“But It’s A Resistant Variety”

 I can remember only a few times in my 25 years in Missouri Extension that someone asked me to look at a crop field because it looked great.  Usually, calls come in about the sick and dying.  Because of the nature of plant diseases, there is usually nothing that can be done by the time I look at a diseased field. 

 After diagnosing the disease problem, the producer will often tell me, “But it’s a resistant variety.”  Then the question becomes, “Resistant to what?”  Resistance is one of the weapons in our arsenal of crop diseases controls.  Many diseases, such as fusarium seedling blight on soybeans, don’t have any resistant varieties. There are highly resistant varieties for other diseases, like soil-borne mosaic of wheat. When selecting a variety, it is important to check for resistance to diseases that are common in Missouri or even in your field or situation.

 There are no varieties that are resistant to every disease.  The key is to prioritize the diseases you are most likely to have and how serious they can be. Look at soybeans. There is resistance for soybean cyst nematodes (SCN), sudden death syndrome (SDS), phytophthora root and stem rot and frogeye leaf disease. For some diseases like phytophthora, there is field tolerance as well as resistance.  A combination of the two works the best. 

 First SCN – have you ever seen SCN in the field or tested for it?  If you have it, what race do you have?  If you did a thorough test and came up with few to none, then that’s not a priority. If your numbers are high, then you should do a race test to see which resistant soybean varieties you should use. 

 Sudden death syndrome has been increasing because it is favored by a cool, wet spring. Should SDS resistance be a priority?  Are you planting after mid-May?  Then, no. If you are planting before mid-May and you’ve had it the field before, then definitely, yes.  What if you planting early but have never had it before?  Because it can be a very devastating disease, it is recommended that you look for resistance to it. 

 Phytophthora likes wet soils. Areas that have had it before are likely to get it again. So if you have a heavy clay soil or wet bottomland, look for resistance to it. In a field with a history of Phytophthora problems, selecting a soybean variety with a combination of resistance and good field tolerance may be the best strategy.  If you can’t find resistance, be sure to use an appropriate seed treatment in these fields. 

 Selecting crop varieties can be complicated:  yields; cold, heat or drought tolerance; insect, disease, herbicide resistance; height, stalk strength; and maturity are a few of the things to look at.  As you can see from above, just disease resistance selection can be complicated.  If you need help sorting through all this, give your University of Missouri Extension agronomist a call. 

Author: Pat Miller, Agronomy Specialist