inoculum increase of rhizoctonia solani ag-2-2 by ...

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Inoculum Increase of Rhizoctonia Solani

INOCULUM INCREASE OF RHIZOCTONIA SOLANI AG-2-2 BY SCLEROTIA FORMATION ON SUGARBEET ROOTS Cheryl A. Engelkes and Carol E. Windels Former Graduate Student and Plant Pathologist, respectively Northwest Experiment Station University of Minnesota, Crookston Severity of Rhizoctonia root and crown rot of sugarbeet varies among seasons and fields in Minnesota and North Dakota. Field experiments conducted at the Northwest Experiment Station (NWES) in 1988-1991 have shown that some of this variation can be attributed to 1) sugarbeet variety and 2) pathogenicity of the AG-2-2 culture of Rhizoctonia solani (=R. solani). Less disease developed on the breeding line FC 712 and the specialty cultivar ACH 184 (both tolerant to Rhizoctonia root and crown rot) than on Maribo Ultramono when sugarbeet roots were inoculated with cultures of R. solani AG-2-2 isolated from diseased sugarbeet roots. More disease developed on sugarbeet, and severity was similar for all three varieties, when roots were inoculated with more aggressive cultures of AG-2-2 isolated from diseased pinto bean and soybean plants. R. solani AG-2-2 has a wide host range. Rotation of sugarbeet with crops not susceptible to this fungus allows the AG-2-2 population to decrease. Close rotation of sugarbeet with bean crops (fababean, navy bean, pinto bean and soybean) results in buildup of the pathogen because all of these crops are susceptible to R. solani AG2-2. R. solani usually survives and overwinters in plant debris in soil; sometimes sclerotia (compact aggregates of fungal biomass) form on surfaces of diseased roots. Sclerotia are dark-brown in color and approximately the size of a pinhead. In 1988, while evaluating Rhizoctonia root rot severity of field inoculated sugarbeet plants at the NWES, sclerotia were observed on roots inoculated with cultures of AG-2-2 originally isolated from different crops.

OBJECTIVES: Production of sclerotia on sugarbeet roots was assessed in field experiments from 1988-1991 to 1) determine effect of sugarbeet varieties differing in tolerance to Rhizoctonia root rot, 2) evaluate cultures of AG-2-2 originally isolated from diseased bean crops and sugarbeet roots, and 3) monitor when sclerotia form on sugarbeet roots following inoculation with AG-2-2.

PROCEDURES: Sugarbeet variety and source of R. solani AG-2-2 - Sugarbeet varieties Maribo Ultramono, ACH 184, and FC 712 (Dr. Earl G. Ruppel, USDA-ARS, Fort Collins, CO) were each inoculated with 17 cultures of R. solani AG-2-2 originally isolated from diseased plants (3 from fababean, 4 from pinto bean, 2 from soybean, and 8 from sugarbeet) and compared to uninoculated control roots. Eight roots were inoculated with each AG-2-2 culture (four replications in each of two field trials). Sugarbeet roots were hand-harvested and evaluated at 9 wk after inoculation in 1988 and at 4 wk after inoculation in 1989. Roots were evaluated for presence of sclerotia. Number of sclerotia was rated as 0 (no sclerotia on root), less http://www.sbreb.org/research/plant/plant91/91p196.htm

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than 100 sclerotia per root, or more than 100 sclerotia per root. Distribution of sclerotia on each root also was rated: present only on healthy root surfaces, present on rotted root surfaces, or present on both healthy and rotted root surfaces. Appearance of sclerotia was evaluated: sclerotia clumped, sclerotia scattered, or sclerotia both clumped and scattered. Clumped sclerotia consisted of individual sclerotia produced close together and ranged in size from larger than 1-mm to less than 5-mm in diameter(1 inch=25 mm). Scattered sclerotia were less than 1-mm in diameter. Data presented on the number of sclerotia on roots is based on the number of sugarbeet roots with sclerotia out of the total number of roots examined. Data on distribution and appearance of sclerotia on sugarbeet roots are presented only for sugarbeet roots with sclerotia. Also, percent rotted surface area of each root was rated on a 0-7 scale (0=healthy, 7=100% of the root surface rotted). Timing of sclerotia formation - Sugarbeet roots of Maribo Ultramono and ACH 184 were inoculated with a culture of R. solani AG-2-2 isolated from pinto bean and one from sugarbeet. Root rot severity was compared to uninoculated control roots. Twelve-week-old roots were inoculated in 1988 and 8-wk-old roots were inoculated in 1989. Sixty sugarbeet roots from two replications were harvested each week and rated for number of sclerotia per root (0, less than 100, more than 100) until 9 wk after inoculation for each year. In 1989, distribution and appearance of sclerotia on each root also was evaluated, as described above. In 1990-1991, sugarbeet roots of Maribo Ultramono and ACH 184 were inoculated at four plant ages with two cultures of AG-2-2 and compared to an uninoculated control. Two inoculation patterns were followed. For the consecutive inoculation, seeds were planted on 14 May 1990 and 9 May 1991 and roots were inoculated at 2wk intervals when they were 6-, 8-, 10- and 12-wk-old. For the simultaneous inoculation, seeds were planted at 2-wk intervals and roots were inoculated on 30 July 1990 and 1 August 1991 when they were 6-, 8-, 10- and 12-wk-old. A total of 180 sugarbeet roots of each of the four ages from three replications were rated in both the consecutive and the simultaneous inoculations for number, distribution, and appearance of sclerotia, as previously described.

