Sample size for assessment of cladode brown spot in prickly pear cactus
Garcete-Gómez, José María | Conforto, Cinthia | Domínguez-Monge, Santiago | Flores-Sánchez, Jorge Luis | Mora-Aguilera, Gustavo | Michereff, Sami Jorge
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. Miúda) 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.
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