Onion Botrytis Leaf Spot

Botrytis Leaf Spot is caused by the pathogen Botrytis cinerea. B. cinerea has a very vide host range and is present in every farming or gardening enviroment. White sunken spots on leaves are usually the first sign of infection; spots are small (0.5 mm up to 6 mm long) and tend to be oval. They sometimes have a light green halo and may appear water soaked. The epidermis around the spots may be silvery. When numerous spots are present, leaf tips die back and whole leaves may be killed. B. cinerea spores land on leaf surfaces and, in the presence of moisture, germinate and produce enzymes that kill leaf tissue. The fungus damages the leaf by causing leaf spotting. Leaf surfaces must be wet by dew or rain for long periods (20 or more hours) for leaf spot to develop. Optimum temperature for germination of spores is 15°C; optimum temperature for mycelial growth is 18°C to 25°C.

Botrytis Leaf Spot Risk Model: B. cinerea infections are related to free moisture. In open field production leaf wetness is a good indicator for this. Dry days will reduce the risk again.
Botrytis cinerea Risk in Austria during May 2008: There are close to 2 weeks left where we would not need a fungicide coverage.