SF Shah, RE Gaunt, BA McKenzie and CM Frampton

PO Box 84, Lincoln University, Canterbury, New Zealand

Background and objective

The objective of this study was to investigate the effect of Alternaria solani on the physiological processes associated with yield in potatoes grown under different environments. Radiation interception is a function of the green leaf area index and extinction coefficient. Haverkort and Bicamumpaka [1] and van Oijin [2] found linear relationship between intercepted radiation and yield in potato infected with Phytophthora infestans with negligible effect on RUE. Generally, necrotrophic pathogens are assumed to reduce the ability of the canopy to capture solar radiation by reducing leaf area duration and radiation interception. Recently Madeira et al. [3] reported a reduction in RUE by the necrotrophic pathogen, i.e., Ascochyta fabae in field bean.

Materials and methods

In 1995-96 and 1996-97 the potato cultivar Ham Hardy was sown in field plots treated individually with three different nitrogen inputs during the season for development of different canopies and yield targets. Early blight infection of sub plot was achieved by inoculating the plants with spore suspensions. Per cent reflectance (PR) was measured with a multispectral radiometer (CROPSCAN, Inc. Fargo, ND, USA). Leaf area index (LAI) and transmissivity (Ti) were measured with a LAI-2000 plant canopy analyser (Li-Cor Inc, Lincoln, NE, USA). Per cent disease severity was measured visually with the help of standard area diagram. Radiation Use Efficiency (RUE) was calculated as the ratio of total accumulated dry matter to the accumulated PAR. Relationship between accumulated PAR and accumulated dry matter were derived by regression analysis.

Results and conclusion

There was a total loss of 26% and 36% in total dry matter (TDM) in 1995-96 and 1996-97 respectively. Early blight reduced intercepted PAR by 9% in both seasons and RUE by 17% and 28% in 1995-96 and 1996-97 respectively. High nitrogen increased RUE by 3% and 16% compared to low nitrogen in 1995-96 and 1996-97 respectively. The effect of early blight on RI and RUE is discussed in relation to methods of calculating RI and RUE. LAI corrected for disease with per cent reflectance ratio was suitable for calculating green leaf area index (GLAI) from diseased crops for further calculating intercepted PAR.

References

1. Haverkort AJ, Bicamumpaka M, 1986. Netherland Journal of Plant Pathology 96, 239-47.
2. van Oijen M, 1991. Potato Research 34, 123-32.
3. Madeira AC, Clark JA, Rossall S, 1994. Journal of Agricultural Science 123, 225-32.