The development of a sequential sampling plan for management of stripe rust (Puccina striiformis West.) in winter wheat (Triticum aestivum L.)

Abstract

Stripe rust caused by Puccinia striiformis West., was studied in three seasons between 1981 and 1984, on the susceptible wheat cv. Rongotea. A sample unit of the top three leaves on a W-shaped sample pattern provided the basis of a reliable and sensitive stripe rust sampling program to detect disease incidence, as measured by low relative variability and high incidence values. A consistent relationship existed between severity and incidence on the top three leaves at incidence below 40%.

The spatial pattern of stripe rust infections on the top three leaves was a slight aggregation of disease foci. This was defined by regression techniques based on mean crowding and mean density, and supported by variance to mean and mean crowding to mean density ratio dispersion indices, and by fitting observed frequency distributions to distribution models. Mean crowding: mean density regressions provided a more accurate and less density dependent description of spatial patterns than the use of mean to variance or mean crowding to mean density dispersion indices or frequency distribution model fits.

A 0.2% severity on the top three leaves was established as an action level for fungicide application, based on a study of severity-yield relationships. A critical period of crop monitoring and applying fungicides for stripe rust was established from G.S. 15 to G.S. 61. No empirical linear or quadratic critical point model of severity-yield loss was fit significantly to observed data consistently.

Based on information on sampling methods, spatial patterns and action levels, a sequential sampling plan was constructed for use in a stripe rust management program. It is recommended sampling begin at the five leaf stage (G.S. 15) and end at anthesis (G.S. 61). Fields would be sampled until a 0.2% severity action level on the top three leaves was detected, estimated by 10% incidence. In the 1984-85 season the management program was compared to scheduled spray programs in commercial fields, and its use resulted in a reduction in sampling time with a high degree of reliability and a reduction in fungicide use compared to scheduled spray programs.