Leaf number, area and chlorophyll content, and specific leaf weight were greater in light-exposed spurs of ‘Hartley’ walnut (Juglans regia L.) than those grown in the shade. Starch content increased early in the season in shaded spurs, but the accumulation ceased while the nuts stored dry matter. In exposed spurs, starch increased steadily until harvest. After harvest, starch level decreased in exposed and shaded spurs. Light intensity did not affect percentage composition of spurs and fruit with respect to carbohydrates or oil content in kernels. Increased exposure to light resulted in higher percentage of return bloom, greater spur growth, and more pistillate flowers per spur the following season.
Quantitative assessments of the degree of susceptibility to experimental infection by Erwinia rubrifaciens Wilson et al. were made on 54 cultivars of Juglans regia L., J. hindsii Jepson, and J. nigra L. on J. hindsii and Paradox rootstocks. Forty-five of the 54 cultivars were susceptible and developed symptoms characteristic of deep bark canker. On Paradox rootstock, ‘Pioneer,’ ‘Early Erhardt,’ ‘Willson-Franquette,’ ‘Sinensisy5,’ ‘Conway-Mayette,’ ‘Scharsch-Franquette,’ ‘Meylan,’ and ‘Sinensis-7’ showed canker extension rates greater than ‘Hartley’; whereas, on J. hindsii rootstock only ‘Sinensis-5’ and ‘Sinensis-7’ showed canker extension rates greater than ‘Hartley.’ All cultivars were ranked into 4 susceptibility groups according to their rates of canker extension: group I (0 mm/day), group II (0.01 to 0.10 mm/day), group III (0.101 to 0.200 mm/day) and group IV (0.201 to 1.0 or greater mm/day). Of 54 cultivars only 8 ranked in group I (non-susceptible); 28 ranked in group II (dry-canker types); 11 ranked in group III (moderately susceptible); and 7 ranked in group IV (highly susceptible). ‘Hartley,’ the only cultivar commonly associated with the disease in commercial plantings ranked in group IV. Cultivars of either J. hindsii or J. nigra were ranked in group I. Erwinia rubrifaciens was recovered from 45 of 54 cultivars, 14 months after experimental inoculation and 23 of 54 cultivars after 26 months. The pathogenic bacterium was recovered from cultivars of group I (that displayed no visible symptoms). It is hypothesized that natural populations of J. hindsii and J. nigra may harbor E. rubrifaciens and may have been the original inoculum source.
Crowded conditions in ‘Serr’ walnut orchards predispose trees to initiate numerous catkins which reduce crop potential in 3 ways: a) catkins replace potentially fruitful buds at several nodes; b) they occasionally induce abscission of the terminal bud; and c) they utilize much reserve food, requisite for pistillate flower set, especially, because the cultivar is protandrous.
Various rates of Wilthin were applied at full bloom to limbs carrying 150 to 250 flowers to study their activity on blossom thinning of `Loadel' peaches. Wilthin applied at 0.75% and 1.0% significantly reduced fruit set to 29% and 30%, respectively, while the control produced 94%. The effectiveness of the 0.75% rate was dramatic, but it is interesting to note that the 1.0% rate did not lead to excessive thinning nor phytotoxicity on foliage or fruit. More extensive studies need to be done to fully determine the potential of this material. However, these results suggest that further testing of Wilthin on a larger scale is warranted.
Annual pruning was compared with nonpruning for 8 years and to two biennial pruning treatments for 4 years in a mature full-canopied `Ashley' walnut (Juglans regia L.) orchard. Light penetration and nut distribution through the canopy was improved by pruning. Nut size and percent edible kernel was consistently lower in nonpruned trees than in trees pruned annually or biennially. Yield from annually pruned trees was not significantly different from that of the nonpruned trees because of the removal of fruitful spurs. Yield of biennially pruned trees was similar to annually pruned or nonpruned trees in the year following pruning, but yield was usually greater during years in which trees were not pruned.
The pistillate flower of walnut is a complex structure (10, 13, 14) and is referred to as a pistil for simplicity. Pistils emerge terminally on shoot or spurs after different degrees of vegetative extension from mixed buds (9, 14). Such growth can range from essentially nil to about 1 m. Pistils are borne on a short peduncle (Fig. 1). Two pistils per peduncle are most common, but one or three, or (rarely) more, can occur. Flowers are wind-pollinated and all cultivars are considered to be cross- and self-fruitful (9, 14). Pistils become receptive very shortly after emergence from the shoot apex when the two stigma lobes begin to separate. Fertilization of ovules is necessary for nut development to maturity, but pollination and fertilization are not required for early growth of the ovary (14). Ovaries of nonpollinated flowers will enlarge at rates similar to those of fertilized ones for several weeks before abscising, when about 1 to 2 cm in diameter.
In both laboratory and field experiments, excessive pollen has been found to be a major cause of pistillate flower abscission (PFA) and reduced yields of sensitive English Walnut cultivars (CVs) (especially “Serr”). In the field, PFA and reduced yields develop when substantial overlap of male and female walnut bloom occurs. PFA and poor yields can be further aggravated when pollenizing CV's have been included into an orchard to maximize pollen availability for the commercial CV Field experiments, conducted in 1992 and 1993, demonstrated that mechanically shaking trees to remove male flowers pre-bloom from either pollenizer CV's or the main CV reduced pollen load, PFA, and substantially improved yields.