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Disease Detection and Losses

Sequential Sampling Plan for Timing Initial Fungicide Application to Control Botrytis Leaf Blight of Onion. P. C. Vincelli, Research assistant, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853; J. W. Lorbeer, Professor, Department of Plant Pathology, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca 14853. Phytopathology 77:1301-1303. Accepted for publication 26 February 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1301.

A sequential sampling plan (SSP) that optimizes sampling intensity for determining disease levels of Botrytis leaf blight in onion fields was developed. With it, lesion counts are made on 15–50 onion plants per field to determine if the mean disease level has reached the critical disease level (CDL) of 1.0 lesion per leaf, an action threshold that calls for the initiation of a fungicide spray program. The SSP was validated by sampling from 15 to 50 computer-simulated lesion counts representing sequentially sampled plants until it was concluded that the disease level was either above or below the CDL. Five hundred conclusions were tested at each of nine disease levels (μ), where μ ranged from 0.2 to 1.8 lesions per leaf at 0.2 lesion per leaf increments. For μ ≥ 1.0 and ≤ 0.6 lesions per leaf, correct conclusions were generally made that the CDL had (≥ 99.8% accuracy) and had not (≥ 97.8% accuracy) been reached, respectively. For μ = 0.8 lesions per leaf, the SSP often led to the conclusion that the CDL had been reached, indicating the conservative nature of the SSP. For μ ≤ 0.6 and ≥ 1.4 lesions per leaf, conclusions were generally reached after sampling 15–35 plants, thereby reducing sampling intensity from the current 50 plants. Similar results were obtained with field data.