Rhizoctonia Root Rot

There's a delicate balance between having the moisture we need and contributing to the negative impacts of root rot

Published online: Jul 06, 2022 Feature Emily Corgatelli, Research Technician, SBQI
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(ED—We are reprinting this article with permission from the Amalgamated Sugar Company. Emily Corgatelli is the author and the photos are provided by Clarke Alder. www.amalgamatedsugar.com)

Rhizoctonia root and crown rot is neither a new nor unfamiliar disease to farmers, particularly in the sugarbeet world. Indeed, the first of many diseases attributed to the fungus Rhizoctonia solani in the U.S. was root rot of the sugarbeet in 1891.

Today, R. solani is known to have an enormous host range, is distributed throughout the world, and continues to be one of the most severe soil borne sugarbeet diseases. Rhizoctonia’s particular threat is its ability to cause damage throughout the season, from early seedling damping-off to later root rot that manifests as continually expanding dark lesions on the beet itself. When found at high levels it causes yield and sugar reductions as well as major issues in storage piles post-harvest. With favorable conditions of warm temperatures and moisture the disease may progress very quickly resulting in a range of economic losses. 

Historically, rhizoctonia caused damage in all areas where sugarbeets were produced, including in the Amalgamated Sugar growing region. There were no options available for chemical control or resistant varieties, so cultural control practices were the only viable option for managing disease. Typically, years that were most affected by rhizoctonia saw warmer temperatures and higher levels of rainfall. Because rhizoctonia has been an issue for so long, it is understood that some level of damage is to be expected every year, and to some extent it has been accepted as “normal.” 

In 2021 we experienced southern Idaho’s hottest summer on record. June was particularly hot with triple digit temperatures in southwest Idaho by just the 3rd. Not only was summer 2021 hot it was also the driest March through July Idaho had seen in around the last 100 years. Additionally, 2021 was also one of the worst years in memory for widespread and damaging rhizoctonia infestations throughout the Amalgamated Sugar growing regions. The chief reason for this can be fairly easily connected to irrigation practices that created consistent damp conditions combined with the exceptionally hot weather. Encouraged by scanty rainfall, it is natural and prudent to focus on ensuring crops aren’t deprived of moisture, however this seems to have unintentionally combined with high temperatures to create the perfect breeding ground for rhizoctonia. Thankfully, because we can readily determine the cause, addressing the situation in the future isn’t as daunting.  

The best and easiest way to practically minimize rhizoctonia considering the factors in the recent outbreak is to properly control irrigation.  Each irrigation type comes with its own issues that must be handled differently. Both sprinkler and pivot irrigation provide consistent and uniform coverage, however they require frequent application leading to consistently damp soil. This allows pathogen to continually develop. Surface irrigation doesn’t need to be done as frequently, avoiding the issue of consistent moisture, but if it isn’t moved routinely and evenly it could result in areas of the field ponding or becoming waterlogged. The wet soil may create anerobic conditions that provide an ideal environment for root rot. If a field has heavy or poorly drained soil, it is even more important to closely monitor irrigation as it is more prone to maintain damp conditions. Although heavy watering is often intentional, especially when concerns about limited water availability may seem of foremost importance, it unfortunately only creates further issues. The best general guideline would be to allow for the top ½” to 1” of soil to thoroughly dry before irrigating again. 

Besides water management, there are other important aspects to a rhizoctonia management plan including weed control, a 3–5 year crop rotation, and an early planting date, along with the use of fungicides and resistant varieties. Rhizoctonia’s massive host range encompasses several common weeds such as pigweed, lambsquarters, and kochia. This means that even with a crop rotation that excludes other host crops, without weed control, rhizoctonia will still survive in a field. An early planting date is especially useful when combating hot weather as it will allow for speedy row closure, shading the soil and reducing soil temperatures. The ability of fungicides or resistant varieties to achieve complete control is very limited, however this does not mean that they aren’t important tools that do have the ability to reduce rhizoctonia impacts under high disease pressure. Ultimately, what determines how important control measures of any kind are, is the pest’s impact on both the yield and the quality of the sugarbeets. 

Rhizoctonia’s impacts are certainly measurable and even significant with just moderate disease levels. Consider a 100-acre field with a yield of 39 T/A and 18% sugar, with a total payment of $219,585. If just 10% of the field was affected by rhizoctonia there would be a loss of approximately 1.5 T/A and 0.4% sugar. This would result in the total payment being reduced by $15,903. With a more significant amount of the field damaged by rhizoctonia the reduction only continues. If 11-25% of the field were damaged by rhizoctonia, 3 T/A and 0.75% sugar could be lost. This would result in a $30,315 reduction in payment. Based on these calculations rhizoctonia is certainly worth working to prevent, especially when many of the control measures are simple and are likely already common practices that may just need alteration. Improved irrigation management has the dual benefit of not only significantly reducing rhizoctonia damage, but also cutting irrigation costs through reduced water usage and less labor inputs. Beyond changes in irrigation practices, the most practical way to handle decisions about more expensive control measures like fungicide applications or resistant varieties, is to evaluate the expected disease pressure early and prior to the beginning of the growing season. If expected pressure is low perhaps resistance traits won’t be necessary and avoiding overwatering could be the key for satisfactory control. But if levels of Rhizoctonia are higher than it may be much more important to consider having one or multiple other tactics like seed treatments, good resistance traits, and carful cultural control practices. 

In one capacity or another, we will unfortunately always deal with rhizoctonia, so the best policy is to plan appropriately to take control measures before it is too late. It seems likely that the coming growing season is going to bring water shortages just as 2021 did, and potentially the same high temperatures. Given this, the best advice is to continue cultural control practices, consider using resistant varieties and fungicides, and most importantly to closely monitor irrigation.  We know that overwatering plays a dominant role in economically damaging cases of rhizoctonia, and luckily this is one factor that we do have the ability to control.