Corn stalk rot, lodging and Bt corn in 2000

Last year, lodging of corn was widespread, with some fields appearing to have 75 percent of the stalks down. According to Iowa Agricultural Statistics records, it was among the worst years for lodging during the past decade. Refer to the September 18, 2000, issue of Integrated Crop Management for some observations made during the season. This winter, I have had opportunities to discuss the lodging situation with many corn producers, extension specialists, and other ag professionals, and I think we can understand to a large extent why the problems were so widespread in 2000. In each field, there may have been a unique set of contributing circumstances, but there are a few general observations that apply to most of the field with problems.

  1. The weather conditions predisposed the plants to stalk rot. The conditions that tend to make plants most susceptible to stalk rot are a wet spring followed by a hot, dry late summer. These weather conditions result in a poorly developed root system and early fungal infection of the roots. Although the spring was drier than normal in Iowa last year, in many areas there were heavy rains during the late spring or early summer. Following pollination, most of the state went through a very dry period for about a month, which leads to considerable moisture stress on the plants while they are attempting to fill grain. With a shallow root system, the plant cannot withstand much moisture stress without a loss in stalk vigor.
  2. Strong winds occurred at just the wrong time. In mid- to late-September, many plants had weakened stalks as a result of stalk rot. Stalk-rot weakened plants can stand for a long time in the absence of a strong wind, but will go down easily when the winds hit. For example, if we compare stalk rot and lodging at our Bt corn plots at the ISU Research Farms at Nashua and Crawfordsville, a high percentage of plants had stalk rot in both locations (Tables 1 and 2). But less than 20 percent of the plants at Nashua lodged, whereas up to 70 percent of the plants at Crawfordsville lodged. The difference? Wind.
  3. Anthracnose stalk rot was more prevalent than in previous years. This prevalence was evident from both visual symptoms and the results of isolating fungi from rotted stalks. The reason for this is not clear, but anthracnose has been increasing in prevalence for several years. It was evident last season that much of the crop residue from 1999 did not decompose very much due to the dry fall and winter of 1999-2000, and this may have contributed to pathogen survival.
  4. European corn borers were not a major factor in the lodging in most fields. In some areas European corn borer injury contributed to the lodging, but this was not a widespread phenomenon. In our research plots at Crawfordsville, planting Bt corn did not alleviate stalk rot problems. At Nashua, however, we did see significantly less stalk rot in Bt versus non-Bt hybrids (Tables 1 and 2).
  5. The relationship between stalk rot and lodging was not always obvious. The combination of moisture stress and strong winds was enough to cause lodging in some plants that were free of stalk rot symptoms. In other cases, the plants lodged at a point above the rotted portion of the stalk, so it was not immediately evident that the lodging was stalk rot-related.
  6. Yield was affected in many fields. Official yield estimates dropped from a record 155 bu/acre in September to the final estimate of 145 bu/acre, largely because of losses due to premature plant death related to stalk rot. On the other hand, several producers reported to me that some of the higher yielding fields had the most lodging, which is not unusual.
  7. Other factors were involved in most fields. In specific fields there also may have been other factors contributing to the stalk problems, including fertility status or compaction. In many fields with high rates of manure, stalk rot problems seemed to be worse. The reason for this is not completely clear, although there are several possible explanations related to the effects of manure on plant growth and soil properties.

Thanks go to George Cummins, Ken Pecinovsky, and Kevin VanDee for their cooperation in the Bt corn plots.

Table 1. Stalk rot and lodging in Bt and non-Bt hybrids at Nashua.

Internal stalk rot External anthracnose Lodging
Hybrid (% of plants) (in.) (% of plants) (% of plants)
Naturally infested plants
DK 525 Bt 90a 11.6b 70a 1.5c
DK 525 95a 15.6a 80a 18.7a
Garst 8600 Bt, It, LL 35b 2.1d 15b 1.0 c
Garst 8600 It 95a 6.7c 60a 14.8ab
N 4640 Bt 100a 10.5bc 95a 3.2c
N 4640 100a 10.3bc 100a 9.8b
Plants manually infested with ECB
DK 525 Bt 75b 7.7bc 60ab
DK 525 100a 13.5a 84a
Garst 8600 Bt, It, LL 65c 2.2d 5c
Garst 8600 It 95ab 5.7c 35bc
N 4640 Bt 92ab 9.9ab 94a
N 4640 100a 12.9a 84a

ECB, European corn borer. In each column, values followed by the same letter are not significantly different. Manually infested plants were not compared with naturally infested plants.

Table 2. Stalk rot and lodging in Bt and non-Bt hybrids at Crawfordsville.

Internal stalk rot External

Hybrid (% of plants) (in.) (% of plants) (% of plants)
DK 647 BtY 80.0a 7.6b 25.0a 70.6a
DK 647 85.0a 4.5b 25.0a 63.6a
Garst 8539 BLt 80.0a 4.8b 25.0a 40.1b
Garst 8539 85.0a 8.6b 45.0a 35.0b
NK 7070 Bt 85.0a 10.8b 35.0a 24.9b
NK 7070 100.0a 16.6a 55.0a 35.0b

In each column, values followed by the same letter are not significantly different. There were no manually infested plants.

This article originally appeared on pages 19-20 of the IC-486 (2) -- February 26, 2001 issue.

Updated 02/25/2001 - 1:00pm