Postharvest Chemical Defoliation and Dormant Hydrogen Cyanamide Sprays Affects Fruiting of Pecan Trees Growing Under Warm and Dry Conditions

Postharvest chemical defoliation and dormant hydrogen cyanamide sprays were evaluated on budbreak, shoot growth and fruiting of ‘Wichita’ pecan trees growing in the Northwest of Mexico. Chemical defoliation was done on December 12 in 2008 and on December 13 in 2009. Hydrogen cyanamide was applied on March 3 in 2009 and on February 24 in 2010. Defoliation treatments were: Non Defoliation (ND), urea, 5% (U); ethephon, 800 mg·L^-1 (E1); ethephon, 1600 mg·L^-1 (E2); and paraquat 150 mg·L^-1 (P). Ethephon sprays were suspended in 2009. Hydrogen cyanamide treatments were: control (-HC) and hydrogen cyanamide 0.5% (+HC). A combination of defoliation treatments with hydrogen cyanamide treatments resulted in ten treatments. In 2010, the effects of treatments were evaluated on budbreak, shoot growth and fruiting of two types of shoots: Vegetative Shoots (VS, did not bear fruit in 2009), and Fruiting Shoots (FS, bore fruit in 2009). Budbreak of FS, on March 26, was present only in those treatments that contained hydrogen cyanamide, ranging from 35% to 65%. Treatments without hydrogen cyanamide did not show up bud expansion in this date. On April 8, 2010, budbreak of FS in ND-HC treatment was 18±8%, but in ND+HC was 53±18%. Terminal shoot growth in FS was 3.2±1.1 cm by April 13 in the ND+HC treatment; however, in the ND-HC was only 1.8±0.2 cm. One month later, shoot growth was 4.1±1.9 in ND+HC and 3.2±0.8 cm in ND-HC. Fruiting Shoots in the treatment E1-HC had the largest shoot growth with 12.7±5.1 cm. Treatment E2+HC had 2.4±1.0 flowers per FS, and the ND-HC and ND+HC had 0.8±1.3 and 0.9±0.5, respectively. Nut yield on 2009 in the ND-HC treatment was 2,510±371 Kg·ha^-1 and the ND+HC treatment yielded 3,010±367 Kg·ha^-1. Highest yields were obtained with the treatment E2+HC (3,803±1,222 Kg·ha^-1) and P+HC (3841±1,221 kg·ha^-1). In 2010, highest yields were obtained with E2+HC (4,278±946 Kg·ha^-1) and P+HC (3,399±1,053 Kg·ha^-1). Highest cumulative yield were presented in treatments E2+HC and A+HC with 8,081±1,486 and 7,241±1,667 Kg·ha^-1, respectively. Cumulative yield of ND-HC treatment was 4,734±770 Kg·ha^-1. Lowest cumulative yield occurred in E2 treatment with 3,089±557 Kg·ha^-1. Alternate bearing (I) varied from 0.07 to 0.24. Highest I values were obtained from U (0.21±0.17), U+HC (0.24±0.17) and E2 (0.23±0.06). Postharvest chemical defoliation with ethephon and paraquat decreased pecan fruiting, but when defoliation is combined with dormant hydrogen cyanamide application, fruiting is increased.