Primary infection of onions by Botrytis aclada can occur via infected seed or infected foliage. On waste heaps, the fungus can go on producing new spores for a long time, which can then infect the new crop in the spring. B. aclada can also overwinter, with sclerotia that can survive in the soil for two years and then release spores in spring. Infection can also be transmitted from neighbouring infected crops. The germ tubes of the conidia first grow on the surface of the leaf and then penetrate through the stomata. Initially, in young leaves, only the epidermis cells are infected, which does not cause any visible symptoms. As the leaves mature and die off, the fungus can develop strongly and sporulate. Flowers can also be infected. In damp conditions, extensive sporulation occurs and the fungus is dispersed by air, insects and growers walking through the crops. In the foliage, the fungus grows towards the neck of the onion, from where the bulb is infected. When topping the haulms of onions, spores are dispersed that can infect the newly cut surfaces. Wounds on the bulbs can also be infected. The symptoms mostly occur after one or two months in storage.
Host plants
B. aclada occurs most commonly on onions and shallots and sometimes on other plants in the Allium family, such as garlic and leeks.
Symptoms
The fungus forms brownish grey mycelium on the dying leaves. Sunken areas and greyish-brown mycelium develop on the bulb.
Conditions
Sporulation and dispersion occur at temperatures between 5 and 25 °C, the optimum being approx. 15 °C, with damp conditions or leaf wetness. Mycelium growth occurs at temperatures between 5 and 30 °C, the optimum being approx. 20 °C. Mycelium growth is optimal at a relative humidity above 95% and quickly decreases at a lower relative humidity. The applied limit value for infection is 80% (Plentinger et al., 2004).
Cultivation measures
Rotate crops every four years at minimum.
Waste heaps should be well covered or removed.
Avoid foliar damage.
Avoid too high a Nitrogen feed during final ripening, as this enhances fungal growth and delays the dying back.
Avoid ploughing infected crop residue under a field that will be used for onion growing.
Harvest in very dry weather.
Accelerate post-harvest drying of onions at 25-30 °C.
