Sample size for assessment of cladode brown spot in prickly pear cactus

Abstract

Cladode brown spot (CBS) is an important disease of prickly pear cactus (Nopalea cochenillifera) in Brazil. No standard method exists for sampling in the field. A study was conducted in 30 fields of prickly pear (cv. Miuda) in Pernambuco state, Brazil, to determine the optimal sample size to assess severity of CBS. In each field, a 0.3 ha (45 × 68 m) test area consisting of 30 rows and 45 plants per row (1350 plants per test area) was arbitrarily chosen. In each test area, 50 plants were sampled using a systematic method and the disease severity was assessed with the aid of a standard area diagram set on 2, 4 and 6 cladodes/plant. The spatial pattern of the disease was examined by spatial autocorrelation analysis and the optimal sample sizes were determined based on three levels of error (5, 10 and 20%). CBS severity in the fields ranged from 0.4 to 8.7%. In 15 fields the disease severity of CBS was >4.0% and in 20 fields the spatial pattern of CBS was aggregated. There was a positive correlation (r = 0.59, P = 0.0001) between disease severity and strength of aggregation, and a negative correlation (r = −0.39; P = 0.0001) between disease severity and sample size (number of plants). There was no significant effect (P = 0.3) of the number of cladodes assessed per plant (2, 4 or 6 cladodes) on sample size. Considering a sample of two cladodes per plant and an acceptable error of 5, 10 or 20%, the optimal sample size was 253, 63 and 16 plants, respectively, for each 0.3 ha of cultivated field.