Carol E. Windels and Rita A. Kuznia
Associate Professor of Plant Pathology and Research Fellow, respectively
Northwest Experiment Station
University of Minnesota, Crookston
Rhizoctonia zeae (=R. zeae) is regularly isolated from unthrifty and diseased sugarbeet seedlings and discolored subcrown internodes of spring wheat in Minnesota and North Dakota. The fungus is related to Rhizoctonia solani, which causes damping-off and Rhizoctonia crown and root rot on sugarbeet. R. zeae is reported as pathogenic to several crops where it causes sclerotial rot and root rot of corn, brown patch of turfgrasses, sheath spot of millet and rice, and damping-off of forage legumes and pine seedlings. The fungus grows most rapidly and is most pathogenic at 90o F on these crops. Sugarbeet and wheat are listed as hosts of R. zeae, but it is uncertain if the fungus is a pathogen of these crops because fungi can infect roots and not cause disease symptoms.
The purpose of this research was to test cultures of Rhizoctonia zeae isolated from sugarbeet seedlings and spring wheat subcrown internodes for 1) optimum temperature for growth and 2) pathogenicity on both crops.
Optimum Temperature for Growth of Rhizoctonia zeae: Eleven cultures of R. zeae isolated from unthrifty and diseased sugarbeet seedlings collected in southern Minnesota and the Red River Valley and 12 cultures isolated from subcrown internodes (healthy as well as discolored) of spring wheat in plots at the Northwest Experiment Station, University of Minnesota were selected for testing. The 23 cultures were inoculated onto solid growth media in dishes 3.5 inches in diameter. Inoculated dishes were placed in incubators set at 68, 75, 82, and 90oF (four dishes/R. zeae culture/temperature). The following day (to allow R. zeae and growth media to equilibrate to incubator temperatures), a baseline was drawn on the bottom of each dish at the margin of fungus growth. R. zeae then was measured from the baseline to the margin of growth after 48 hr.
Pathogenicity of Rhizoctonia zeae on Sugarbeet and Spring Wheat:
Sterile soil was inoculated with R. zeae (11 cultures from sugarbeet and 12 from spring wheat) using an inoculum-soil layer technique. Four pots of soil were inoculated with each culture of R. zeae and 25 seeds of sugarbeet ('Maribo Ultramono') were planted per pot. Pots were placed in an incubator at 82oF and evaluated at 3 wk after planting for percent stand. Roots also were washed and examined for discoloration and root rot. The trial was repeated.
Inoculum of the same 23 cultures of R. zeae was increased on a mixture of sterile sand and cornmeal for 10 days and then mixed (5% weight) into a sterile greenhouse soil mix (field soil:peat:sand;3:1:1,v/v/v). Soil inoculated with each culture of R. zeae was placed in six pots (4.5-inch diameter), planted with 12 seeds of wheat ('Marshall') per pot, and placed in the greenhouse at 82oF. Plants were evaluated at 3 wk after planting for percent stand and fresh weight of tops (stems and foliage) and roots. The trial was repeated.
Optimum Temperature for Growth of Rhizoctonia zeae: Optimum and equal growth of R. zeae occurred at 82 and 90oF compared to lower temperatures (Figure 1). Cultures from both crop sources (sugarbeet and spring wheat) grew equally well at all temperatures tested.
Pathogenicity of Rhizoctonia zeae on Sugarbeet and Spring Wheat:
Sugarbeet: Results were similar in both trials, so results of only one trial are presented. Eight out of 12 cultures of R. zeae from wheat and 6 out of 11 from sugarbeet significantly (P=0.05) reduced sugarbeet stand as much as 33% compared to the uninoculated control at 3 wk after planting (Figure 2). No discoloration was observed, but R. zeae occasionally was isolated from symptomless roots grown in soil inoculated with the fungus.
Spring Wheat: Results were similar in both trials, so results of only one trial are presented. Four out of 12 cultures of R. zeae from wheat and 2 out of 11 from sugarbeet significantly (P=0.05) reduced wheat stands as much as 57% by 3 wk after planting compared to the uninoculated control (Figure 3A). Three out of 12 cultures of R. zeae from wheat and 3 out of 11 from sugarbeet significantly reduced top weights of wheat as much as 45% compared to the uninoculated control (Figure 3B). None of the cultures from wheat or sugarbeet significantly reduced root weights of wheat compared to the uninoculated control (Figure 3C). Discoloration of subcrown internodes was not observed.
Cultures of Rhizoctonia zeae ranged from being nonpathogenic to moderately pathogenic on sugarbeet and wheat, regardless of original crop source. Three cultures from spring wheat (#10,11,12) and two from sugarbeet (#10,11) were consistently pathogenic to both crops. The three pathogenic cultures (and the other nonpathogenic cultures) from spring wheat originally had been isolated from healthy and discolored subcrown internodes. All cultures of R. zeae grew optimally at 82-90oF, which corresponds to results of other scientists working with the fungus. Thus, all pathogenicity tests were conducted within this temperature range to favor maximum pathogen activity.
Whether R. zeae causes stand losses or root rots on sugarbeet and spring wheat in the field is uncertain. Soil temperatures most favorable for activity of R. zeae (82-90oF) do not occur when most of the spring wheat and sugarbeet crop is planted (mid-April to early May) in Minnesota and North Dakota. In the field, R. zeae may not infect roots until soil temperatures increase, or roots may become more resistant to R. zeae as they mature. Some cultures of R. zeae were not pathogenic and others were moderately pathogenic, which suggests that the fungus may not be significant in production of these crops. However, when crops are stressed by adverse environmental conditions, pests, misapplication of pesticides, etc. (and thus are more susceptible to opportunistic fungal pathogens), R. zeae could more easily infect roots and exacerbate damage. This may explain why R. zeae was isolated from plants that were unthrifty or weakened by other, more aggressive pathogens.
Rhizoctonia zeae is a relatively weak pathogen and may cause disease on sugarbeet and spring wheat growing under stress conditions.
We thank the Sugarbeet Research and Education Board of Minnesota and North Dakota for funding providing in support of this research.
1994 Sugarbeet Research and Extension Reports. Volume 25, pages 95-99.