Effect of early blight (Alternaria solani) on healthy area duration and healthy area absorption of potatoes (Solanum tuberosum) grown in Canterbury New Zealand with different nitrogen application and stress from potato cyst nematode (Globodera rostochiensis)

Abstract

Two experiments were conducted over the 1995/96 and 1996/97 growing seasons, at Lincoln University and New Zealand Institute for Crop & Food Research, Lincoln, Canterbury to assess the effects of nitrogen and potato cyst nematode (Globodera rostocheinsis) on the development of early blight epidemics. The objective was to measure effects on photosynthetically active radiation (PAR) intercepted by crops under multiple pest constraints, and also to compare the effect of early blight, nitrogen, and potato cyst nematode on integrated values for healthy leaf area duration (HAD) and healthy leaf area absorption (HAA). Epidemic development of early blight was rapid in high nitrogen fertility plots and in plots with 30 nematode eggs/ml of soil in 1995/96, but in 1996/97 neither nitrogen nor potato cyst nematode significantly affected the epidemic development. In these trials the intercepted PAR (HAA) was derived from the integral over time of green leaf area index (GLAI) which was used as a measure of HAD. HAD and HAA were derived from leaf area index (LAI) corrected by two methods, the percent reflectance (PR) ratio or percent yellowing (PY). This paper explains the detailed correction methods used and shows that LAI corrected with PR ratio is the best way for calculating GLAI from a diseased crop, which can then be used for calculating HAA. Early blight and potato cyst nematode significantly reduced HAD and HAA whereas nitrogen significantly increased HAD and HAA in both growing seasons. The effect of disease on HAD and HAA was not altered by nitrogen or potato cyst nematode in both growing seasons.