Judith M. Kolkman1 , Rebecca J. Nelson1, 2, and Gary C. Bergstrom1
Sections of Plant Pathology and Plant-Microbe Biology1, and Plant Breeding and Genetics2 – School of Integrative Plant Science – Cornell University
What to know about BMR silage corn and diseases
Brown midrib (BMR) corn is a market class within silage corn that is desirable due to its significantly decreased lignin content. As the name suggests, midrib veins of BMR corn leaves have a distinctive brown color. Decreased lignin is desirable in corn silage because it increases feed digestibility for ruminant animals. BMR corn carries naturally derived mutations in single genes that affect the plant’s lignin biosynthetic pathway.
The biosynthetic pathway that produces lignin also makes compounds that contribute to active plant defense mechanisms. Some of these active defenses include small molecules called secondary metabolites that confer resistance against pests and diseases. Structurally, lignin is a major component of the cell wall and serves as a barrier against fungal pathogens. Lignin is also actively produced to strengthen cell walls that are being attacked.
To date, six BMR mutations have been identified in corn, and are designated as bm1 through bm6. The causal gene has been identified for five of the BMR mutations. Lines carrying two of these mutations, bm1 and bm3, are used as inbred parents for the production of commercially available hybrids known as BMR1 and BMR3, respectively. Commercial BMR silage corn hybrids have been gaining in popularity.
There is concern that the same bm gene(s) that confer greater digestibility to BMR silage hybrids may also confer increased susceptibility to fungal diseases. Some of these hybrids are more vulnerable to stalk lodging. Northern leaf blight severely affected commercial BMR hybrids in 2013 and other recent growing seasons.
To determine the effect of the brown midrib mutations on disease susceptibility, we used replicated trials across multiple years to test the reaction of bm1 – bm4 mutants in a uniform inbred line background, W64A, to leaf, stalk and ear diseases (Fig. 1, Fig. 2 and Fig. 3). Corn lines containing the four BMR mutations were all found to have heightened susceptibility to foliar fungal diseases, including northern leaf blight, gray leaf spot and anthracnose leaf blight (Fig. 1 and Fig. 2).
Figure 4 depicts a dramatic increase in W64A with the bm3 mutation. The lines were also found to be more susceptible to the foliar bacterial disease, Stewart’s bacterial wilt (Fig. 2). After two years of trials, our evidence suggests that BMR corn inbreds have higher susceptiblity to anthracnose stalk rot as well (Fig. 3). Additionally, the bm1 and bm3 containing inbreds were more susceptible to Gibberella ear rot, caused by Fusarium graminearum, when compared to their non-BMR counterparts (Fig. 3).
The benefits of BMR silage corn are huge for the dairy industry. While individual hybrids may vary, BMR corn, appears to be more susceptible to diseases than non-BMR corn. The degree of susceptibility does vary by bm mutation and specific pathogen (Fig. 2 and Fig. 3). Breeders are constantly working to improve disease tolerance, and disease ratings should be factored into hybrid choices. BMR hybrids in the market show a wide range of suceptibilities to individual diseases.
How to manage diseases in BMR silage hybrids
Knowing that BMR silage corn can be more vulnerable to foliar, stalk, and ear diseases means that a proactive and integrated strategy is needed to maintain optimal plant health in these hybrids. Elements of integrated management include:
Be aware of corn diseases on your farm and in your area. Scout your fields annually for foliar diseases from tassel emergence through grain formation. Check for ear rots (by pulling back husks) and stalk rots (squeeze lower stalks or attempt to push stalks over) prior to harvest. Seeing diseases even late in the season gives you an indication of what pathogens may survive in corn residues into the next growing season and helps you to plan rotations and select hybrids.
Fungi that cause anthracnose, gray leaf spot, northern leaf blight, and Gibberella ear rot and stalk rot survive between crop seasons in corn residues on the soil surface; therefore rotation of corn with non-host crops can help to reduce the spore inoculum potential for these diseases. Northern leaf blight has been the most widespread and injurious foliar disease in New York in the past decade and can be a problem anywhere in any given year.
Consider disease susceptibility when selecting BMR hybrids. Select BMR hybrids with the least susceptibility to specific diseases that have been problematic on your farm or in your region. If disease risk is extreme, e.g., in a humid river valley with a history of severe gray leaf spot, it may be preferable to grow non-BMR hybrids with documented resistance.
BMR hybrids, especially BMR1 and BMR3, have the potential to have severe ear rot and mycotoxin contamination in years with persistent moisture during silk emergence. Be sure to check seed company guides for the latest disease ratings for BMR hybrids.
Apply foliar fungicide based on disease detection and forecast risk. There is a wide choice of foliar fungicide products labeled for control of fungal leaf blights in New York. Table 3.5.1 in the 2020 Cornell Guide for Integrated Field Crop Management (https://www.cornellstore.com/2020-PMEP-Guide-for-Integrated-Field-Crop-Mgmt) notes the relative efficacy of labeled fungicides against different corn diseases. To slow down the development of resistance to fungicides in pathogen populations, it is best to use products with different modes of action (FRAC groups) in alternating years or to apply combination products with more than one mode of action.
The optimal timing for applying foliar fungicides is between tassel emergence (VT) and brown silk (R2) stages. Observation of foliar fungal diseases in the middle leaf canopy (at lowest ear level) and a forecast of significant precipitation in the following week are the best indicators that fungicide application will be result in disease suppression and yield increase. Suppression of foliar diseases also helps to preserve stalk health, standability, and quality, including lower levels of fungus-produced mycotoxins.
Consider longer term and regional effects of growing BMR hybrids. Year after year of growing a susceptible BMR hybrid can increase the disease inoculum load in a particular field and locale, thus affecting neighboring fields of non-BMR silage, dent, and sweet corn. Occasional rotation out of BMR corn should be considered.
BMR silage corn is increasing in popularity and acreage as it provides a high quality, digestible feedstock for dairy nutrition. Its positive attributes need to be balanced with proactive disease management to insure plant health and sustained productivity in dairy cropping systems.
This work was supported by the USDA National Institute of Food and Agriculture Hatch accession #1004040.