Arvydas (Arv) Grybauskas, Extension Plant Pathologist, University of Maryland; arvydas@umd.edu
Fusarium head blight (FHB) or head scab is a serious disease of small grains that does not develop every season. As a consequence we tend to drop our guard or just operate as we always have done because it is not front and center in our minds. The last big one was now two seasons ago, and only a few growers experienced vomitoxin (DON) levels above 1 ppm last season. Vomitoxin is the toxin produced by the fungus that could result in rejection of loads at the elevator or mill. Diseases are always the result of a combination of factors. We have a general rotational sequence of field crops that puts us at risk of scab every season. A serious outbreak of scab is highly dependent on weather conditions just prior to flowering through grain development. Weather is obviously something we cannot control but that does not mean there aren’t things we can do to reduce the risk of scab and ensure a harvestable crop.
We have been conducting scab management research with funding from The Maryland Grain Producers Utilization Board, the US Wheat and Barley Scab Initiative and the University of Maryland Experiment Station, which illustrates the importance of integrating management practices. The data that follows demonstrates that:
1) No one tactic (ie. fungicide or resistant variety) is sufficient in a serious outbreak;
2) Fungicides, although vastly improved and still an important tactic, provide the smallest increment in reducing DON;
3) Distance to a source of inoculum (primarily infested corn stubble) has the largest effect on scab, followed by resistant varieties and fungicides.
4) Management of scab is best achieved by integrating practices that minimize in-field inoculum and take distance to a local source into consideration, selecting varieties with some resistance, and using fungicides when needed.
The management research was conducted using 6 wheat varieties representing the range of resistance to scab that was known and was commercially available. To keep things brief we will focus on DON levels to illustrate the effects of resistance and other management practices. Varieties were ranked on the basis of DON levels based on trials conducted over several season in inoculated nurseries and naturally infected State wheat trials. A percentile was then calculated so that a variety with a 0.9 percentile means it is in the 90th percentile or is in the top 10% of resistant varieties tested. A variety that had a percentile of 0.1 means it had high levels of DON on average and is in the bottom 10% of tested varieties. Wheat trials were planted with all six varieties at the Wye and Beltsville facilities of the University of Maryland Experiment Station. All sites were turbo-tilled and each location had a planting where the previous crop was corn and another planting where the previous crop was soybean. A major difference between locations is that wheat planted into soybean stubble at the Wye was surrounded by corn, whereas at Beltsville the nearest corn stubble was about 300 ft to the south for the soybean-wheat rotation. This difference greatly affected scab development and DON levels. At the Wye wheat following soybeans had DON levels only 10% lower than wheat following corn, whereas at Beltsville DON levels were 40% lower in wheat following soybeans (Table 1). This illustrates that the most important source of spores for scab development is within the field, and that nearby sources provide background levels of spores that can still infect a crop regardless of rotation. Nevertheless, by reducing the spore load with a rotation away from corn (or wheat) even in a very disease favorable year and a highly susceptible variety, DON levels are close to 1 ppm and could be mixed with clean seed to be marketed.
Variety selection is clearly a very important tool to reduce the potential for DON levels that could make or break your season. Even varieties in the middle percentiles have a huge advantage over highly susceptible ones. Yet no variety by itself under high disease potential consistently had DON levels below 1 ppm. Fungicides provided another increment of DON reduction averaging about 40%. Yet 40% of 2 ppm is still above the stringent FDA guideline of 1ppm for human consumption. Combining resistance to DON from a variety that is in the 75th percentile or above with the recommended fungicides can get us very close to 1 ppm or at least to levels that could be mixed with clean seed to market. Combining all three tactics, rotation with resistance and fungicides when needed is the best way to keep scab and DON from destroying your crop. Rotation and variety selection is a pre-season management decision. The time is now to make the best management choices, a table on varietal rankings based on DON levels is provided to help make the choices.
Table 1. Scab Management Research 2009 – Effect of rotation, cultivar and Prosaro fungicide on DON (ppm) in wheat. Dr. A. Grybauskas and E. Reed, Univ of Maryland.
Highest risk of scab following no-till corn. Wye 2009 soybean site within 100 ft of corn stubble, therefore risk was practically unchanged. Whereas at Beltsville, source of spores (corn stubble) further from soybean site and disease reduced dramatically.
Summary of Vomitoxin (DON) Levels Due to Scab Infection and Percentile Rankings of Cultivars in Wheat Trials in Maryland
1The proportion of cultivars that have higher levels of DON under the same conditions, eg. 0.9 = 90th percentile where 90% of the population had higher levels of DON, or entry was in the top 10% of the population. Percentiles calculated over all available trials.
Red Text top 30% based on DON assessed over all trials.
Data from Dr. Jose Costa, U of MD; includes inoculated and misted scab nurseries 2008 & 2009, and naturally infected State Wheat trials at Queenstown and Keedysville in 2009.
Summary prepared by Dr. Arv Grybauskas, U of MD (revised 5/19/2010).
Note: Resistance to the visual blighting, bleaching of spikelets and poor grain fill is not completely correlated to resistance to DON development. Thus some low DON cultivars still suffer yield loss due to scab. For example, Pioneer 26R15 has greater resistance to symptom development than Chesapeake but Chesapeake tends to have lower DON levels.