Assessing the Results

By Greg Stewart, OMAFRA Corn Specialist and Heather Griffiths, OMAFRA IPM Specialist

As yield results from the 2000 corn crop are stacked up, some growers have been reasonably happy with the fact that yields, although not outstanding, were not as low as they might have been. Others have been discouraged by the fact that where they anticipated a poor crop - it was exactly that. In assessing the underlying causes of poor crop performance, it is easy to point to the weather and the host of problems experienced in getting the crop planted, up and growing. Many also want to understand how the weather during the heart of the growing season may have affected corn yield potential. Specifically, did below normal temperatures disadvantage the crop to a point that it could not catch up and/or did poor weather conditions during grain filling result in low leaf photosynthesis and thus low yields and low test weights?
Table 1 outlines key weather factors as observed at the Elora Research Station over the last two growing seasons. Although these results are from a single weather station, they represent trends that will be similar in most areas of the province. Crop heat units for 2000 were actually close to normal throughout June, July and August, although they were considerably behind last year’s numbers, particularly in July. Corn that was planted during the first week of May and that got off to a reasonable start was not put at any significant disadvantage by temperatures. The result, for the most part has been reasonable yields on the early corn. However, corn planted late in May or early June that needed some above average heat to speed its development along simply did not get it.

Solar radiation during the June-July period was lower than last year and somewhat behind the average for the last 6 years. One theory that has been set out was that low yields and low test weights were caused by a cloudy August combined with a significant amount of leaf area being consumed by leaf diseases, most notably rusts. However, Table 1 indicates that solar radiation was normal in August.

Leaf Diseases
Leaf disease should not be discounted in lowering the photosynthetic activity in this year’s corn plants. Dr. Thys Tollenaar, Department of Plant Agriculture, University of Guelph, recorded photosynthetic rates in hydroponically grown corn plants both in 1999 and 2000. Interestingly, there was nearly a 25 per cent drop in photosynthesis measurements taken this year compared to last year (see Table 2). Tollenaar acknowledges these plants were somewhat infected by rust, but also points to other factors such as the status of the plants, temperature, radiation, etc.

Although not strictly weather related, photosynthetic rates were most likely below normal in a large share of the corn crop either in July because of low sunlight or in August because of leaf diseases.

Greg Shaner from Purdue University points out that rust damages the plant in two general ways. The fungus is a parasite, and it derives all of its nutrients from the host plant. Carbohydrates and other nutrients that would normally sustain vegetative growth and fill the grain are diverted into the fungus. Also, at the site of each pustule, the leaf epidermis is torn open. This means that the ability of the plant to regulate its water economy through stomatal action is compromised. A heavily rusted plant can show symptoms of moisture stress under hot, windy weather, even when soil moisture would be regarded as adequate. A severely rusted plant suffers both from lack of nutrients moving into the grain and from lack of water.

Sudden Senescence
One of the peculiarities of corn development this season was the sudden aging of corn plants during the last half of the growing season. This sudden aging, or ‘senescence’, was characterized by being earlier than normal, by developing rapidly in the plant, and by leaves senescing from both the bottom and top of the plant. In some cases, this top leaf death made one think that the crop had been hit by a frost. To appreciate the phenomenon of early senescence, consider the movement of carbohydrate in the plant during the grain-fill period. The demand for carbohydrate is at the ear, the supply of carbohydrate is the leaf. If the demand outstrips the supply, carbohydrates are drawn from other areas of the plants, i.e., stalk, shank and husks. Dr. Tollenaar refers to this as the corn plants’ own cannibalization. In fact, in studies where he removed leaves from corn plants prior to the grain filling period, he could alter the supply/demand balance to the point where the corn plant senesced prematurely (see Figure 1). Tollenaar is quick to point out that early senescence is not a new disease, but rather a physiological response that can be exaggerated by leaf or stalk diseases. In 2000, low photosynthetic rates in July and August caused by low irradiation, leaf disease, nitrogen stress, etc., could have contributed to the plant leaves not being able to adequately meet the demands of the filling ear, thus resulting in remobilization of carbohydrates from other parts of the plants. This imbalance within the plant may result in early leaf senescence, low test weight and yield, as well the depletion of soluble carbohydrates from the stalk, which often results in greater infections by stalk rot organisms and a high percentage of broken stalks.

Recommendations
A few reminders surface as a result of observations in 2000.
1) We cannot overestimate the importance of getting corn planted early and off to a good start in order to deal more effectively with the stresses that will occur in some form or another throughout the season.
2) Significant differences exist in hybrids’ susceptibility to leaf disease and stalk rots, but be careful not to swing too far in that direction when selecting 2001 hybrids. A good genetic package with high yield potential needs to be the foundation upon which other specific disease resistant traits are added; not the other way around.

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