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Cercospora sp. leafspot and defoliating arthropods are major pests which reduce the yield of peanuts (Arachis hypogaea L.) in the southeastern United States by decreasing the amount and effectiveness of photosynthetic surfaces. Nevertheless, in the past, yield reductions have been empirically predicted from season‐end yields without considering the intermediate effects of leafspot or defoliation on canopy photosynthesis. Information on short and long term responses of crop growth processes to these pests is vital to the development of croppest management models which dynamically simulate crop and insect interaction. Our objective was to determine canopy photosynthesis and characteristics of peanut foliage layers in response to leafspot, defoliation, and combinations of disease and defoliation. Measurements from a field experiment included canopy C exchange rate (CER), photosynthetic uptake of 14CO2, leaf area, and light interception by leaves in three canopy layers. The upper 42% of the canopy leaf area intercepted 74% of the light and fixed 63% of the total 14CO2 taken up by intact canopies. Removal of 25% of the total leaf area, primarily from the upper half of the canopy, reduced 14CO2 uptake by 30% and canopy CER by 35%. In 1977, severe leafspot damage reduced leaf area index (LAI) by 80%, 14CO2 uptake by 85%, and canopy CER by 93%. Canopy CER values measured in 1978 were reduced 35 and 65% for medium and high leafspot damage treatments, respectively. Photosynthesis of diseased canopies was reduced not only by loss of leaves which abscissed as a result of infection, but also because diseased leaves which remained on the plants were less efficient in fixing CO2. Modeling the effects of insect defoliation and leafspot on peanut plant growth will require the fraction of LAI lost relative to LAI of 3 or 4, as well as the location and photosynthetic rate of the leaves remaining after insect defoliation or disease.

Defoliation during reproductive growth of soybean [Glycine max (L.) Merr.] may significantly reduce yields. The objective of this study was to elucidate the relationships among organism‐induced defoliation, CO2 exchange rates, and reproductive growth in fieldgrown soybean. Soybean (‘Bragg’) plots were sprayed a factorial design with diflubenzuron (0.035 kg a.i./ha) and a benomyl‐maneb mixture (1.12 a.i./ha) on two dates, first at early flowering (5 August) and again at early pod set (19 August). Diflubenzuron application prevented defoliation by velvetbean caterpillar (Anticarsia gemmatalis Hubner). By 12 September, LAI was reduced from 5.5 to 2.8, leaf dry weight from 185 to 110 g/m2, and midday light interception from 94 to 72% in defoliated as compared to non‐defoliated plots. While canopy specific leaf weight (SLW) increased in all treatments during the experimental period, SLW was 1.0 mg/cm2 higher in defoliated than non‐defoliated plots. On 4 September defoliated plots were sprayed with diflubenzuron (0.035 kg a.i./ha) and carbaryl (0.56 kg a.i./ha) prevent further leaf loss.Canopy CO2 exchange was measured at various photosynthetic photon flux densities (PPFD) to develop photosynthesis light response curves. Net canopy photosynthesis at 1,500 μE·m−2·sec−1 PPFD (PN1500) declined steadily in all treatments during seed growth. PN1500 and plant dark respiration, respectively, were reduced by 6.8 and 3.3 mg CO2·dm−2·hour−1 in defoliated compared to non‐defoliated plots. Soil CO2 efflux rates did not differ significantly (P < 0.05) between treatments. Differences in pod growth rates (7.6 and 9.21 g·m−2·day−1 and yields (434 and 502 g·m−2) between defoliated and non‐defoliated plots, respectively, were attributed to differences in canopy photosynthetic capacity. There was no apparent effect of defoliation on seed growth duration or seed abortion. Rather, yield reduction in defoliated plots was related primarily to slowing of the individual seed growth rate. Balance of carbon flux to seeds showed relationships between instantaneous canopy CO2 exchange measurements and sequential harvest data.

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