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The heat balance method was used to estimate transpirational sap flow through 60- to 75-mm-diameter stems (trunks) of 3-year-old peach [Prunus persica (L.) Batsch. cv. O'Henry] trees under field conditions. On rare occasions, heat balance estimates agreed well with independent lysimetric measurements, but on most occasions, heat balance estimates of sap flow were unrealistic in both direction and magnitude. In some cases, the errors in sap flow approached two orders of magnitude and were always the result of a calculation involving division by a very small and sometimes negative temperature differential between the stem surface temperature above and below the gauge heater. The occurrence of negative temperature differentials under positive transpiration conditions may be inconsistent with a fundamental assumption in the heat balance model, namely that temperature differentials are solely a consequence of the dissipation of energy supplied to the gauge heater. In the absence of heating power applied to the gauge, temperature differentials exceeding - 1C were correlated with the rate of change in stem temperature, indicating that ambient conditions themselves can impose a bias in gauge signals and, hence, influence gauge accuracy. Our results suggest that the effect of ambient conditions on gauge signals should be critically evaluated before considering heat balance estimates of sap flow as reliable under any given conditions.
Abstract
Resistance in Pisum sativum to Aphanomyces euteiches was evaluated in the laboratory by the number of oospores formed in excised root tips inoculated with zoospores of the pathogen. Significantly lower mean numbers of oospores formed in root tips of 3 moderately resistant breeding lines than in those of 3 susceptible cultivars. Results were reproduceable if test procedures were adequately controlled. A sample size of 40 root tips gave 95% confidence in detecting a 40 oospore/root tip difference between means of resistant and susceptible genotypes.
Lower mean numbers of oospores formed in excised root tips of genotypes which were resistant in greenhouse tests. Inoculum concentrations of 10 to 100 zoospores/plant caused severe disease in susceptible cv. New Era, while greater amounts of inoculum were needed to cause comparable disease in 2 resistant breeding lines.
A kiwifruit vineyard converted to an organic farm was compared to a conventionally farmed vineyard from 1990 through 1992. February or March applications of composted chicken manure (organic plot) or NH4N O3 plus CaNH4 (NO3)3 through microsprinklers during the growing season (conventional plot) were applied to give equal rates of N. Soil analyses indicated no differences in nutrient or salt levels. Nitrogen leaf levels from the organic plot were consistently lower than those from the conventional system but were not deficient. Leaf concentrations of sodium and chloride increased over the three-year period in the organic plot, but not to phytotoxic levels. Organically grown fruit was as firm or firmer than conventionally grown fruit at harvest and four months after harvest. Damage from latania scale or omnivorous leaf roller was minimal in both plots until 1992, when the organic plot had 3.9% scale compared to 0% in the conventional plot. An economic analysis comparing the short-term profitability of the two systems will be presented.
Abstract
An inexpensive chamber for controlled freezing of large container-grown plants up to 2 m in height was constructed using liquid nitrogen as a refrigerant. A microcomputer-based system was developed to control the cooling sequence and to collect data on tissue temperature, air temperature, and exotherms. Versatile software was written that allowed the programmed rate of temperature drop to be based on either tissue temperature or air temperature.
Hand thinning is a necessary and costly management practice in peach (Prunus persica) production. Stone fruit producers are finding it increasingly difficult to find a workforce to manually thin fruit crops, and the cost of farm labor is increasing. A new “hybrid” string thinner prototype designed to adjust crop load in vase or angled tree canopies was evaluated in processing and fresh fruit plantings in varying production systems in four U.S. growing regions in 2009. Data were uniformly collected across regions to determine blossom removal rate, fruit set, labor required for follow-up green fruit hand thinning, fruit size distribution at harvest, yield, and economic impact. String thinner trials with the variable tree forms demonstrated reduced labor costs compared with hand-thinned controls and increased crop value due to a larger distribution of fruit in marketable and higher market value sizes. Blossom removal ranged from 17% to 56%, hand thinning requirement was reduced by 19% to 100%, and fruit yield and size distribution improved in at least one string-thinning treatment per experiment. Net economic impact at optimum tractor and spindle speeds was $462 to $1490 and $264 to $934 per acre for processing and fresh market peaches, respectively. Case study interviews of growers who thinned a total of 154 acres indicated that commercial adoption of string-thinning technology would likely have positive impacts on the work place environment.