As crops start to emerge, now is a good time to get out and do early-season scouting to see how things went with planting and to also check for any early season crop disease and insect issues. Knowing how things are looking can help you make more informed management decisions. Here are some tips and things to keep your eyes out for while doing early season scouting.
Before you head out to the field, there are several tools that are a must-have for early-season crop scouting. First, a tape measure to take stand counts. As the saying goes if you don’t measure it, you can’t improve it. Another important tool is a seed digger, trowel, or spade to dig up seeds or plants to evaluate planting depth, seedling diseases and below-ground feeding insects, like seed corn maggots. My personal preference is a square-bottom spade, you can easily push it into the ground a few inches from the furrow, pop open the furrow and look at planting depth, seed placement, sidewall compaction and rooting issues. A recent ICM News article “Crop Scouting Basics” outlined a thorough list of tools and resources for crop scouting.
The first principle of crop scouting is to determine what is normal and what is not 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. Two resources that may be of help are IPM 76, A Farmer’s Guide to Corn Diseases (https://store.extension.iastate.edu/product/14600) and IPM 77, A Farmer’s Guide to Soybean Diseases (https://store.extension.iastate.edu/product/A-Farmers-Guide-to-Soybean-Diseases).
Favorable Conditions for Seedling Diseases
Generally speaking, 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 and Outreach Plant and Insect Diagnostic Clinic for confirmation. More information can be found at: https://hortnews.extension.iastate.edu/pidc.
Due to dry conditions during planting across most of Iowa this spring, we don’t anticipate seedling diseases to be a big issue, but you may find isolated pockets depending on soil drainage. Knowing what disease(s) is present can help you choose hybrids and varieties that have good disease scores in the future and can guide decisions on the use of fungicide treated seed.
Many insects can pose a potential threat to seedlings and small crops during the early growing season. For the purposes of this blog 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 of which can contribute to yield loss.
Because insects are small, mobile, and sometimes nocturnal feeders, it may be difficult to actually 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 is 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.
This crop_scout_insect_pest_calendar_degree_days.pdf provides a handy reference as to when to scout and pest control windows. It was developed by Brian Lang, extension field agronomist located in NE Iowa. If you are located in southern Iowa, you may anticipate earlier insect pest activity based on warmer temperatures.
Using Economic Thresholds and Economic Injury Levels
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 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 (ET) and economic injury levels (EIL) 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, ISUEO 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 VE-V2. If corn is V7-V8, treat if there are greater than 8 larvae per plant, 25% of leaf area is removed and larvae are less than three-quarters of an inch in length.
Consult the ISU publication, CSI 14, Field Crop Insects (https://store.extension.iastate.edu/product/13725) 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 Extension field agronomist, crop consultant or retail agronomist. For updates on insect trapping and crop disease information visit the ISUEO Integrated Crop Management web page at https://crops.extension.iastate.edu.
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