F, then a fungicide may be considered.
Keywords: Agronomy, pest, disease, corn, soybean