Biotic Stress and Yield Loss

Y RUE(t)RI(t)[1 X]dt [11.12]where RUE(t) equals the radiation use efficiency, RI(t) equals the amount ofradiation intercepted by the plant, and [1X] equals the amount of healthy leaf areaat any point in time (t)dt. If Equation 11.8 is substituted for RI then biomass productionor yield can be calculated with Equation 11.13.Y RUE(t) ((I o(1 e kLAI(t) )) (1 X(t))) (t)dt [11.13]Powdery mildew infested leaf area reduces the RI, but what remains uncertain iswhether powdery mildew reduces the RUE of the remaining healthy leaf area.Studies have shown a linear negative correlation between powdery mildew infectedsmall grain crops and the healthy leaf area. 34–39 However, Rabbinge et al. 40 reportedthat at maximum light intensities the net assimilation rate of powdery mildewinfested leaves was less than what could be accounted for by the percentage of leafarea infested. These data indicate that powdery mildew of winter wheat has an effecton RI but also on RUE. Haigh et al. 41 did not observe the same dramatic reduction innet assimilation when oats were infected with Erysiphe graminis f. sp. avenae, nordid Balkema-Boomstra et al. 42 when spring barley, Hordeum vulgare, was infectedwith Erysiphe graminis f. sp. hordei. Consequently, the different effects of powderymildew on the crop RUE probably are species-specific between pathogen and smallgrains host.Necrotic lesions of late blight of potato on leaves and stems often destroy theentire leaf or stem. Haverkort and Bicamumpaka 43 observed a linear relationshipbetween potato tuber yields and radiation intercepted over the entire growing season,regardless of the severity of late blight symptoms observed. Measurements of netphotosynthetic rates of green potato leaf tissue from healthy and infected potatoplants also supported this relationship. 44 There was no effect of the RUE of leaveswith necrotic lesions on the stems. These studies provide evidence that Phytophorainfestans does not reduce the RUE of the plant, but only limits the net assimilation ofthe plant by reducing the LAI.Infection of rice with Pyricularia oryzae not only reduces the RI with the developmentof lesions on the leaves, but also causes a reduction in the RUE of the remaininghealthy leaf tissue. 45, 46 Similarly, Alternaria alternata infections of cottonreduced the assimilation and transpiration rates more than what could be explainedby infected leaf area. 47 Pyricularia oryzae and Alternaria alternata are bothpathogens known to produce toxins that lead to plant stress, leaf chlorosis, and earlysenescence of infected leaves. If the amount of time between infection and leaf senescenceis short, the ultimate effect on the plant is loss of RI. If the process takes longer,then part of the outcome should be a reduction in RUE as well as RI. Plant virusesalso have a significant effect on photosynthetic metabolism and probably lower theRUE of the leaves. 4811.4.4 DISEASES OF ROOT AND STEMPathogens that infect plant roots and stems usually restrict the flow of water to the