Development of Varieties with Enhanced Resistance to Common Scab Preliminary Results (2000-2005) Félix Navarro1, Bryan Bowen2 Jiming Jiang3, Horia Groza4 and Jiwan Palta3 UW Potato Breeding Program, Department of Horticulture INTRODUCTION Common Scab of Potato, a disease caused by the bacterium Streptomyces scabies continues to be a serious problem for growers in the north central and other potato producing regions of the United States. This disease adversely affects the marketable yield of potato crops, either through occasional reduction of gross yield, or more importantly, reducing the skin quality. The incidence and severity of scab varies with location and season. Cropping history, soil moisture, and soil texture are largely responsible for this variability (Haynes et al. 1997). Lowering the soil pH under 5.2 has been proposed to reduce scab problem, but this practice will greatly reduce the choices of crops that can be used in rotation with potatoes. Furthermore, other species, Streptomyces acidiscabies produces scab diseases at pH lower than 5.2 (Bonde and McIntyre, 1986; Lambert and Loria, 1989). Another strategy to control common scab is maintaining the soil near field capacity during the 2-6 weeks following tuber initiation. However, in sandy soils like in the Central Sands of Wisconsin, this is difficult and may increase other disease problems (Pérombelon, 2002). Crop rotation does not provide an effective control to this disease because the bacterium S. scabies can survive for years in the absence of potatoes (Vrugging, 1976). Moreover, crop rotation makes consolidation of production fields difficult. Chemical control also has not provided reliable management tool. Resistant varieties are the best option to manage common scab when available. Only a limited number of cultivars such as Russet Nugget and Russet Burbank of russet type and the chipping cultivars Superior and Pike have shown acceptable resistance to this disease (Rieman, 1962; Holm et al., 1992; Loria et al. 1997; Jansky and Rouse, 2003). However, the standard chipping cultivars such as Atlantic as well as most recently released varieties (e.g. Snowden) are susceptible. Resistance to common scab has been detected in wild species. Results from scab evaluations by the Potato Germplasm Introduction Station indicate that out of 95 accessions that showed resistance to common scab, 48 corresponded to S. phureja subsp. phureja, 25 to S. tuberosum subsp. andigena, and 13 to S. chacoense subsp. chacoense. Breeding programs have included these species in the background of numerous breeding lines and some resistant varieties, e.g. Russet Nugget have resulted. Experimental strategies are needed to overcome the challenges posed by environment by location and year interactions in the evaluation of scab 1 Assistant Scientist- 4181 Camp Bryan Afon, Rhinelander, WI 54501. E-mail: [email protected]. 2 Researcher- 4181 Camp Bryan Afon, Rhinelander, WI 54501. E-mail:[email protected]. 3 Professor, Department of Horticulture, University of Wisconsin, 1575 Linden Drive, Madison WI 53706, Email: [email protected]. 4 Former University of Wisconsin Assistant Scientist. E-mail: [email protected]. resistance in breeding programs (Haynes et al. 1997; Bramwell et al. 1998; Bouchek- Mechiche et al., 2000; Jansky and Rouse, 2003). Starting in 2006 we have began anew effort in breeding for scab resistance. The objective of our study is to estimate the level of resistance in Wisconsin potato breeding lines and to use this information as a selection criterion to develop elite lines with future variety potential. We report here scab evaluation results from 2002 and 2005 seasons of investigation. In these two years it was possible to separate resistant and susceptible lines. MATERIALS AND METHODS The experiments were carried out during years 2000, 2002, 2004 and 2005 in a field with high scab potential at the UW-Rhinelander Agricultural Research Station. A randomized complete block design including 3 replications was used; plot size consisted of 8 hills 2000-2004, 4hills in 2005. Each year, 45 to 92 different lines as well as resistant and susceptible checks were evaluated. This included russets, reds and chipping lines. Line performance was evaluated by the incidence (% of tubers affected) and severity (% of lesion coverage) of scab. A combined, equally weighted scab index was developed from the incidence and severity values, where 1=no scab and 9 = worst scab (60-100 of tubers in a plot covered with pitted scab. Statistical Analyses The statistical model to study the variability among the lines was Yijk = µ + ri + varj(typek) + eijk. Scab index data were normalized using Box-Cox transformations. Best linear unbiased predictors (BLUP) for the line performance and