Goss’s Wilt was reported in several areas of Manitoba during the 2016 growing season. Goss’s Wilt is a bacterial disease and CANNOT be controlled by a fungicide. Managing Goss’s Wilt include weed control, tillage and most importantly rotation and hybrid selection (genetics!). And with the seed ordering season quickly upon Manitoba corn growers, here are 3 questions you should ask your seed dealer about Goss’s Wilt ratings. The more information you have, the more informed decision you can make.
But before that, some key points:
- there is no third party data available for Manitoba hybrids;
- ratings will likely change over time as more years of testing are completed, in different locations and conditions;
- resistance does not equal immunity! Plants don’t have immune systems and therefore can’t be immune to any disease. Depending on the level of disease pressure, hybrids that are rated as resistant/tolerant can still be infected to some degree. If disease pressure is high (i.e. high inoculum levels, conducive environmental conditions for a long period of time), yield loss due to Goss’s Wilt can still occur in the best rated hybrids.
But First! Before you start asking your seed dealer questions, if you experienced Goss’s Wilt this year perhaps there’s a few questions you can ask yourself (or your neighbor if they had Goss’s Wilt). Was Goss’s Wilt present in every corn field, just one or a few? What were the levels of Goss’s Wilt in individual fields? Do you (or your neighbor) know the resistance rating of those hybrids, both exhibiting symptoms or not exhibiting symptoms? Are you keeping good field notes? While there is no third party data available, you could start making subjective on-farm comparisons (but at the same time recognizing the limitations of those comparisons).
Question 1: What is the rating scale used? Since there is no universal system for determining Goss’s Wilt ratings in Manitoba, there can be differences between companies and their hybrid ratings. For some companies, a rating scale of 1 to 9 is used, where 1=Poor and 9=Excellent. However, other companies use the same 1 to 9 scale, but 1 = Resistant and 9 = Susceptible. Then there are others that only use a 1 to 5 scale. So read the fine print….what does a 3 really mean? And remember, since there is no universal system in Manitoba, you can only really compare between hybrids within a single company.
Question 2: How is the testing done to establish the ratings? Ask if the testing is done under natural infection or through disease nurseries with inoculation. Relying on natural infection to determine ratings is not as dependable as disease nurseries with inoculation (and wounding). Goss’s Wilt typically shows up in patches and can be very weather –dependent. Also, Goss’s Wilt needs an entry point, often caused by hail, wind damage, etc. No symptoms under natural infection may not indicate resistance, but instead conditions weren’t conducive for infection, i.e. escape. Artificially inoculated nurseries may be resource intensive, but provide a better chance for determining resistance levels of hybrids being evaluated.
Question 3: Where is the testing done to establish the ratings? For some companies, testing is done in the United States, while other companies have established trials in Manitoba. Why would this be important? There is variability in the pathogen population, where strains are separated into groups based on DNA analysis. Further research is on-going at the University of Manitoba with funding provided by the Manitoba Corn Growers Association and Growing Forward 2 to determine the strains of Goss’s Wilt present in Manitoba. We are only beginning to understand the pathogen population here in Manitoba so there is more research that needs to be done to fully understand the role of host resistance. In the meantime, testing conducted with disease nurseries and inoculation, either here or elsewhere, is a good step to provide information on hybrid resistance ratings.
Remember, resistance ratings to Goss’s Wilt is only one of many hybrid characteristics producers should consider when choosing their hybrid!
Written by: Pam de Rocquigny, Provincial Cereal Crops Specialist & Holly Derksen, Field Crop Pathologist, Manitoba Agriculture
For more information on Goss’s Wilt, visit Manitoba Agriculture’s website at https://www.gov.mb.ca/agriculture/crops/plant-diseases/goss-wilt.html