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Pecan Breeding & Genetics: Somerville, TX

PECAN DESCRIPTORS

  • Growth Characteristics
    • Phenology of vegetative growth
      • Bud growth (rated 1=dormant, 2=swell, 3=inner scale split, 4=burst, 5=first leaflet expansion, 6=25% expansion, 7=to 50% leaf expansion, 8=to 75% leaf expansion, 9= fully expanded leaves)
      • Leaf drop (rate as percent of leaves remaining, or use scale of 1=dormant, 2= to 25% remaining, 3= to 50% remaining, 4= to 75% remaining, 5= over 75% remaining.)
      • Leaf condition on date (0=defoliated, 1= senescent, 2=mature, 3= immature, 4= active growth)(Defoliation date is often useless, since all defoliate at first freeze some years)
      • Cold hardiness - Need objective methods (e.g. differential thermal analysis, electrolyte leakage).
    • Tree form
      • Trunk diameter (measured at uniform height (2cm in greenhouse, 1 ft for young seedlings in the field, at 3 ft for grafted trees in orchard, dbh for native trees). For small trees, diameter is measured with calipers. For large trees, diameter is recorded using calibrated circumference tape during dormant season.
      • Tree height/canopy width (measured in dormant season with clinometer; canopy width measured across widest point in axis of row).
      • Secondary growth (+ or -, noted in August, or appropriate date of record)
      • Leaf color (rate on 1-10 from dark green to brown, based on Munsell Color Chart for Plant Tissue)
      • Nutrient uptake based on foliar analysis (mid-leaflet pair of mid-leaf from mid-shoot of current season’s growth in sun, sampled in July)
  • Disease and Pest Resistance
    • Diseases
      • Pecan scab (Fusicladosporium effusum [Partridge & Morgan-Jones]), rate in June and August for nuts, leaves (lamina only) using Hunter-Roberts scale (Pecan Quarterly 12(3):3-6) (see Rating Leafscab and Rating Nutscab)
      • Vein spot (Gnomonia nerviseda Cole) rate on basis of lesions on rachis (not lamina) where 1=0 lesions, 2=1-5 lesions, 3=6-10 lesions, 4=>10 lesions.
      • Downy spot (Mycosphaerella caryigena Demaree and Cole)
    • Insects
      • Pecan nut casebearer (Acrobasis nuxvorella Neunzig)
      • Hickory shuckworm (Cydia caryana Fitch)
      • Pecan weevil (Curculio caryae Horn)
      • Yellow aphid complex
      • Black pecan aphid
  • Flowering and Yield Characteristics (Pecan flowering scoring, Pecan flowering description)
    • Dichogamy (record first and last dates of pollen shed and pistil receptivity and calculate early, mid or late season pollen shed, pistillate receptivity)(Pollen adherence is the best criterion of receptivity)
    • Nuts per cluster (mean of at least 10 clusters, counted in September)
    • Percent terminals with clusters(count 50 terminal branches around the canopy in September, and express the number with fruiting clusters as a percentage of total)
    • Nut development phenology
      • Cotyledon size (longitudinal cut, 1=1/4 extended, 2=1/2 extended, 3=3/4 extended; 4 to base)
      • Cotyledon development (longitudinal cut, 1=water 2=gel (+water), 3=dough (-water)
      • Shell hardening (radial cut at tip, middle and base of nut, perpendicular to suture)
      • Shuck dehiscence (date of 75% split (valves separated to allow visibility of the nut)
      • Harvest season (actual date of harvest)
      • Individual tree yield (weight of sound, marketable nuts)
    • Nut quality characteristics (based on 15 nut samples taken at harvest, with 10 being destructively measured, and 5 being used as voucher)
Dr. Warren Chatwin, Research Geneticist (Plants)
Google Maps icon USDA-ARS Pecan Breeding & Genetics
10200 FM 50
Somerville, TX 77879
phone: 979-845-0212
email: warren.chatwin@ars.usda.gov