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F. V. Hebard Research Farms Meadowview, VA Fred@acf acffarms Nov 12, 2010

Some conclusions from five years of testing resistance to Phytophthora root rot in chestnut in South Carolina. F. V. Hebard Research Farms Meadowview, VA Fred@acf.org www.acffarms.org Nov 12, 2010. Phytophthora root rot resistance screening data for parents and some backcross progeny.

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F. V. Hebard Research Farms Meadowview, VA Fred@acf acffarms Nov 12, 2010

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

  2. Phytophthora root rot resistance screening data for parents and some backcross progeny.

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

  4. Utilization of resistant/susceptible cutoff of 5 for Phytophthora root rot resistance in backcross progeny

  5. Segregation of Phytophthora root rot resistance in backcross progeny as revealed by test crosses

  6. 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 B3s. • 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. • F2s from controlled pollinations need to be tested.

  7. 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 B3 and B3-F2 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.

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