This article appears in the August/September 2018 issue of Sugar Producer.
In many sugarbeet-growing areas, root rots are the most important limiting factor for production. One of the most common is Rhizoctonia root and crown rot (RRCR) caused by Rhizoctonia solani. This disease not only causes yield losses in the field but can result in severe decay in the storage pile as well. R. solani is also responsible for post- and pre-emergent damping off in seedlings, which often occurs when planted into warm soils because the pathogen prefers temperatures of 70 to 95 degrees. Seedlings will form water-soaked, dark tissue just below the soil surface, which will eventually spread to the hypocotyl. Early planting will help prevent seedling death due to cooler soil temperatures during seedling development.
In older plants, the disease can manifest as a root or crown rot. Crown rot usually begins when Rhizoctonia-infested soil is thrown onto crowns by weather or agricultural practices. Root rots start at or below the soil line. Wilting of foliage is observed for both crown and root rot, followed by yellowing of the tissue and leaf death. The leaves will remain attached to the plant, and the base of the infected petioles can form dark brown to black lesions.
On roots, infected tissue, which has a well-defined margin between healthy and diseased tissue, is dark brown to black. This tissue necrosis includes the formation of sunken lesions, cracks and/or cankers in the root. In severe cases, roots can be rotted completely. It is recommended that growers dig up wilted beets and cut open the root to confirm RRCR.
There are several ways to manage this disease. Rotations are necessary to prevent buildup of inoculum in the soil. Sugarbeets should be grown in a minimum of three-year rotations—every four to five years where RRCR has been a problem. Small grains such as wheat and barley are not hosts of R. solani, and are therefore good rotational partners with sugarbeets. Beans, corn, potatoes and alfalfa are all alternative hosts for R. solani. Severe losses have been documented in sugarbeets following these crops; it is therefore best to avoid these in rotations. Resistant varieties are available but usually come with a reduced yield and quality. They may also lack resistance to diseases such as Rhizomania, curly top, Cercospora, or Fusarium yellows and therefore may not comply with local beet cooperative requirements.
Other agricultural practices have been shown to have a positive impact on RRCR management. These include limiting excessive hilling, which brings infested soil in contact with the crown and petioles. Reducing compaction and managing irrigation, which prevent soils from becoming too dry or waterlogged, help minimize disease in the field. Crop residues aid in Rhizoctonia survival and proliferation in the field, so it is important to ensure that straw and other residues are allowed to decompose before plowing. A high plant density allows for earlier row closure and increases shading, which lowers soil temperatures. As RRCR prefers warm temperatures, increasing plant populations lowers disease losses. Additionally, providing proper fertility and controlling weeds that act as alternate hosts help limit disease.
Chemical controls including seed treatments, in-furrow and band fungicide treatments have shown to be effective against RRCR. Newer seed treatment options using active ingredients such as metconazole, tolclofos-methyl, penthiopyrad, triticonzole, sedexane, and fluxopyroxad have shown good efficacy against the disease when compared with previous popular products. However, these usually only last 35 to 45 days at the most, so it is important to follow up with a banded treatment at the four- to eight-leaf stage. Azoxystrobin, applied either as a banded application or an in-furrow treatment, is most commonly used and has been experimentally shown to give the most consistent control.
In Montana and other states, RRCR has been increasing in severity in recent years mostly due to the use of more susceptible varieties and the dramatic increase in corn acreage. As this happens, it becomes essential to keep a record of which fields have a history of RRCR so that proper steps can be taken to manage the disease early in the season.
Frankie Crutcher is a plant pathologist and assistant professor at Montana State University’s Eastern Agricultural Research Center in Sidney, Mont.
Jessica Rupp is a Montana State University assistant professor of crop pathology, specializing in sugarbeets and seed potatoes.