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Onion Botrytis Leaf Blight

Botrytis leaf blight (Botrytis squamosa) is a fungal disease that occurs in many of the onion growing areas of the world. The causal organism, Botrytis squamosa, causes leaf spots (lesions) and maceration of leaf tissue resulting in leaf dieback and blighting. The lesions are whitish in color, from 1-5mm in length, and most are surrounded by greenish-white halos that appear water-soaked when first formed. The centers of the lesions usually become sunken, straw colored, and may develop a characteristic slit that is oriented lengthwise in the lesion. Older onion leaves are more susceptible than younger leaves to lesion formation and blighting. Under favorable environmental conditions (high rainfall, extended periods of leaf wetness, high relative humidity, and moderately warm temperatures 9°C - 25°C), Botrytis Leaf Blight can result in reduced onion bulb growth and yield.

Botrytis Leaf Blight Infection: Some infection can occur with only seven hours of leaf wetness if temperatures are near the optimum range of 15°C to 20°C. As temperatures deviate from this optimum range, longer periods of leaf wetness are required for infection. For example, temperatures at 9°C and 26°C are conducive to infection, provided that leaf wetness periods increase to 10 hours. Levels of infection should be regarded as being most severe if leaf wetness periods approach 14 hours, provided that temperatures are in the optimum range. Severe levels of infection can also occur at 9°C and 26°C if leaf wetness periods last for nearly 20 hours.

Decision should be made in combination with a method of forecasting the presence of significant amounts of inoculum of  B. squamosa. The presence of inoculum depends on the age of the culture in the field and observed symptoms before.

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The Botrytis Leaf Blight (Botrytis squamosa) Infection Model used by FieldClimate.Com points out Infections and severity. An infection of the weakest severity is fullfilled after the minimum time needed for spore germination and infection. As longer the leaf wetness will persist as more spores will participate in the infection.

In FieldClimate.com we calculate 4 Severity levels:

1) insignificant
2) slight
3) moderate
4) severe potential infection periods.