Wheat Fusarium Head Blight (Head Scab) in Relation to Tillage and Previous Crop
by Donald Hershman
Fusarium Head Blight (FHB), also known as Head Scab, is a potentially serious disease of wheat in Kentucky. Losses can be very heavy across a large area, as occurred during 1991; or losses can be minimal state-wide, as occurs in most years. However, in every year there are some fields in Kentucky that are severely damaged by FHB.
University of Kentucky Extension Publication PPA 38, Head Scab of Small Grains in Kentucky, details the occurrence, symptoms, general epidemiology, and management of FHB in Kentucky. That publication is available at county Extension offices and through the Department of Plant Pathology on the web at: http://www.ca.uky.edu/agc/pubs/ppa/ppa38/ppa38.pdf. The purpose of this fact sheet is to present information regarding the relationship between FHB, tillage, and previous crop.
Effect of Tillage and Previous Crop on FHB
Kentucky wheat producers have not generally embraced no-till wheat production. There are a variety of reasons for this situation, but one factor is the generally accepted fear that FHB is significantly enhanced in no-till production systems. The majority of wheat in Kentucky is, in fact, planted behind corn. Thus, there is a biological basis for the fear of enhanced FHB when wheat is produced no-till following corn. This is because the primary FHB causal fungus, Fusarium graminearum, also infects corn and causes Gibberella stalk and ear rot. Thus, superficially, it makes common sense that leaving corn stalks on the ground would enhance FHB in a subsequent wheat crop.
Although there is no denying that leaving corn stalks on the soil surface may result in higher populations of F. graminearum in fields, there is no evidence to suggest that FHB is significantly impacted by tillage of corn stalks. In fact, a three year survey of 230 wheat fields during 1998 - 2000, as well as a great many observations, suggest that tillage is a secondary factor affecting FHB levels in Kentucky. The overriding factor in all cases appears to be weather. Specifically, if weather is not favorable for FHB development, then it doesn't make much difference what the previous crop was or what the tillage system is. Conversely, if weather is highly conducive to FHB, and the crop is at a susceptible stage (e.g., flowering), a high level of FHB is likely to develop regardless of previous crop and/or tillage.
How can the above situation exist considering that F. graminearum is a pathogen of both wheat and corn? There are probably many different contributing factors, but the main one is thought to be related to the agricultural systems we have in Kentucky. For example, in any given year, we grow about 1.2 million acres of corn. According to a survey conducted recently by the Kentucky Integrated Pest Management Program, the average sized corn field in the state is just over 39 acres. Simple mathematics indicates that this translates into A LOT of small corn fields being widely scattered throughout central and western Kentucky. Add to this the fact that spores of F. graminearum are wind-borne, and it is easy to imagine that if weather is favorable for FHB, almost every acre of wheat will be exposed to significant levels of infectious spores. F. graminearum also grows in soybean and wheat stubble, so this adds to the overall high levels of F. graminearum inoculum we have in Kentucky.
The next obvious question is why don't we have FHB epidemics more often if F. graminearum is so widespread? The answer to this is that wheat is only highly susceptible to infection by F. graminearum during and shortly after crop flowering; this period of susceptibility lasts for only 4 - 5 days. If the weather is not favorable for infection during this period, it makes no difference how many spores blow onto the wheat head. Now, if you add highly variable flowering dates and durations to the picture, it is easy to imagine that most crops in most years simply ESCAPE infection.
The take home message is that FHB is not highly impacted by previous crop or tillage level in Kentucky. When weather is favorable for FHB, and crops are in susceptible stages, an abundance of spores of F. graminearum assures a high levels of FHB, regardless of previous crop or tillage. When weather is unfavorable for FHB during susceptible stages, the crop escapes infection. Weather is the decisive factor. Escape of FHB is also common because of the relatively short period of susceptibility for wheat. Farmers should not use fear of FHB as a reason for not giving no-till wheat a try.
This non-link between tillage and FHB may not be the same everywhere. Specifically, any factor or situation that would reduce the widespread occurrence of wind-blown spores of F. graminearum in an area, such as might occur where fewer, larger fields are the norm, might make the tillage of residue, especially corn residue, more important in the overall FHB picture.
Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!
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UNIVERSITY OF KENTUCKY, KENTUCKY STATE UNIVERSITY, U.S. DEPARTMENT OF AGRICULTURE, AND KENTUCKY COUNTIES, COOPERATING