Biotic Stress and Yield Loss

general relationship between LAI and the amount of RI by the crop holds true for allcrops (Figure 11.1).11.3.3 RADIATION USE EFFICIENCYWatson 20, 21 demonstrated that the biomass production of different crops could beexplained by measuring the LAI over time. When the LAI was integrated over theentire growing season, the leaf area duration (LAD) correlated well with yield.Monteith 22 took this one step further and suggested that biomass production of acrop was directly proportional to photosynthetically active radiation (PAR) by greenplant tissue or LAI and the radiation use efficiency (RUE) of the plant could be calculatedas:RUE (Total dry matter / M 2 ) / RI / M 2 [11.9]The RI is the amount of irradiant energy intercepted by the plant. The plant transformsthis energy by means of photosynthesis to produce assimilates for growth andproduction of seed. From this equation, Monteith showed that barley, potato, sugarbeet, and apple produced about 1.4 g of carbohydrate per MJ of solar energy interceptedby healthy foliage. In the previously mentioned work of Khurana andMcLaren, 19 when the PAR or RI was integrated over the entire growing season andplotted against the total dry weight a significant positive linear relationship wasobserved. The RUE for potato was 3.4 g of carbohydrate per MJ of solar energy. Instudies where potatoes were grown in dry conditions, the RUE was lower comparedto potatoes grown in a year with adequate rainfall, resulting in lower yields for thesame RI as observed in other studies. 2311.4 EFFECTS OF DISEASES ON RI AND RUEFrom the insights of Boote et al. 24 and Johnson 25 and summarization by Madden andNutter, 11 foliar plant pathogens have been categorized into two groups: (1) those thatreduce the amount of foliage or RI by a plant, and (2) those that reduce RUE of thefoliage (Figure 11.2). Those pathogens that reduce the amount of foliage or RI do soby consuming foliage, accelerating leaf senescence, reducing plant stands, and/orstealing light. Then there are pathogens that interfere with the RUE of the photosyntheticprocess in the leaf by consuming assimilates, reducing the photosynthesis rate,and reducing plant turgor. Both categories of pathogens ultimately reduce the netphotosynthetic ability of an infected plant compared to a healthy noninfected plant.11.4.1 RADIATION INTERCEPTEDUntil the late 1980s, the majority of studies investigating the relationship betweendiseases and crop yield attempted to correlate only disease severity or incidence at asingle point or multiple points in time with yield or AUDPC and yield. 12, 26 Most ofthe correlations only had limited accuracy and did not predict yields at different locationsor years. In 1987, Waggoner and Berger 27 helped clarify the relationship