A story about resistance to pecan scab

USDA 75-8-5

    I was out looking at the pecans in our cultivar trials and came across something I thought was pretty interesting. We have two USDA clones under trial that originate from the same cross; Osage x Creek. The clones are numbered 75-8-5 and 75-8-9. The 75 represents the year (1975) the cross was made, the 8 represent the parents (in this case, Osage x Creek), and the final number is the number given to the seedling tree within all seedlings of that cross.

USDA 75-8-9

       USDA 75-8-5 is pictured above while USDA 75-8-9 is pictured at left. Wow, the difference in scab susceptibility hit me like a brick. Seeing this obvious difference between siblings in the amount of scab found on the nuts made me starting thinking about the inheritance of scab resistance. This took me on a quest to look up the ancestry of these two USDA clones.

       Above is a flow chart of the ancestry of USDA 75-8-5 and USDA 75-8-9. The parents of these clones were the same: Scab resistant, Osage and scab susceptible, Creek. Going back another generation I found that the scab resistance of Osage originated from its Major parent. Looking at the ancestry of Creek, I found scab susceptibility in all cultivars that make up the Creek side of the family tree.
    I don't think that scab resistance is a single gene inheritable trait because we see such a wide range of reactions to the scab fungus. However, a quick look at the family tree would lead me to expect that only a portion of Osage x Creek crosses would demonstrate some level of resistance to scab.
    Even though 75-8-5 looked scab free in our trials in 2013, this USDA clone has demonstrated only mediocre scab resistance under conditions of  high humidity in southern Georgia; a location of much higher scab pressure than SE Kansas (my location).