Insect pests of corn are often involved in the disease cycles of ear rot and stalk rot diseases. The European corn borer is particularly well known for its involvement in these diseases.
The interaction between corn borer larvae and pathogenic fungi is threefold:
- wounds on the plant made by corn borers are open infection sites for fungi;
- corn borer larvae act as vectors of pathogenic fungi (they carry the spores directly into the wounds); and
- the larvae cause stress on the plant that makes it more susceptible to infection and disease.
Because of these interactions, it is logical that some disease control might be achieved through the control of European corn borers, either by insecticide or genetic engineering. We (and others) have studied the effects of Bt transformation on certain corn diseases since 1994, and it has become clear that there are some disease-control benefits. Like any management practice, there are limitations to the benefits provided by Bt corn.
||Stalk rot symptoms developing from European corn borer tunneling site.
||European corn borer damage and fungal infection in non-Bt (left) and Bt hybrids following manuyal infestation with second-generation corn borer larvae.
Much of our research has focused on Fusarium ear rot because it is the most common ear rot disease in the Midwest and it is closely associated with insect damage to ears. Although this disease does not cause a great deal of yield loss, it reduces grain quality and the fungi can produce mycotoxins, such as the fumonisins. We also have studied stalk rots caused by Fusarium, Gibberella, and Colletotrichum (anthracnose).
Table 1. Percentage of corn ears with Fusarium ear rot in Bt and non-Bt hybrids in 1996, following manual infestation with second-generation European corn borer. The * indicates a significant difference between Bt and non-Bt hybrids.
||Fusarium ear rot
Table 2. Mean stalk rot severity (cm of stalk with symptoms) in Bt and non-Bt hybrids in two Iowa locations in 1997. The * indicates a significant difference between Bt and non-Bt hybrids. Specific hybrids differed between locations.
|Novartis (Ciba Seeds)
Fusarium ear rot is heavily influenced by second-generation corn borer control. For this reason, Bt hybrids that express Bt proteins in kernels late in the season (usually YieldGard hybrids) have demonstrated the best control of this disease. Results from 1996 are a good example (Table 1). When the plants were manually infested with corn borers, the level of Fusarium ear rot was high in the non-Bt hybrids, but was much lower in some Bt hybrids. Sometimes control is seen even in hybrids that do not express the Bt protein in kernels. Gibberella ear rot probably will not be affected by Bt transformation because this disease often infects ears with or without insect damage.
Researchers at Cornell University have reported reduced stalk rot severity in Bt hybrids, and in our experiments, we have seen similar results. But this effect has not been as consistent as the effect on Fusarium ear rot. In 1996 and 1997, in fields with minor stalk rot problems, some Bt hybrids displayed significantly less stalk rot (Table 2). In an experiment in Centerville, IA, in 1997, three of four Bt hybrids had significantly less stalk rot than their non-Bt counterparts. In one field in 1997 where stalk rot was severe (Dallas Center), the Bt hybrid had an advantage in only one of the four comparisons.
Stalk rot fungi can enter corn plants through insect injuries, but they often enter through the roots instead. When this happens, stalk rot will develop regardless of insect injury. Therefore, when conditions are very favorable for stalk rot, we expect Bt hybrids to experience stalk rot severity similar to non-Bt hybrids.
In 1997, there were reports that some Bt hybrids experienced greater stalk rot and lodging than their non-Bt counterparts. Some of this lodging occurred in plants that were apparently free of stalk rot. This is an unexpected observation that will require close attention in the near future. In evaluating this situation, we need to remember some important factors about stalk rots and lodging: (1) in 1997, inadequate soil moisture during grain fill, strong winds, and rapid drydown led to stalk rot and lodging in all types of hybrids; (2) physical and physiological stresses often cause lodging even in the absence of stalk rot; (3) higher yield causes both physical and physiological stresses that affect stalk strength and stalk rot development; and (4) Bt hybrids are not genetically identical to their near-isogenic, non-Bt counterparts (they may differ in stalk rot susceptibility).
Stalk rots and ear rots require an integrated management approach, and Bt hybrids may be one factor in their control. However, it is important to use other methods as well because Bt hybrids will not provide adequate control under all conditions.
This article originally appeared on pages 1-3 of the IC-480 (1) -- January 19, 1998 issue.