By Chelsea Harbach
Field crop health matters throughout the entire growing season. The types of diseases on crops depend on the point in the growing season and the environmental conditions. Many later-season diseases result from soilborne pathogens over which in-season fungicide applications have no control. Most commercial varieties will have ratings for the performance of their selections against various diseases, including those caused by soilborne pathogens.
It is exceedingly important to understand what soilborne diseases affect your crop so that you may plan accordingly for years to come by selecting more tolerant or resistant varieties. HOWEVER, time is a limiting factor in accurately identifying what is causing plants to be sick in your field.
In the Plant & Insect Diagnostic Clinic, we are prone to receiving less-than-ideal samples for diagnostics. This becomes more problematic the later we get into the growing season, even leading up to and after harvest. To get an accurate diagnosis, ideally, we receive plant tissue that includes living and sick tissue, as that includes the “zone of transition,” which is where we are most likely to successfully recover any pathogen associated with the symptoms. When we receive completely dead tissue, there is essentially nothing we can do with it. Quickly after plant tissue dies, there is a succession of microbes on the tissue moving from any pathogenic bodies to those that are saprophytic (feeding on dead tissue).
IDEAL SCENARIO: Season-long scouting up to the point of harvest of the ENTIRE field- allows you to detect early senescence in plants, which are viable candidates for diagnostics as long as they are not completely dead. The early-senescing plants are diagnosed with a disease. Now, you can plan accordingly with cultivar selection the next time you grow that crop in that field to try and suppress late-season disease development genetically.
UNIDEAL SCENARIO: You harvest a field and see on your yield monitor an area that yielded lower than the rest of the field. You collect bits of the plants from the back of the combine and bring them in for diagnosis. We cannot tell anything about what caused these plants to underperform (likely senesce early) at this point, with very few exceptions. This is not helpful for us or you.
Also of note, many soilborne late-season diseases have similar aboveground symptoms on soybeans. Getting an accurate diagnosis is the only way to ensure you can plan accordingly for years to come to protect your future yields.
Source : iastate.edu