Biotic Stress and Yield Loss

to associate the effects of disease epidemics with physiological functions of the cropto improve predictions of yield loss. The assessment of early and late leafspot severityand defoliation two to three weeks before harvest measured the relative amountof defoliation caused by both diseases throughout the year. 12 For any single year, theregression lines provided good estimates of yield. The slopes were similar but couldnot be transferred from year to year. The data established a close relationship betweenyield and defoliation or infection. This established a relationship and effect betweenthe foliar disease, healthy green leaf tissue, and yield. By indirectly quantifying theeffect of plant disease on the relative quantity of foliage and function of the differentgrowth stages of the crop, the studies began to account for photosynthesis, assimilateproduction, and biomass accumulation.11.3.1 LEAF AREA INDEX (LAI)To directly account for the assimilate production of healthy green foliage, the leafarea of healthy foliage must be measured. Measuring the leaf area would account fordifferences in abiotic and biotic stresses on plants from year to year. The stressesmanifest themselves in healthy foliage, which accounts for biomass productionand yield. Plants grown in unfavorable abiotic and biotic conditions have a lowerleaf area, relative to plants grown in favorable conditions. Typically, plant leaf areais measured per square meter of soil surface and referred to as the leaf area index(LAI).LAI leaf area M 2 / 1 M 2 of soil surface [11.2]11.3.2 BEER’S LAWKnowing the LAI of a crop, the amount of radiation intercepted by the crop can beestimated if the incident solar radiation also is measured. Beer’s Law predicts theamount of solar radiation not intercepted by plant foliage at any level in a crop assumingthe crop canopy is uniformly distributed. 18 This means that the crop canopy ishomogeneous through all layers of the canopy. The average irradiance not interceptedby foliage decreases exponentially with increasing depth in the canopy. Beer’sLaw predicts the amount of sunlight to reach the soil surface plane as:I I o(e LAI ) [11.3]where I is the amount of sunlight that reaches the soil surface and I ois the amount ofirradiance immediately above the plant canopy. The algebraic transformation ofBeer’s Law into the amount of radiation intercepted (RI) by the crop foliage isIf we substitute I o(e LAI ) for I then:RI I o I [11.4]