Corn Breeding
Breeding Corn With Improved Resistance
by Dr. Lana Reid, Research Scientist - Corn Breeding, Agriculture and Agri-Food Canada

Figure 1. Typical mould growth on an ear of corn infected with gibberella ear rot.

Gibberella ear rot (pink mould/rot) reduces yields and contaminates grain with mycotoxins including deoxynivalenol (DON, vomitoxin) (Fig. 1). Fungal spores are blown or splashed onto silks and grow down the silks to infect the kernels, or the spores can enter the ear through wounds created by insects or birds. The most feasible control method is to use resistant hybrids; unfortunately only a few hybrids with moderate resistance are available. This may be about to change thanks to the work of a team of researchers led by Dr. Lana Reid at the Eastern Cereal and Oilseed Research Centre (ECORC) of Agriculture and Agri-Food Canada (AAFC).

Since 1986, an ear rot resistance breeding program has been conducted at ECORC. Because of the sporadic nature of ear rot epidemics, the first step was to develop field inoculation techniques which consistently separate resistant from susceptible plants. Two techniques were developed, one for assessing silk resistance and one for kernel resistance. Using these techniques, the team was able to find a few corn varieties with intermediate levels of resistance and initiated a breeding program to develop new inbreds with improved resistance and adaptation to Ontario.

Figure 2. Disease severity ratings after silk (blue bars) and kernel (purple bars) inoculation of 8 inbreds released by ECORC with improved resistance to gibberella ear rot. B73 and CO354 are susceptible checks. CO325 and CO272 were some initial sources of kernel and silk resistance, respectively, used in the breeding program. Higher values for disease severity indicate more infection. In a breeding program, good silk resistance is associated with disease ratings of 2 or less (blue horizontal line) while good kernel resistance is associated with ratings of 3 or less (purple horizontal line).

So far, ECORC has released eight inbreds with improved resistance to ear rot. The first three were released in 1997 and were called CO387, CO388 and CO389 (Fig. 2). This international coding designates a ‘C’ for Canada and an ‘O’ for Ottawa. CO387 to CO389 have moderate silk resistance but inadequate yield performance. This is always a challenge for corn breeders: most developing inbreds have to give up something to have improved resistance, and unfortunately the yield genes are usually the first to go. This is often referred to as the resistance-yield barrier (Figure 3). In 1999, three more inbreds were released: CO430, CO431 and CO432 were derived from a population made up of five commercial hybrids with moderate levels of resistance. CO432 has the best combining ability for yield of the three inbreds. In 2000, CO433 was released. In 2002, an inbred (CO441) with very high resistance to ear rot and excellent combining ability for yield was released (Figure 4). ECORC had managed to combine silk with kernel resistance and competitive yields. These 8 inbreds are the first of their kind to be released from a public breeding program. This research was supported in part by the Ontario Corn Producers’ Association and Ontario Pork. Further information on AAFC’s inbred releases and breeding program can be found at the ECORC website: http://res2.agr.ca/ecorc.

Figure 3. Association between disease severity and yield. The breeder must select inbreds in the top left hand corner, i.e., those with high yield capacity and low disease scores. Only in recent years has ECORC started to develop inbreds that fit this category, thus they are just starting to break the resistance-yield barrier that is often a problem in breeding for pest resistance.

ECORC Corn Breeding Program
An inbred is produced by self-pollinating a hybrid cross or population for 7 or more generations. Self-pollinating involves manually taking the pollen from the tassel of a plant and placing it on the silks of the ear of the same plant. If a commercial hybrid is self-pollinated, the offspring will vary in height as well as in many other traits. If you continue to self-pollinate the offspring over several more generations, you will develop a much smaller plant (due to decreased vigour) that is uniform for most traits. If this plant is cross-pollinated to a plant developed from a different hybrid (i.e., take the pollen from the tassel of the first plant and put it on the silks of the second plant), you will produce a hybrid plant that will be much more robust and higher yielding than either of the parental inbreds. This is called hybrid vigour or heterosis, and explains why almost all commercial hybrids are crosses between two inbreds. The objective of corn breeding is to select for certain traits, such as disease resistance, during the inbred development and

still retain the genes for yield and good agronomic performance. In the ECORC ear rot breeding program, this means that development of new inbreds with ear rot resistance takes about 7-10 years, and all self-pollinated plants must be inoculated each year. As well, the developing inbreds must be tested for hybrid yield several times. In addition, mycotoxin analyses are performed; fortunately, there is a high correlation between visual symptoms and mycotoxin levels.

Figure 4. Two different hybrids inoculated through the silk channel with gibberella ear rot. The hybrid on the left is a susceptible commercial hybrid. The hybrid on the right is a cross between CO441 and a commercially available inbred.

Like most public breeding programs, ECORC only releases inbreds. The inbreds are released free of charge to all seed corn companies. If a company decides after several years of testing to use the inbred in a commercial hybrid, a commercialization agreement is signed and ECORC receives royalties on seed sales. Obviously, this is an honour system and it is up to the commercial seed company to inform AAFC that they are marketing a hybrid with an AAFC inbred.

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