By Carol Windels, Jason Brantner, Albert Sims & Carl Bradley*
The spreading of spent lime on sugarbeet fields around Minnesota and eastern North Dakota has increased significantly in recent years — with a primary motivation, in many instances, being to help manage Aphanomyces root rot.
For those growing planting sugarbeets in Aphanomyces-infested fields, management recommendations typically consist of planting early, using tolerant or partially resistant varieties treated with Tachigaren®, and employing certain cultural practices (e.g., cultivation and improved drainage) to avoid or reduce disease pressure. But when the inoculum densities are high and the soil is warm and wet, these measures may not be enough — with the result being poor-yielding or even abandoned fields.
That’s where spent lime from the region’s beet factories comes into play. The seven factories in the Red River Valley and southern Minnesota generate about 500,000 tons (dry weight) of spent lime annually. Some has been stockpiled for as long as 20 years. Along with increasing soil pH and supplying crop nutrients, spent lime also has been shown to reduce Aphanomyces on sugarbeets.
But how long does it take, following application, for spent lime to have an effect on Aphanomyces? And, for how long does that influence persist?
Those were the core questions addressed in field trials at two Red River Valley locations where several rates of spent lime had been applied eight years previously. Our goals were to (1) determine the lime’s long-term effects on Aphanomyces diseases and (2) measure the lime’s effect on sugarbeet yield and quality.
One of two trials was established in a commercial field near Hillsboro, N.D., in October 2003, and the other in a field near Breckenridge, Minn., in April 2004. At that time, the Hillsboro site had a history of moderate Aphanomyces root rot (a soil index value of 48, with 100 being highest possible level) while the Breckenridge field had severe Aphanomyces (soil index value of 98).
Spent lime treatments (each replicated four times) at Hillsboro were at rates of zero, five, 10, 20 and 30 tons per acre on a wet-weight basis. (That corresponds to dry weights of zero, 3.3, 6.5, 13 and 19.5 tons, respectively.) At Breckenridge, the rates were zero, five, 10, 15 and 20 wet-weight tons/acre (zero, 2.7, 5.3, 8.0 and 10.6 tons dry-weight basis).
To allow the lime treatments to stabilize in 2004, corn was sown across the four experiments at Hillsboro, while spring wheat was planted at Breckenridge. Sugarbeets have since been grown in one experiment each year from 2005 to 2011. Corn, wheat and soybeans (and, at Hillsboro, fallow) were rotated onto the other three experiments in the other years. Each experiment plot was one acre in size.
In 2011, two Roundup Ready® sugarbeet varieties were sown in one experiment at both locations. Those plots were last planted to sugarbeets four years earlier, in 2007. Standard fertility and production practices were followed to obtain maximum sucrose yield and quality. Here are the results from those two 2011 beet experiments:
• Hillsboro -- As of June 2011, more than seven and a half years after the lime was applied, soil pH was 7.2 in the non-limed plot, 7.7 in plots treated with five and 10 tons of (wet) lime, and 7.8 where treated with 20 and 30 tons of lime. Those values were very similar to the pH values determined in July of 2004, nine months after the spent lime had been applied. Aphanomyces soil index values were high (ranging from 75 to 100) and were statistically the same in the limed plots and the non-limed control.
• Breckenridge — In June 2011, just over seven years since the lime was applied, soil pH was 6.4 in the non-limed control plots. It was 1.3 points higher — 7.7 — where limed had been applied at 15 and 20 (wet) tons per acre. The pH values remained nearly identical to those measured just six months after the lime was added in 2004. Maximum Aphanomyces soil index values of 100 occurred across nearly all treatments at the Breckenridge site.
Table 2 shows the 2011 sugarbeet data from Breckenridge. Seedling stands at 35 days after planting were not significantly different among non-limed and any of the limed plots. But by harvest, the numbers of harvested roots were significantly higher for all rates of lime compared to the non-limed control. Low numbers of roots and severe Aphanomyces root rot in all treatments resulted in very poor yields, although yield, recoverable sucrose per acre and revenue per acre did increase along with liming rates. Overall, however, economic returns were very poor.
What can be concluded from the 2011 findings? There are three main points:
• In the eighth growing season after application of spent lime, there was a significant reduction in Aphanomyces root rot — and higher yields — at Hillsboro, where disease pressure was moderate.
• At Breckenridge, where disease pressure was intense and prolonged (several weeks), the addition of lime also reduced Aphanomyces root rot and increased beet yield compared to the non-limed control. However, even the best management practices resulted in insufficient disease control and non-economic yields.
• Soil pH levels initially increased with application of spent lime and have remained relatively stable across the several years since the lime was applied. Also, Aphanomyces soil index values are high at both locations, regardless of the rate of spent lime applied to the soil seven to eight years ago.
Under prolonged environmental conditions that are highly favorable for disease (especially in soils with high populations of Aphanomyces), even the best management practices — plant resistance, Tachigaren seed treatment, good soil drainage, early planting and application of factory lime — fail. Under such conditions, the pathogen predominates, overriding plant resistance and all other management strategies. In a more-typical season, however, the same management practices are effective, economic strategies for managing this disease.
* Carol Windels, Jason Brantner and Albert Sims are professor (plant pathology), research fellow (plant pathology) and associate professor (soil science), respectively, with the University of Minnesota Northwest Research & Outreach Center, Crookston. Carl Bradley, currently extension plant pathologist with the University of Illinois, formerly was with North Dakota State University.