Botrytis leaf blight of onion, caused by Botrytis squamosa is one of the most chronic and problematic disease of onion. The fungus overwinters as sclerotia (compact mass of fungi capable of surviving unfavourable environmental conditions). These are produced on infected onion bulbs left in cull piles, on mother bulbs stored for seed production, and on bulbs left in the field.

These sclerotia produce spores which are dispersed through the air to newly planted onions, and land on leaf surfaces. In the presence of moisture, the spores germinate. The leaf surface must be wet from dew or rain for 20 hours or more for leaf spots to develop. The optimal temperature for spore germination is 15°C and 20-25°C for growth of the fungus.

During the onion growing season the fungus produces spores that are carried by wind to infect new onion plants. Spores are produced on nights with high humidity and moderate temperatures (4-25°C) with an optimum temperature for sporulation of 13°C. The spores mature early in the morning and are dispersed during the day. They remain viable for about 4 days. For germination the spores require free water and the optimal temperatures 15-20°C. Rain is not needed for infection. When dew occurs continuously during the night and morning, it is sufficient for penetration/infection. When there is rain early in the morning, it has a negative effect on the infection possibilities of the fungus. After the fungus is established, it completes its life cycle in 11 to 15 days. New spores infect new plants or leaves.

Purple blotch can infect all above-ground parts of the plant in addition to the bulb. Initial symptoms appear on older leaves, usually late in the summer as spores are blown from infested debris. Older leaves and plants are more susceptible to infection. Optimum infection and disease development of A. porri on onion leaves occurred following a long period of humidity >90%, or dew deposition and temperatures of 20-25°C. Almost no infection occurs below 5°C. Bulbs can be infected at harvest if the pathogen enters neck wounds. The fungus is disseminated within and among fields by splashing water and wind, and overwinters in and on infested crop debris. The pathogen may also be seed-borne.

During the onion growing season the fungus produces spores (conidia) that are carried by wind to infect new onion plants. Spores are produced on nights with high humidity and moderate temperatures (4-25°C) with an optimum temperature for sporulation of 13°C. The spores mature early in the morning and are dispersed during the day. They remain viable for about 4 days. For germination the spores require free water and the optimal temperatures 15-20°C. Rain is not needed for infection. When dew occurs continuously during the night and morning, it is sufficient for penetration/infection. When there is rain early in the morning, it has a negative effect on the infection possibilities of the fungus. After the fungus is established, it completes its life cycle in 11 to 15 days. New spores infect new plants or leaves.

Stemphylium vesicarium infects herbaceous plants as onion, garlic, asparagus, and tomato. The symptoms that develop on each host are quite different. In onions the infection usually remains restricted to the leaves and does not extend to the bulb scales. Sometimes fruiting bodies may appear as small, black, pinhead-like, raised bodies in infected tissue. Pseudothecia of Pleospora allii develop during the winter on plant debris infected by S. vesicarium.

Stemphylium vesicarium requires the presence of free water for infection and moderate temperatures. Spore germination and infection on onion occurs at favourable temperatures with a leaf wetness period exceeding 8 hours