Some conclusions from five years of testing resistance to Phytophthora root

rot in chestnut in South Carolina

F. V. HebardResearch FarmsMeadowview, VAFred@acf.orgwww.acffarms.orgNov 12, 2010

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

Parent

TypeParent ID

Disease Severity Rating, 0 is least James Index

of Resistance*0 1 2 3

Chinese Princeton, MA 29 2 0 0 96.8

Chinese Princeton, MA 7 1 0 1 83.3

Chinese Princeton, MA 7 8 2 0 67.6

Chinese Missouri (Ken Hunt) 12 5 2 0 78.9

Amer (op) Irradiated 0 0 0 20 0.0

Amer (op) Pennsylvania 0 0 0 12 0.0

F1 x A Leffel Gtld 171 x Amer (op) 2 4 7 19 18.0

F1 x A Leffel Gtld 85 x Amer (op) 5 10 10 9 36.8

F1 x A Leffel Gtld 90 x Amer (op) 2 11 14 7 32.4

F1 x A Leffel Gtld 224 x Amer (op) 0 1 2 28 3.2

F1 x A Leffel Gtld 404 x Amer (op) 0 3 6 22 9.7

* 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

CrossBackcross

ParentNumber of

Plants

James Index of

ResistanceResistant

JB216 x Lee Gragg Amer (op)ican

JB216 38 5 no

JB216 x KYHART1 JB216 66 2 no

Hyko x JB575 JB575 20 56 yes

Boston SB1 x JB575 JB575 49 37 yes

Ada1 x BX39 BX39 11 0 no

Andover x BX39 BX39 39 0 no

M22fr(CoYo44) x CH34 CH34 16 8 yes

Mo Fr--A(CoYo 95) x CH34 CH34 32 4 no

Powell 35A x IL858 IL858 44 10 yes

GAWAL x IL858 IL858 28 5 no

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

CrossBackcross

Grand Parent

Backcross Parent

Number of Plants

James Index of

ResistanceResistant

JB216 x Lee Gragg American GR171 JB216 38 5 no

JB216 x KYHART1 GR171 JB216 66 2 no

Hyko x JB575 GR171 JB575 20 56 yes

Boston SB1 x JB575 GR171 JB575 49 37 yes

MG-Pugel x AG200 GR97 AG200 85 11 yes

R6So--A(AaYo 53)x AG247 GR97 AG247 57 5 no

Manahan 3 x AG247 GR97 AG247 36 3 no

Ada1 x B3119 F295 B3119 18 15 yes

Frye Mtn. x B3176 F295 B3176 13 19 yes

Ada1 x B343 F295 B343 30 3 no

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.

Cross SourceNot Resistant Resistant

# Trees Mean # Trees Mean

B2 Clapper 9 2 4 8

B3 Clapper 19 2 4 8

B2-F3 Clapper 2 0 4 27

B1 Graves/Mahogany 0 . 1 34

B2 Graves/Mahogany 7 3 8 22

B3 Graves/Mahogany 10 1 2 17

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.