analysis was conducted using thermal time instead of real time expressed as the number of days. The thermal time, reported as degree days, was calculated for each growth chamber. In this case, each chamber had the same air temperature response during the
Andrés Javier Peña Quiñones, Melba Ruth Salazar Gutierrez, and Gerrit Hoogenboom
Arthur Villordon, Christopher Clark, Don Ferrin, and Don LaBonte
crops, the most common approach used for harvest scheduling is based on the relation of harvest date with accumulated degree days often in combination with other factors ( Everaarts, 1999 ; Perry et al., 1997 ). Well-characterized degree day
Diane E. Dunn, Janet C. Cole, and Michael W. Smith
The objective of this study was to determine the most advantageous time to collect cuttings of Chinese pistache, a commonly recommended ornamental shade tree that is difficult to propagate by cuttings. In 1993, calendar date and degree days (daily mean temperature -7.2C) were used to estimate an appropriate cutting time. The greatest percentage of rooted cuttings occurred in male cuttings harvested on 13 May 1993 (397 degree days) and treated with 17,500 mg·liter-1 IBA or in male cuttings harvested on 20 May 1993 (482 degree days) and treated with either 8750 or 17,500 mg·liter-1 IBA. In 1994, cutting time was associated with calendar days, degree days, and morphology. The most rooted cuttings (44%) were from green softwood cuttings taken on 9 May 1994, which was 380 degree days from orange budbreak using a threshold temperature of 7.2C. Orange budbreak was characterized by separation of the outer bud scales such that the orange, pubescent inner bud scales were visible. Cuttings taken on 9 May 1994 and treated with 8750 mg·liter-1 IBA produced the most primary and secondary roots and the longest primary roots per cutting. Male Chinese pistache cuttings should be collected from green softwood or red semi-softwood stems when about 380 to 573 degree days have accumulated after orange budbreak. Chemical names used: indolebutyric acid (IBA).
Gil Simmons and Bill B. Dean
Carrot (Daucus carota) L.) seed quality is affected by the environment in which it matures. Substantial differences in germination from year to year and from field to field have been recognized for many years for umbelliferae seed. Part of the explanation for low germination appears to be the harvest of immature seed. Data was collected for two years, from fields of the cultivars Chantenay and Nantes. Approximately 550 growing degree days were accumulated from anthesis until maturity for seed from the primary umbel. Growing degree days were calculated using a 10°C base temperature and without truncating for temperatures in excess of 35°C. Secondary, tertiary, and quaternary umbel seed maturity sequentially followed primary umbel seed. Secondary and tertiary umbels produced approximately 80 percent of the total seed yield while the primary and quaternary umbels produced approximately 20 percent. Seed maturity was determined by measuring the germination rate. Immature seed germinate at a slower rate than mature seed. The implications of these results for obtaining high quality carrot seed will be discussed.
Matthew L. Richardson and Dewey M. Caron
Various instruments and contract services can be used to calculate degree-days. This study compared instruments and services to the Wescor Biophenometer, an instrument used by cooperators of the Southeast Pennsylvania IPM Research Group (SE PA IPM RG) throughout Delaware and southeastern Pennsylvania for 10 years. Instruments evaluated in the study were the Wescor Biophenometer Datalogger, Avatel HarvestGuard, Avatel Datascribe Junior, Davis Weather Monitor II, Accu-Trax, and the HOBO H8 Pro Temperature Data Logger. The services were SkyBit and national weather data. Different combinations of instruments and services were used at three locations in Pennsylvania and four locations in Delaware over a 2-year period. We checked the degree-day accumulation of each instrument and service weekly and made statistical comparisons among the instruments and services at each site. To further construct a comparison of the instruments, we noted distinctive qualities of each instrument, interviewed the manufacturers, and received feedback from SE PA IPM RG members who used the instruments. We evaluated the instruments' algorithms, durability, cost, temperature sampling interval, ease of use, time input required by the user, and other distinctive factors. Statistically, there were no significant differences in degree-day accumulations between the Biophenometer, Harvest-Guard, Datascribe, Weather Monitor II, Skybit, or weather service data. However, cost and time required to access/interpret data and personal preference should be major considerations in choosing an instrument or service to measure degree-days.