RESULTS: General observations on sclerotia formation - Sclerotia formed on roots of field-grown sugarbeet plants that were inoculated with AG-2-2 cultures of R. solani in 1988-1990. In 1991, no sclerotia formed on sugarbeet roots. The 1991 trial was planted in a different location (soil type=Glyndon very fine sandy loam) than in 19881990 trials (soil type=Bearden silty clay loam). Sugarbeet variety - Since uninoculated control roots rarely were infected by naturally occurring populations of R. solani, data are presented only for roots inoculated with R. solani. When results are combined across cultures of AG-2-2, Maribo Ultramono had the highest percent of roots with sclerotia and root rot was most severe (Table 1). As percentages of sugarbeet roots with sclerotia decreased, there was a decrease in root rot ratings. ACH 184 had an intermediate number of roots with sclerotia and intermediate root rot ratings. On FC712, sclerotia formed on the lowest percentage of roots and the variety was the most root rot tolerant. Source of R. solani AG-2-2 - Among the three bean sources of R. solani (cultures from pinto bean, soybean, fababean), sclerotia formed on similar percentages of sugarbeet roots (Table 2). However, cultures from pinto bean and soybean were more pathogenic than the cultures from fababean. Cultures of AG-2-2 originally isolated from sugarbeet produced the highest percentage of roots with sclerotia compared to AG-2-2 cultures from bean crops and also caused less root rot (similar to cultures from fababean). http://www.sbreb.org/research/plant/plant91/91p196.htm

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Sclerotia of R. solani AG-2-2 formed predominantly on rotted root surfaces (Fig.1A). Sclerotia occurred more frequently on healthy surfaces of sugarbeet roots when inoculated with the less pathogenic cultures from fababean and sugarbeet compared to the more pathogenic cultures from pinto bean and soybean. Appearance of sclerotia on inoculated sugarbeet roots was affected by AG-2-2 culture (Fig. 1B). All cultures of AG-2-2 primarily formed scattered sclerotia (less than 1-mm diameter) on root surfaces. Occasionally, individual sclerotia formed close together (clumped) on sugarbeet roots. These clumped sclerotia (greater than 1-mm or less than 5-mm diameter) readily separated into smaller aggregates when scraped off roots. Clumped sclerotia occurred less frequently on sugarbeet roots inoculated with pathogenic cultures from pinto bean and soybean compared to the less pathogenic cultures from fababean and sugarbeet. Interactions between sugarbeet variety and source of R. solani AG-2-2 - There was an interaction between sugarbeet variety and source of AG-2-2 culture that affected formation of sclerotia on sugarbeet roots (Fig. 2). When the three varieties were inoculated with cultures from fababean, sclerotia formed on similar percentages of roots of each variety. However, when the three varieties were inoculated with cultures of AG-2-2 from pinto bean, soybean, and sugarbeet, there was a decrease in number of sclerotia that formed as tolerance of the sugarbeet variety increased. This effect was most pronounced with cultures from pinto bean, which caused the highest root rot ratings (Table 2) compared with cultures from the other sources. Greater percentages of sugarbeet roots inoculated with cultures of R. solani AG-2-2 had less than 100 sclerotia per root (Fig. 3A) compared to more than 100 sclerotia per root (Fig. 3B) from 1 to 9 wk after inoculation. Sclerotia formed on roots as early as 1 wk after inoculation with the AG-2-2 culture from sugarbeet, while sclerotia did not form on roots until 3 to 4 wk after inoculation with the culture from pinto bean (Fig. 3A).At 2 wk after inoculation, the AG-2-2 culture from sugarbeet formed more than100 sclerotia per root on Maribo Ultramono (root rot susceptible) and ACH 184 (root rot tolerant) (Fig. 3B). By 7 and 9 wk after inoculation, the AG-2-2 culture from pinto bean formed more than 100 sclerotia per root on both varieties (Fig. 3B). Sclerotia usually were scattered on rotted root surfaces (data not presented). When beets were planted 14 May 1990 and inoculated at 2-wk intervals when 6-,8-, 10- and 12-wk-old (consecutive inoculation), no sclerotia formed on roots inoculated when 6-wk-old (Fig. 4A). Plants that were 8wk-old when inoculated had the highest percentage of roots with sclerotia. Sclerotia formed on an intermediate percentage of sugarbeet roots that were 10- and 12-wk-old when inoculated. Sclerotia usually were distributed on rotted surfaces of diseased sugarbeet roots. Most sclerotia were clumped (greater than 1-mm or less than 5mm diameter) or were in clumped and scattered aggregates (less than 1-mm diameter). Data are not presented on these sclerotial characteristics because of the low number of sclerotia present on sugarbeet roots. For beets planted at 2-wk intervals and inoculated on 30 July 1990 when plants were 6-, 8-, 10- and 12-wkold (simultaneous inoculation), more sclerotia formed by 8 wk after inoculation (Fig. 4B) than when roots were inoculated at 2-wk intervals in the consecutive inoculation (Fig. 4A) Inoculation of 8-wk-old plants resulted in the highest total percentage of roots with sclerotia by 8-wk after inoculation. Sclerotia formed on an intermediate percentage of roots that were 10- and 12-wk-old when inoculated. Fewest sclerotia formed on roots that ere 6wk-old at inoculation. Sclerotia first formed at 4 wk after inoculation of 10-wk-old roots inoculated with either culture of AG-2-2 (data not shown). Both clumped and scattered sclerotia usually were distributed on rotted root surfaces (data not presented).

