Development of a reliable greenhouse screen for Rhizoctonia Crown and Root Rot to identify resistant germplasm and develop a non-destructive rating method.
For over 30 years, the USDA-ARS Crop Germplasm Committee has organized the screening of Beta germplasm for resistance to multiple critical diseases. The CGC effort screens between 30-50 Plant Introductions (PI’s) from the USDA National Plant Germplasm System each year for multiple diseases. Fort Collins has consistently conducted the Rhizoctonia Crown and Root Rot (RCRR) resistance screening for over 50 years. The evaluated germplasm results are summarized in the Germplasm Resource Information Network (GRIN) and reported in Plant Disease Management Reports (PDMRs). The Disease Index (DI) reported for each evaluated accession represents a mean of the replicated plot-level DI ratings, which consist of approximately 50 – 75 plants each per accession. In this study, we report a reliable greenhouse (GH) RCRR screening protocol that effectively triples our annual screening capacity. The GH protocol also allows for a nondestructive rating procedure to collect individual DNA and allow resistant plants to survive both the screening process and vernalization. This allows the plants to generate seed for further plant breeding experiments that are very difficult to accomplish with field evaluations alone. This GH protocol has been optimized with sugar beet and now also applied to a core collection of 64 Beta vulgaris ssp. maritima (Bvm) accessions. The Bvm RCRR screen consisted of 1330 plants (1 plant/6” pot) in one large GH bay with 3 large benches. Twenty plants for each of the 63 PI’s, (1 PI did not have sufficient germination) and 70 check plants (35 resistant & 35 susceptible) were planted on the same day. After germination, and thinning, all plants of all PI’s and checks were randomly positioned in 133 blocks of 10 plants across a large GH bay for the remainder of the experiment. The six Bvm PIs with the lowest mean DI and most harvestable individuals (scoring 0-3) are from 4 countries: Denmark (2 PIs), France (2 PI’s), Italy and Germany. PI 540677 and PI 540674 both from Denmark, and 1 from France (PI 504276) have 5 plants surviving from each population (n=20). These resistant plants have also survived induction. The top 6 accessions have a mean DI GH score of 20 plants that range from 4.4 to 5.6. The same 6 accessions have previously been evaluated in field trials and have DI scores recorded in GRIN ranging from 3.0 to 6.1. This suggests the GH screen is sufficiently uniform and of an appropriate disease intensity to separate resistant plants from susceptible plants. The GH screen shows uniform pressure over a diverse set of Bvm PIs. The reduced GH variability compared to field evaluations that vary year to year, is preferable to making accurate selections in the greenhouse. This new protocol for screening both sugar beet and crops wild relatives provide an additional tool to expedite RCRR resistance trait discovery via germplasm screening, genetic mapping, and marker assisted selection.