The Cropping Systems column in the April edition of Wallaces Farmer included an article titled “Crop scouting improves management decisions.” That article contained information explaining the basic principles of crop scouting, economic thresholds and available resources. This article will focus on scouting for early-season diseases and insects.
The first principle of crop scouting is to determine what is normal and what isn’t normal. Knowing what a healthy plant looks like is key to identifying seedling diseases. Plants communicate in color and growth. When scouting for seedling diseases look for yellowing, wilted, stunted, dead or missing plants. In corn, look for discolored or rotten mesocotyls, seminal roots, and nodal roots. In soybeans, look for seedlings that pull easily from the soil, discolored or rotting root tissue, and lesions that form on the taproot or hypocotyl.
While there may be nothing you can do to fix the problem this year besides replanting if necessary, scouting for seedling diseases can help you make management decisions for future years.
Conditions ripe for disease
Generally, certain weather and soil conditions favor specific pathogens. Cool and wet soils favor fusarium and pythium; warm and wet soils favor phytophthora; and warm and moist soils favor rhizoctonia. It should be noted that a lab diagnosis is needed to confirm what pathogen is actually causing the symptoms. Plant samples can be sent to the ISU Extension Plant and Insect Diagnostic Clinic for confirmation.
Knowing what disease is present can help you in the future, as well as choosing hybrids and varieties that have good disease scores and using fungicide-treated seed.
Scouting for insects
Many insects can pose a potential threat to seedlings and small crops during the early growing season. Here, we will define “early growing season” from planting through the first week in June; however, in reality it has more to do with crop size, or stage and degree days (thermal units) than an actual calendar date. Insect injury can cause stand loss and loss of leaf tissue (photosynthetic capacity); both can contribute to yield loss.
Because insects are small, mobile and sometimes nocturnal feeders, it may be difficult to find the culprit, so scouting for signs of plant damage such as leaf feeding, plant cutting, or missing plants is important.
Insects feeding belowground can cause thin or irregular stands and poor germination. It’s important to dig up seeds or seedlings to determine if an insect pest is present. Some insects, such as cutworms, can clip plants at the soil surface; some insects may be nocturnal and burrow into the soil during the day, making it difficult to identify the culprit. Leaf feeding is more obvious, and sometimes the insect may be present. However, if you see shot holing, you may need to cut the plant open to determine what is feeding in the whorl.
In addition to the insects listed in table, wireworms, slugs, millipedes and isopods can all contribute to plant injury or stand loss.
Economic thresholds, injury
Depending on the amount of damage done, the growth stage of the crop, and the size and number of insects, treatment may be warranted to prevent additional damage or loss of a stand. In some cases, treatment may only be necessary in some field areas, as insect pests often move in from field edges and waterways.
Some common insect pests have economic thresholds and economic injury levels established to help make treatment decisions. An ET is the point at which action should be taken to avoid reaching the EIL. The EIL is the lowest population density that will cause economic injury, or where yield loss is equal to the cost of control.
For example, Iowa State University recommends insecticide treatments for true armyworm are warranted if 10% or more of the plants are injured and the larvae present are less than three-quarters of an inch in length when corn is at the VE to V2 growth stage.
If corn is V7 to V8, treat if there are greater than eight larvae per plant, 25% of leaf area is removed, and larvae are less than three-quarters of an inch long. Consult the ISU publication Field Crop Insects, CSI 14, for additional information on insect identification, scouting and management trips.
In summary, early-season crop scouting can help you make management decisions for possible replant situations, treatment that may include a whole field or only border rows, and future seed and seed treatment decisions. Don’t forget to use all resources available to you such as reference materials, your ISU Extension field agronomist, crop consultant or retail agronomist.
For updates on insect trapping and crop disease information, visit the ISU Integrated Crop Management web page.
Rieck-Hinz is the ISU Extension field agronomist covering north-central Iowa. Reach her at firstname.lastname@example.org.