DISCUSSION: http://www.sbreb.org/research/plant/plant91/91p196.htm

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These studies demonstrate that R. solani AG-2-2 formed sclerotia on sugarbeet roots as soon as 1 wk following inoculation in 1988-1990. Age of sugarbeet roots at time of inoculation affected sclerotia formation, with sclerotia forming more readily on roots that were 8-wk-old when inoculated compared to 6-, 10 and 12-wk-old roots. Roots that were 6-wk-old when inoculated frequently rotted very quickly, so there probably was inadequate time for sclerotia to form. Sugarbeet variety affected the number and timing of sclerotia formation. Varieties with tolerance to Rhizoctonia root rot, such as ACH 184 and FC 712, resulted in the formation of less inoculum as sclerotia and/or as rotted sugarbeet debris. Root rot tolerant varieties not only reduce root rot, also may have root surfaces that are less favorable for sclerotia formation compared to root rot susceptible varieties. Source of AG-2-2 culture affected the number and timing of sclerotia formation. Cultures originally isolated from sugarbeet and fababean caused less root rot than cultures from pinto bean and soybean. However, cultures from sugarbeet produced more sclerotia sooner after inoculation than did cultures from bean crops. Slow development of root rot may allow more time for sclerotia to form on roots. In situations where sclerotia do not form and/or roots rot quickly, R. solani would be expected to survive within infected roots. The importance of sclerotia in long-term survival of R. solani in soil is uncertain. In 1990, we reported that sclerotia of R. solani AG-2-2 did not survive for 2 years when removed from sugarbeet roots and buried in soil. If sclerotia had remained associated with beet roots, rather than being removed, they may have survived better. Removal of sclerotia from roots also broke up clumps of sclerotia. Larger clumped sclerotia may have more stored energy reserves for longer survival and have less surface area exposed to competing microorganisms in soil compared to small, scattered sclerotia. Based on observations of Rhizoctonia root rot and crown rot throughout the Red River Valley and in southern Minnesota, sclerotia are not consistently formed on root surfaces. Also, sclerotia are not easy to see because of their small size, dark color, and typical scattered distribution over sugarbeet root surfaces. It would be easy to overlook AG-2-2 sclerotia that form on healthy root tissues,such as occurred with the less pathogenic cultures from sugarbeet and fababean. It is unknown why no sclerotia formed on inoculated sugarbeet roots in 1991. The 1991 trial was conducted in a different location (soil type=Glyndon very fine sandy loam) than in 1988-1990 (soil type=Bearden silty clay loam). Greenhouse experiments are underway to determine if sclerotia formation is inhibited in the Glyndon soil type.

CONCLUSIONS: 1. Sclerotia formed on sugarbeet roots infected by AG-2-2 cultures of R.solani. 2. Sclerotia formed on a higher percentage of sugarbeet roots of Maribo Ultramono than on ACH 184 and FC 712. 3. Cultures of AG-2-2 originally isolated from sugarbeet produced sclerotia more abundantly and sooner after inoculation of sugarbeet roots than did AG-2-2 cultures from bean crops.

ACKNOWLEDGEMENTS: The Sugarbeet Research and Education Board of Minnesota and North Dakota; Northwest Experiment Station; and College of Agriculture, University of Minnesota; are gratefully acknowledged for funding http://www.sbreb.org/research/plant/plant91/91p196.htm

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provided in support of this research. 1991 Sugarbeet Research and Extension Reports. Volume 22, pages 196-203. Questions/Comments

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