Some conclusions from five years of testing resistance to Phytophthora root rot in chestnut in South Carolina F. V. Hebard Research Farms Meadowview, VA Nov 12, 2010

Phytophthora root rot resistance screening data for parents and some backcross progeny. Parent Type Parent ID Disease Severity Rating, 0 is least James Index of Resistance* 0123 ChinesePrinceton, MA ChinesePrinceton, MA ChinesePrinceton, MA ChineseMissouri (Ken Hunt) Amer (op)Irradiated Amer (op)Pennsylvania F1 x ALeffel Gtld 171 x Amer (op) F1 x ALeffel Gtld 85 x Amer (op) F1 x ALeffel Gtld 90 x Amer (op) F1 x ALeffel Gtld 224 x Amer (op) F1 x ALeffel Gtld 404 x Amer (op) * Index =[ ( “ 0 ” + 1/2 ” 1 ” + 1/4 ” 2 ” )/n] * 100.

Basis for setting resistant/susceptible cutoff at 5 (or maybe 10!) for Phytophthora root rot resistance in backcross progeny

Utilization of resistant/susceptible cutoff of 5 for Phytophthora root rot resistance in backcross progeny Cross Backcross Parent Number of Plants James Index of Resistance Resistant JB216 x Lee Gragg Amer (op)ican JB216385no JB216 x KYHART1JB216662no Hyko x JB575JB yes Boston SB1 x JB575JB yes Ada1 x BX39BX39110no Andover x BX39BX39390no M22fr(CoYo44) x CH34CH34168yes Mo Fr--A(CoYo 95) x CH34CH34324no Powell 35A x IL858IL yes GAWAL x IL858IL858285no

Segregation of Phytophthora root rot resistance in backcross progeny as revealed by test crosses Cross Backcross Grand Parent Backcross Parent Number of Plants James Index of Resistance Resistant JB216 x Lee Gragg AmericanGR171JB216385no JB216 x KYHART1GR171JB216662no Hyko x JB575GR171JB yes Boston SB1 x JB575GR171JB yes MG-Pugel x AG200GR97AG yes R6So--A(AaYo 53)x AG247GR97AG247575no Manahan 3 x AG247GR97AG247363no Ada1 x B3119F295B yes Frye Mtn. x B3176F295B yes Ada1 x B343F295B343303no

Frequency and level of resistance to Phytophthora root rot in chestnut backcrosses The fraction of trees resistant to Phytophthora root rot declines with backcrossing; there is no evidence we are selecting for resistance. Mean resistance among trees that retain resistance does not appear to decline with backcrossing, but more tests are needed, especially in Graves B 3 s. The rate of decline with backcrossing in the fraction of resistant trees is compatible with a single gene controlling resistance, but perhaps not the lower level of resistance in Clapper compared to Graves/Mahogany. F 2 s from controlled pollinations need to be tested. CrossSource Not ResistantResistant # TreesMean# TreesMean B2B2 Clapper9248 B3B B 2 -F 3 Clapper20427 B1B1 Graves/Mahogany0.134 B2B2 Graves/Mahogany73822 B3B3 Graves/Mahogany101217

Conclusions With simple inheritance, not every backcross line needs to have resistance to Phytophthora root rot, because, with enough resistant lines, resistance could sweep through the population. This would be easier if the population were reasonably homozygous for blight resistance Retaining lines susceptible to Phytophthora root rot will help preserve the genetic diversity we have so laboriously added. Markers bracketing resistance could be used this winter to probe the rest of our B 3 and B 3 -F 2 populations to assess whether a sweep is possible, before spring planting season. There may not be enough lines with resistance in Clapper progeny, and they may not have sufficient levels of resistance.