Comparison of nut removal and hull rot at harvest between fully irrigated and stressed nonpareil almond trees

Field experiments were conducted on 'Nonpareil' almond trees to evaluate nut removal under different moisture stress levels from 1992 to 1998. The first five years an orchard using drip irrigation was evaluated. All treatments received full or 100% ET (evapotranspiration) daily until first hull split occurred. Irrigations then were applied differentially with one treatment receiving 100% ET and the second treatment receiving 50% ET. The last two years an orchard with micro sprinkler irrigation was used. After shaking the trees at harvest nuts left on the tree were counted and the number of spurs killed by hull rot (Rhizopus stolonifer and Monilinia spp.) were evaluated. Results from the trials in the drip irrigation orchard showed that reducing ET to about 50% for two to four weeks starting when first hull split occurred reduced the number of nuts remaining on the tree after shaking 59, 76, 80, 65, and 77% during 1992 to 1996 respectively. 'Nonpareil' shoots killed by hull rot were reduced 100, 86, 79, 59, and 80% by the moderate stress during the same five years. In 1997 trials were moved to a commercial almond orchard using micro sprinkler irrigation and evaluated on peach and peach-almond seedling hybrid rootstocks. Water was reduced one week before first hull split. Leaf pressure bomb tests showed no difference in moisture at early hull split. There was no difference in the number of nuts left after shaking on the peach rootstock and a 38% non significant reduction on the peach almond rootstock. Further calculations revealed that moisture levels were 120% ET and 60% ET on the full irrigation and stressed trees. Trials in 1998 were maintained near the same 120% and 60% ET levels but the stress treatment was started two weeks before first hull split. Nuts remaining on the almond trees were reduced 70% on peach almond and 78% on peach rootstocks. Hull rot strikes were reduced 75% on peach almond and 92% on peach from the stress. These data suggests moisture stress that reduces ET to approximately 50 to 60% to create a two to four week stress period at early or first hull split increases the number of nuts that shake off the tree at harvest. The stress at early hull split also reduces the number of spurs killed by hull rot.