• Interactions of air temperature and rainfall can have complex effects on crop growth.
• Crops are adapted to our growing conditions but weather extremes can cause crop symptoms.

Optimum crop growth requires a balance of crop physiology, nutrients, and water.  Water, in particular, has a critical role in the health and productivity of the crop.  Our experience shows this clearly but does not account for how complex this really is in practice.  Following are some comments about the balance between the crop and its environment.

Evaporation and Transpiration

Crop foliage is capable of absorbing moisture during rainfall and even high humidity but, for corn, soybeans, wheat, and other crops, the roots are primarily responsible for supplying the water needs of the plant.  The foliage of these plants conserves and regulates water movement by opening and closing stomates on leaves.  Evaporation of water and osmotic balance helps create a "pull" of moisture through the crop vascular tissue, while also affording a natural cooling effect for the plant.  Meanwhile, capillary action in the vascular system helps create a "push" of water.  Adequate nutrient supply helps maintain the osmotic balance in the plant.  This system can become impaired with either excessive moisture, or deficit moisture conditions.

Adverse Crop Symptoms

Under hot and dry conditions, the foliar demand for water can exceed the ability of the roots to supply needed water, even when soil moisture is adequate.  Under these conditions, temporary wilting can result.  This usually does not have a lasting effect, except when the plant experiences excessive stress during critical reproductive growth phases.  Severe heat or dry weather can cause breaks in the normal movement of water in the plant.  If this period of stress is lengthy, irreversible yield loss can occur.  Here is a table showing how four consecutive days of corn wilting can effect yield at different growth stages:

Anything which impairs root growth can also affect the ability of the root system to supply needed water to the plant.  This includes soil compaction, insect feeding, disease, or mechanical injury.  Crop symptoms during hot, dry weather may also develop when weather suddenly shifts from mild and moist, to hot and dry.  Diagnostic evaluation of crop injury should always include careful examination of both above-ground and blow-ground plant growth, as well as documentation of climate factors.

Heat Effects on Crops

During periods of hot, dry weather, our focus is usually on the lack of water for the crop.  Occasionally, however, heat can be a limiting factor by itself.  Though rare in our area, high temperatures can directly damage plant tissue.  This is especially true when the moisture supply to the crop is limited, because transpiration has a powerful cooling effect on the foliage.  When the water supply is limited, leaf stomates close to conserve moisture.  This limits transpiration and foliage cooling.  Then, with air temperatures in the 90s or 100s, various amounts of leaf scorching or scalding may result.

Soil temperatures rarely increase to lethal levels for the crop.  However, surface soils will sometimes dry and heat up to the point where crop symptoms develop.  One example of this includes "heat canker," where the crop stem at the soil surface develops a canker.  This can be common with beans and other crops grown on sandy soils, and it may be confused with diseases, such as Rhizoctonia stem rot.  A second example is a form of "rootless or floppy corn," where initial corn nodal root development is inhibited when these roots cannot penetrate into hot, dry, surface soils.  The result is that the corn may fall over due to lack of nodal roots, or the corn may eventually wilt and die if moisture conditions don't improve enough to allow nodal root development.

Contact your FS Crop Specialist for your agronomic information.