David Wees, Philippe Seguin, Josée Boisclair, and Chloé Gendre
placed outdoors a few days before transplanting to harden them off. See Table 2 for dates of propagation, planting, and harvesting. Table 2. Dates of procedures, length of growing season, cumulative growing degree days (GDD), and rainfall for
Chilling and heating effects on budbreak of pecan [Carya illinoinensis (Wangenh.) K. Koch] trees were examined by linear regression analyses from experimental data and from records of budbreak dates over a wide geographic range. The results demonstrate that budbreak in pecan is under the interactive control of heating and of chilling. Heat required for budbreak varies inversely with chill accumulation, and budbreak may occur with no chilling once sufficient heat accumulates. Variability in budbreak increases dramatically when there are fewer than ≈ 100 chilling degree days. Heating degree days with daily minima <2.2C are inefficient; 3.9C is the most efficient heating and chilling base. At base 3.9C and the daily minimum heating temperature of 2.2C, heating degree days required for budbreak of a composite of cultivars can be predicted from chilling degree days over a wide geographic range by the relationship, Log Y = 2.7190-0.0216 √X.
Matthew G. Blanchard and Erik S. Runkle
/b 1 ) and the amount of thermal time (units of degree-days) that were required from VI to flower (°C·d −1 = 1/b 1 ) in each Odontioda clone ( Roberts and Summerfield, 1987 ). Results During Year 1, plants of both clones displayed
J. Steininger and C.C. Pasian
`Butter Pixie' and `Horizon' Asiatic lilies (Lilium spp.), were grown at several temperatures. The phenological events of visible shoot (VS), visible flower bud (VB), and open flower (OF) were recorded daily. Based on these events, phenophases from VS to VB (VS:VB), from VB to OF (VB:OF), and from VS to OF (VS:OF) were defined. Daily rates of development to complete a phenophase increased with temperature. Nonlinearity was obvious for all phenophases around 25 °C for `Horizon' and 27 °C for `Butter Pixie'. A piece-wise linear regression change point model was fitted to each dataset. The base temperature (Tb), the temperature at which the nonlinearity occurred (Ti), and the temperature for fastest development (To) could then be determined. Tb for the phenophase VS: OF was -0.4 °C for `Butter Pixie' and 3.0 °C for `Horizon'. Ti for `Butter Pixie' was 25.7 °C for VB:OF and 26.1 °C for the phenophase VS:OF. However, Ti for `Horizon' was found only for the phenophase VS:OF. To complete the phenophase VS:OF, 1102.2 degree days (°Cd) were predicted necessary for `Butter Pixie' and 833.2 °Cd for `Horizon'. Predicted time of events was compared with observed values. Subdividing VS:OF into VS:VB and VB:OF and using their respective Tb and TU reduced the average prediction error from 2.13 to 1.87 d for `Butter Pixie' and from 2.39 to 1.86 days for `Horizon'.
Robert J. Dufault
The objective of this research was to determine the least variable method to predict the dates of the first and last broccoli (Brassica oleracea L. var Italica) harvests based on heat unit summation using coefficients of variation (cv). The method with the lowest cv for predicting first harvest was to sum, over days from planting to harvest, the difference between the growing season mean (GSM) temperature and a base temperature of 7.2 °C. If the GSM maximum (max) temperature, however, was >26.7 °C, an adjusted max temperature was calculated by first subtracting 26.7 °C from the GSM max temperature and then subtracting the GSM mean temperature. Then the growing degree days (GDDs) were summed by subtracting the base temperature of 7.2 °C from the average of the GSM minimum (min) and adjusted max temperatures. This method produced a cv of 3.96 compared to 4.13 for the standard method of summing over the entire growing season, the mean temperature minus the base temperature of 4.4 °C. The method with the lowest cv for predicting last harvest was to sum, over days from planting to harvest, the difference between the GSM max temperature and a base temperature of 7.2 °C. If the GSM max temperature, however, was >29.4 °C, the base temperature was subtracted from 29.4 °C and not the actual GSM max temperature. This method produced a cv of 3.71 compared to 4.10 for the standard method of summing over the growing season, the mean temperature minus the base of 4.4 °C.