Biotic Stress and Yield Loss

tion. Two weeks before harvest, the percent reflectance positively correlated withyields from four separate studies. The reflectance data were not analyzed collectivelyacross all four studies because there were dramatic differences in yield betweenstudies. If percent reflectance data were collected periodically throughout thegrowing season in the four separate peanut studies and related to LAI, then the RIintegrated over all the observation dates may have allowed all the data to be analyzedtogether. Theoretically, this would demonstrate that RI as affected by late leafspotdefoliation and different environmental conditions could explain most of the yielddifferences. Additionally, if disease severity was rated each time the spectralreflectance was measured, then the effects of leafspot on spectral reflectance and LAIcould be correlated.Dudka 66 used a hand-held radiometer to measure the reflectance of soybeanswith varying levels of sclerotinia stem rot caused by Sclerotinia sclerotiorum.Disease incidence correlated positively with reflectance at 706 nm and negatively at760 nm. It was inferred that yield data negatively correlated the incidence of sclerotiniastem rot.The studies using the handheld radiometers demonstrate that remote sensing andimage analysis have utility in quantifying the effects of disease on yield loss. Becausethe remote sensing was limited to a single finite point in time, the data collected onlycorrelated well with each study and the relationships could not be integrated acrossstudies. Instead, if the spectral measurements were collected over the entire growingseason and related to LAI, then the RI by the crop could be integrated and calculatedfor the entire season and correlated to yield. This would then allow data from differentstudies to be collectively analyzed.11.8 SUMMARYWhether a pest attacks the roots, stem, or leaves, the effects ultimately will manifestthemselves with a reduction in leaf area. The concept is simple, but often overlooked.Using Beer’s Law and LAI, if a crop had an LAI of six, it could lose one half the LAIbefore the plant would see a dramatic reduction in RI (Figure 11.1). High LAI providesprotection against loss biomass production and yield when leaves are destroyedby diseases or biotic stress, because of the relationship between LAI and RI. In theearly growth stages of an annual plant, the LAI is less than three, and the loss of LAIhas a significant effect on RI and net assimilate production (Figure 11.1). Early seasondisease epidemics that affect HLAI and RI may only delay the crop maturity, ifcontrol practices are implemented to stop the epidemic. Delayed maturity mightreduce crop yields if the incident radiation between the normal maturation time andthe delayed time is dramatically less due to weather and angle of the sun. For this reasonit is important to quantify the severity of infestation and on what portion(s) of thecrop, to record the growth stage, and measure the LAI if the effects will be correlatedwith crop yields.Integrating RI and RUE over a growing season provides an accurate measurementof total biomass production of a crop as outlined by Monteith 22 andGoudriaan. 67 Differences in abiotic and biotic effects on the crop are then reflected