University of Florida, Quincy, FL, USA

Background and objectives
Stem rot of groundnut caused by Sclerotium rolfsii causes major losses of yield in the south-eastern portion of the peanut belt in the USA [1]. Because of the non-random distribution of natural sclerotial inoculum in soils, measuring the actual pod losses to stem rot in a given cultivar is difficult. Potential pod yield losses to stem rot in commercial cultivars had not previously been fully quantified and it was therefore difficult to compare the benefit of using plant resistance to prevent these losses. The research reported here quantitates the yield loss to stem rot in eight runner and six Virginia type cultivars. The benefits of resistance in preventing yield losses are also shown for seven partially resistant breeding lines compared to five of the commercial cultivars.

Material and methods
Techniques were developed to inoculate two plot rows adjacent to two uninoculated rows to allow a paired plot analysis of pod losses [2]. Inoculum was grown on sterile oat seed in the laboratory using an isolate of the pathogen that was pretested for virulence (SR8). Inoculum (160 ml/row by volume) was applied to two rows of each 4-row plot and the two adjacent rows were uninoculated. Rows were 0.91 m apart by 6.1 m long. The two uninoculated rows were treated twice with a soil fungicide (thifluzamide or fluazinam) to minimize the potential for contamination by the pathogen. Three replications of each genotype were used in a randomized complete block design. Stem rot incidence and severity were evaluated when plots were inverted using a commercial digger and pod yields were determined. Disease incidence was assessed using the number of disease loci per 12.2 m and severity was assessed using a 1-5 scale in which 1=healthy plants and 5=plants dead or dying with most of the pods lost to stem rot. Fourteen commercial cultivars were tested for yield loss to stem rot at one location over 3 years. Yield loss of seven breeding lines with varying levels of resistance were compared to five commercial cultivars over 2 years at 2 locations.

Results and conclusions
Yield loss of the commercial cultivars varied from a low of 34% for NCV11 to a high of 65% for GK7. The mean yield loss for the commercial cultivars in 3 years of testing was 54%. The resistant breeding lines varied in yield loss from 23 to 39%. The breeding line with the lowest yield losses in these tests was UF 81206-2. When commercial cultivars were compared to the resistant breeding lines, yield losses ranged from a low of 39% for Southern Runner to 65% for Florunner. Mean yield loss of the resistant breeding lines in these comparative tests was 18% less than for the commercial cultivars. The correlation between stem rot severity and pod yield loss for the cultivar/breeding line tests was high and significant (r= -0.74, P<0.001). The methods used in these tests allowed us to quantitate pod yield losses to stem rot and get a fairly accurate evaluation of the value of the resistance in the breeding lines for preventing pod yield losses to stem rot. These methods are now being used at the final level of screening for stem rot resistance in our elite breeding tests.

References 1. Melouk HA, Backman PB, 1995. Peanut Health Management, pp. 75-82.
2. Shokes FM, Rozalski K, Gorbet DW, Brenneman TB, Berger DA, 1996. Peanut Sci. 23, 124-128.