"Direct seeding is responsible for the high levels of disease we have had in the past few years. We never had disease when there was still summerfallow in the area. I have to cultivate my stubble to bury the residue so it won't contaminate next year's crop. My neighbour, who zero tills, is responsible for contaminating my fields with Fusarium". Ever hear one of these statements? Probably more than once.
The disease in the last few years that has been causing the most concern for eastern and particularly south eastern farmers has been Fusarium Head Blight (FHB). Fusarium species are found throughout the prairies, but the debilitating F. graminearum that is responsible for FHB has been mostly limited to Manitoba, South East and Eastern Saskatchewan.
Fusarium Head Blight affects wheat, barley, corn and other small cereal grains. It can cause huge losses in yield and quality through the production of a fungal toxin. There are numerous species of Fusarium, but F. graminearum is the only prevalent species that has the ability to produce a mycotoxin, known as vomitoxin or deoxinyvalenol (DON). Even low levels of contamination can downgrade the seed to become unacceptable for either human or animal consumption.
The perception that direct seeding creates the perfect conditions for Fusarium manifestation is still quite prevalent. Some recommendations include tillage as a means of controlling Fusarium. These recommendations are based on long-standing beliefs and not science. There are many unknowns about Fusarium control and infection, however, direct seeding does not increase the severity of FHB in tolerant varieties. Results of the last three years of research in the south east area of SK where FHB is well established indicate that environmental conditions, susceptibility of the crop, and tillage practices were the most important factors influencing the development of FHB
In 1999, Dr. M.R. Fernandez and Dr. R.P. Zentner initiated a 5-year study in eastern Saskatchewan to identify agronomic factors that may be associated with FHB-infected fields. The objectives of this project are to identify risk production factors that might lead to the development of FHB and to develop recommendations to producers for managing this disease. About 200 cereal fields have been sampled annually for the presence of FHB and species identification. A database has been developed with the results obtained from these analyses and from detailed information on agronomic practices provided by the cooperating producers.
After three years of the study, resistant varieties grown with zero tillage did not have FHB severity any greater than conventionally tilled fields and FHB severity was less than minimum-till fields. Possible reasons why minimum-till, but not zero-till, resulted in an increase in FHB levels in relation to conventional tillage are presently being investigated.
In Manitoba, where disease pressure is higher, zero tillage did not increase FHB severity over conventionally tilled soil. Studies on FHB conducted at the Brandon Research Centre by Dr. Debra McLaren and Dr. Byron Irvine assessed the impact of tillage and rotation on Fusarium head blight of wheat. Preliminary results indicate that there were generally no differences in levels of FHB between conventional and zero tillage regimes under Manitoba conditions.
Last summer's drought created conditions that would both help control the spread and facilitate the spread of Fusarium. For an infection to occur three factors have to be in place. 1. Disease inoculum has to be present in the susceptible flowering or heading stage. 2. Warm, moist conditions are required to create an infection. 3. A susceptible host has to be present. Eliminating one of these factors will eliminate the risk of Fusarium Head Blight.
The relatively drier, cooler climates in Saskatchewan compared to Manitoba during flowering has prevented Fusarium from becoming a major management concern. F. graminearum is found mainly in the Black Soil Zone, thus indicating the strong correlation between precipitation and infection. The disease creates the greatest infection when rainfall occurs in July, corresponding to the flowering season. Even with low inoculum levels, enough rainfall will create high levels of infection. With limited rainfall, temperature and inoculum levels will increasingly determine the level of infection. Warmer temperatures with humidity or precipitation will increase the severity of an infection.
A fortunate side effect of drought is an elimination of the environmental conditions necessary for infection and a reduction in the inoculum present. The range and severity of the Fusarium infected area decreased in 2002 over previous years. A Fusarium survey conducted by extension agrologists and the Crop Protection Lab of Saskatchewan Agriculture and Food found F. graminareum in fewer locations and in lower concentrations than previous years. Infected samples were primarily from the south east and east central areas. It should be noted that samples were taken at the soft dough stage. Wet, humid conditions prior to harvest but after sampling provided conditions favourable for greater levels of infection.
In Saskatchewan, the South East has had the most severe Fusarium problems. This area has the highest average temperatures in the province and high average precipitation for the month of July. East Central and North East Saskatchewan are also high rainfall areas but experience cooler temperatures.
The effect of global warming will play a significant role on Fusarium severity and the area affected by F. graminearum. Increasing temperatures will favour an increase in the severity of infections and increase the areas affected. On the other hand, rainfall is projected to increase in the winter and decrease in the summer. Decreased July rainfall will favour a decrease in the severity of FHB. However, predictions of increased severity of weather patterns and greater fluctuations in precipitation will create conditions in some years that favour severe disease outbreaks. Increasing temperatures and July rainstorms will also facilitate the spread of F. graminearum westward.
One way to control damages from F. graminearum is to seed unaffected crops or crops tolerant to Fusarium. Ultimately, breeding is seen as the best solution for Fusarium head blight control in susceptible crops. Prairie adapted lines are currently being bred with a gene to provide low levels of Fusarium infestation. Lines have been developed that are providing good levels of Fusarium resistance, however there have been other problems associated with the varieties. Quality problems and other agronomic problems are preventing the lines from being registered. While genetics can provide a valuable tool in production, good agronomics will always be necessary.
There are current varieties that provide more Fusarium resistance than others. Hard red spring wheat is generally more tolerant than Canadian Prairie Spring, extra strong or durum wheat varieties. Two-row barley is generally less susceptible than 6-row barley varieties. Within each crop category are differences in variety susceptibilities. Consult seed guides and seed companies for varieties suitable for your region.
Often the best way to deal with a problem is to avoid it. While growing cereal crops in Saskatchewan is necessary, not all cereal crops are equally susceptible. By avoiding spring wheat and barley, you are eliminating most of the problem. However, wheat is an important crop for rotations. Eliminating wheat as an option will limit your ability to develop a sustainable rotation. Fortunately FHB can usually be avoided by growing winter wheat instead of spring wheat crops. Winter wheat develops and matures earlier than all spring wheats. It is not fully understood why winter wheat is less affected by Fusarium, however, the cooler temperatures during flowering appear to limit the infection.
If conditions are favourable for infection, your only option may be a fungicide control. However, since it is rare that environmental conditions are favourable for infection for extended periods you can limit the damage to susceptible crops by alternating seeding dates. More specifically, by alternating seeding dates of similar crops you will vary the flowering period. This risk management technique may not guarantee you will miss out on all infection, but it will decrease the risk of losses to all fields.
Penny Pearse, Provincial Plant Disease Specialist, SAFRR
Dr. Myriam Fernandez, Plant Pathologist, SemiArid Prairie Agricultural Research Centre, AAFC
Dr. Debra McLaren, Crop Production Pathologist, Brandon Research Centre, AAFC
Elaine Wheaton, Senior Research Scientist, Saskatchewan Research Council, Saskatoon, SK