Photosynthesis of Peanut Canopies as Affected by Leafspot and Artificial Defoliation

Cercospura 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 25y0 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 SO%, 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.