Peaches require a time of cool temperatures before normal springtime growth begins. Inadequate chilling can result in a delayed and extended bloom period, poor fruit shape and soft sutures. This increases the chance for disease and insect management problems. Decisions about chemical applications depend upon how the growing season progresses. Mathematical models of earth's daily heat wave are used to develop peach orchard management algorithms using available spreadsheet programs.
The model assumes that daytime high temperature occurs two hours after solar noon and the morning minimum temperature occurs at sunrise. The daytime temperature wave follows a sine curve and a log-log expression models the nighttime cooling curve. Vapor pressures are calculated with the Clausius-Clapyeron equation. In a humid climate, the lowest daily (minimum) temperature can be used as an estimate of dewpoint temperature. Daytime relative humidity is calculated at air temperature assuming that the dewpoint does not change from the morning minimum temperature. Dewpoints after sunset are assumed to be the next morning's minimum temperature. Wet bulb temperature is estimated from dewpoint and air temperature.
Chill hours are calculated from the daytime and nighttime heating equations as hours below a critical temperature (7 C). Utah Chill Units are calculated at hourly time steps from sunrise and totaled for the day ending at sunrise. Degree Days are calculated from daily maximum and minimum temperatures. Degree hours are determined from hourly temperatures.
Bacterial populations grow exponentially in time when vegetation is wet. Leafwetness is assumed from the time relative humidity reaches 85% in the evening until relative humidity falls to 65% the next day. Four categories based upon the time of precipitation are used to account for leafwetness due to rainfall or morning fog. These categories are: morning hours defined as sunrise to solar noon; afternoon hours defined as solar noon plus one hour to solar noon plus three hours; around noon defined as solar noon plus or minus two hours; and all day as sunrise to sunrise the next day.
Controlled temperature tests verified growth coefficients for Bacterial Spot (Xanthomonas Arbicola pv. Prunii) developed from reported doubling times. The growth coefficient is a function of temperature, quadratic from 0 C to 28 C, constant from 28 C to 33 C, and a linear decrease to 0 from 33 C to 40 C.
The effective temperature for bacterial growth is the temperature during leafwetness. Thus effective temperature during wetness times is either dewpoint temperature (condensation during dewfall) or wetbulb temperature (evaporation during rain). This effective temperature determines the growth coefficient in the exponential growth equation. Population is calculated in hourly steps and spray recommendations are made when the population reaches predefined levels. Population is initialized at one after a spray application.
Special Session -- Weather Data Requirements for Integrated Pest Management