Hot, dry weather when corn is starting to tassel and pollinate creates crop stress.

At the start of this week, the weather forecast for Iowa was for daytime highs in the 90s and even in the 100s across the state, and low nighttime temperatures in the 70s to 80s with little chance of rainfall. This forecast raises concerns on what impact such weather conditions can have on crops, especially with the earlier-planted corn starting to tassel and pollinate.

Iowa State University Extension cropping systems agronomist Mark Licht wrote a blog on the ISU Integrated Crop Management website July 15, as this week was starting off hot and dry. With weather information provided by state climatologist Justin Glisan, they discussed current weather conditions and the outlook.

They also explained what a flash drought is and what impact the hot, dry weather would have on Iowa’s 2019 corn and soybean crops. Here’s what they have to say about the hotter and drier weather—although some areas of the state (especially southwest Iowa) ended up getting a couple inches of rain on July 16.

Current weather, outlook

This week began with a persistent, upper-level blocking ridge sitting over the Midwest, preventing airflow and causing a stagnation in our weather pattern. The warmer temperatures and little rainfall can be attributed to this blocking high. The six- to 10-day and eight- to 14-day weather outlook showed higher probabilities of above-average temperatures and below-average precipitation. The one-month outlook is still showing more below-normal temperatures and above-average precipitation, but it was last updated June 30.

What is a flash drought?

One term that has been thrown around recently is that we may see what is called a “flash drought.” A flash drought can occur when relatively short periods of anomalously warm air resides over a location, coupled with rapidly decreasing topsoil and subsoil moisture. There are varying definitions of a flash drought, but there are two general varieties named by the physical mechanisms that create them: heat wave and precipitation-deficit flash droughts.

Flash droughts can form relatively fast over a period of one to two months (hence the name “flash”). Another factor that can make a flash drought worse is less cloud cover. That allows more solar radiation (sun) to reach the Earth’s surface, which leads to higher evapotranspiration rates from crops in fields.

Flash droughts differ from classic droughts in that a classic drought forms over a longer seasonal timescale and can be classified in multiple ways. Think of a drought as creeping up, while a flash drought happens over a much shorter time frame.

Flash droughts are basically brought on by weather and surface conditions that begin to impact crops and livestock. Given the lack of rainfall over the last 30 days, combined with the forecast for unseasonable dryness and warmth, flash drought considerations are being monitored by climatologists and agronomists.

What does this mean for crops?

A flash drought can impact corn and soybean crops in the following ways:

Corn. Soil moisture availability is influenced by precipitation as well as rooting depth. April and early-May planted crops will have a deeper root system compared to late-May and June planted crops. A deeper rooting depth means more soil water available for crop uptake. Additionally, if root systems are restricted due to soil compaction or insect injury like corn rootworm feeding, this can also make the corn much more susceptible to experiencing moisture stress.

Leaf rolling is often the first distinguishable symptom that the crop is under stress. Leaf rolling is a protective response that limits the exposed leaf area, which in turn reduces transpiration. Corn leaf rolling periodically on hot afternoons does not mean a yield loss is occurring. However, if there are 12 or more hours of consecutive leaf rolling, a grain yield loss will likely occur.

A lot of the April planted cornfields and even some of the May planted fields are now in mid-July, tasseling and beginning to pollinate. Pollination is a critical period for corn development and yield and is a time of high amounts of water uptake (0.3 to 0.4 inch per day).

In addition to leaf rolling, moisture stress on corn plants can also slow or delay silk elongation. This raises concerns because the forecasted higher temperatures will increase heat units or growing degree day (GDD) accumulation, which will speed up crop development. The combination of limited soil moisture and higher temperatures could lead to pollen shed prior to silk emergence, resulting in either having a poor “nick” of the pollen and silks or missing the nick.

High temperatures can reduce pollen shed and viability. Pollen shed is limited when temperatures rise above 86 degrees and temperatures greater than 100 degrees can kill corn pollen. Consequently, this usually limits pollen shed to mid-morning and early evening.

Pollen shedding occurs over a five- to 14-day period and “fresh” pollen is available daily. Silk elongation can last up to 10 days, and silks remain receptive up to 10 days following emergence (viability does decrease as the silks age). This can help to minimize the impact and reduce the concern with short-term heat waves.

Soybeans. Effects of hot, dry conditions on soybeans currently are less severe compared to the effects on corn, which is entering pollination. Soybeans respond to drought stress by flipping their leaves so the underside is turned up. Vegetative growth can also be diminished, and flowers or pods may be aborted. However, soybeans can set new flowers and pods when conditions improve.

Pests to watch for

If conditions stay hot and dry, keep your eyes open for spider mites in both corn and soybean fields, as this pest becomes more of a concern with hot and dry weather.

Hot and dry conditions are not very favorable for foliar disease development, but high humidity combined with warm temperatures may be favorable for gray leaf spot development on corn. So far, disease pressure has been low. Common rust, eyespot and gray leaf spot have been the main foliar diseases observed in cornfields.

Source: Rod Swoboda, WallacesFarmer