their corresponding variances were estimated using REML in SAS. Data were transformed back to 1-9 scab index for reporting. RESULTS In years 2000 and 2002, scab pressure did not allow separation of resistant and susceptible lines and cultivars. In years 2003 and 2005 significant differences for scab performance separated resistant lines and cultivars from the susceptible ones. The best scab resistance in the russet types was observed for Freedom Russet (W1836-3), W1879-1rus, W3132-1rus and W5785-5rus with equal or better scab resistance than Russet Burbank. MegaChip (W1201) was one of the chipping varieties with outstanding scab resistance. Our results for MegaChip agree with results from the 2004 New Brunswick Chip variety trial, 2005 Michigan and Idaho (US Potato Board 2005-Snack) and 2005 National Scab Trial. Enhanced resistance to scab, in addition to high specific gravity and chip quality are important attributes of MegaChip. Of the red varieties tested, W2275-7, a sister line to Villeta Rose (W2275-3R) was the only one with better performance than Dark Red Norland in 2003. PROSPECTS FOR THE FUTURE To be more efficient we need to develop a bigger number of lines that have scab resistance and good performance for yield and quality. Our plan to achieve that goal include the use of additional locations with high scab potential and the evaluation of a bigger number of progenies at earlier stages in the breeding scheme. Implementation of these should increase our chances of detecting more scab resistant lines that may be considered for release through SpudPro in the future. Table 1. Scab performance of breeding lines, Rhinelander Agricultural Research Station, 2003. Breeding Scab P- Line 1-9 Value Comments from Yield Trials from WI and Out of State ROUND WHITE W2955-4 2.8 0.0006 Nice, midsize tubers, knobs, yellow flesh, variable size, gravity 1.074 W3002-2 2.8 0.0006 Nice, midsize, growth cracks, specific gravity 1.079, deep bud end W2492-3 4.2 0.0236 good size round, rough, growth cracks, specific gravity 1.082 W2033-8 4.4 0.0452 Excellent yield, color, nice smooth mid size tubers, specific gravity 1.091 W1201 4.6 0.0618 MegaChip, early bulk, good chip color from 45ºF, specific gravity 1.094 ND5822-C7 4.8 0.1005 Good size, ON, rough, med deep eyes, WI:NCRT03 MSH095-4 4.9 0.1261 Medium specific gravity, yield similar to Atlantic, Chip from 45ºF SFA03 W2143-1 4.9 0.1207 Flat, netting and smooth, specific gravity 1.086, some vascular discoloration W2889-1 5.1 0.1939 Pink eye, yellow skin, light netting, specific gravity 1.070 MSE221-1 5.3 0.2735 Nice, uniform: NCRT03, WI, flat oval, knobby 02 W2203-2 5.6 0.4705 Oval, smooth tubers that may become green, high specific gravity 1.092 V0056-1 5.7 0.5478 Some pink rot (ON), small tubers (WI): NCRT03 B0766-3 5.9 0.6788 Very nice: NCRT03, WI, NCRT01 W2768-3 5.9 0.6925 Flat, smooth, netting visible, specific gravity 1.084 W2982-1 6.0 0.7909 Oval shape, smooth, specific gravity 1.076 W2310-3 6.1 0.8993 Flat, too oval, Specific gravity 1.084 W2768-9 6.1 0.8404 Rough, flat, oval, specific gravity 1.076 W2114-5 6.2 0.9098 Low yield, medium spec. gravity, shatter-bruise W2615-2 6.3 0.9718 White color, specific gravity 1.078 W2962-2 6.3 0.9098 Flat, light netting, large tubers, specific gravity 1.082, vascular discoloration W2310-11 6.4 0.8721 Oval, small, like a prism, specific gravity 1.075 Norvalley 6.5 0.7974 Smooth tubers, variable size and shape:NCRT03 V0379-2 6.5 0.7772 Good size, uniform, not smooth: NCRT03, ON,WI W1443 6.6 0.6527 Atlantic type, early bulking, good yield & chips, net. NCRT04 W1773-7 6.6 0.6951 Fresh harvest chipping, good size, yield and quality, sg=1.087. SFA04 W5784-3 6.6 0.6736 Yellow skin, yellow flesh, smooth, light netting, sg 1.082 W2265-25 6.7 0.5833 Deep bud end, high specific gravity 1.090, some hollow hearts W3011-1 6.7 0.5741 Greens, small size tubers, specific gravity 1.089 Snowden 6.9 0.4032 Standard variety, deep eyes, small-mid size tubers: NCRT02, s. gravity 1.088 Note: P-values < 0.05 indicate that scab performance is statistically different to the overall mean. Scab scale 1=best, 9=worst. Scab performance of boldfaced lines was statistically better than the rest. NCRT= North Central Regional Trial. SFA=Snack Food Association/US Potato Board Chip Variety Trial. Table 1 (Cont’d). Scab performance of breeding lines, Rhinelander Agricultural Research Station, 2003. Scab P- Breeding Line 1-9 Value Comments from Yield Trials from WI and Out of State ROUND WHITE (Cont’d) W2978-2 6.9 0.4446 Round, nice looking, deep stolon insertion, specific gravity 1.076 W2978-3 6.9 0.4504 Smooth, white, specific gravity 1.075 W2978-3 6.9 0.4504 Smooth, white, specific gravity 1.075 ND2470-27 7.0 0.3510 Very nice, good size, round: NCRT03, ON, WI.