Sequential Sampling Plan for Timing Initial Fungicide Application to Control Botrytis Leaf Blight of Onion

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Abstract
A sequential sampling plan (SSP) that optimizes sampling intensity for determining disease levels of Botrytis leaf blight [B. squamosa] in onion [Allium cepa L.] 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 (.mu.), where .mu. ranged from 0.2 to 1.8 lesions per leaf at 0.2 lesion per leaf increments. For .mu. .gtoreq. 1.0 and .ltoreq. 0.6 lesions per leaf, correct conclusions were generally made that the CDL had (.gtoreq. 99.8% accuracy) and had not (.gtoreq. 97.8% accuracy) been reached, respectively. For .mu. = 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 .mu. .ltoreq. 0.6 and .gtoreq. 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.

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