Onion downy mildew is caused by the fungus-like Oomycete organism Peronospora destructor, which first infects the leaves and later bulbs of onions. It is worse in cool wet seasons and in wet areas.
If uncontrolled, the causes heavy crop losses.
Disease Cycle
The fungus can reproduce in 11-15 days, providing spores for infection of nearby onion plants.
This mildew sends its spore-bearing stalks out through the leaf stomata (the breathing pores on the leaf surface). The leaf appears green and normal except for a velvet-like growth that appears purplish gray. The mildew only forms one crop of spores on any given piece of leaf tissue, and once the spores have been formed and released, the tissue collapses and dies. This is what causes the blasted appearance in a mildew-infected field.
Spores are splashed by rain, and are also carried for long distances on the wind.
Extended periods of leaf wetness are required for spore production and infection, so severe outbreaks of downy mildew are likely to occur during wet periods.
The airborne spores remain viable for just a short time, but the fungus can also produce a second spore type (a resting spore) within the affected plant tissues. These resting spores are much more resilient, and are released into the soil as the diseased material rots down. They are likely to survive within the soil for a long time.
The fungus can remain dormant within affected bulbs, producing spores that spread the disease.
Symptoms
The disease is characterized by pale–green, yellowish to brownish areas of irregular size and shape (oval to cylindrical) on infected leaves or seed stalks. These areas may consist of alternating yellow and green layers of tissue.
Infected leaves become girdled in the region where mildew develops and they eventually collapse. The dead leaf tissue is rapidly colonized by purple blotch, which is dark in color and obscures Downy mildew.
Dead leaf tips can be seen within defined regions in a field.
Bulb tissue, especially the neck, may become spongy and the bulb may lack storage quality.
Management Strategy
Chemical method
This involves the use of chemicals (fungicides) and is the most common and reliable method. However, in order to avoid resistance build up by the fungus, it is recommended that fungicides with different active ingredients be alternated during the crop’s season.
The following fungicides are fit for use in both prevention and eradication of Downy mildew disease;
- FORTRESS GOLD 720WP 40g/20l
- GEARLOCK TURBO 250WP 25g/20l
- KATERINA 720SC 50ml/20l
- PROPELLER 722SL 50ml/20l
- SACRIFIDO 125EC 20ml/20l
- TOWER 720WP 50g/20l
- TRINITY GOLD 425WP 50g/20l
- COMRADE 450SC 20ml/20l
- PYRAMID 700WP 50g/20l
- ABSOLUTE 375SC 10ml/20l
- CADILAC 800WP 50g/20l (for prevention purposes)
Non-chemical control
- Avoid overcrowded crops, damp conditions and sheltered sites
- Maintain good weed control to ensure airflow through the crop
- Avoiding overhead irrigation method to keep the foliage dry.
- Remove and dispose infected plants. Do not compost them.
- It is important not to allow any bulbs to remain in the soil from year to year. The fungus remains dormant in infected bulbs, producing spores in the spring which spread the disease to newly sown plants
- Any soft, moldy or otherwise suspect onion sets should not be planted
- Rotations with non-host crops
Note
- Whenever spraying, it is advisable to mix the fungicide with INTEGRA 3ml/20l, which improves the efficacy of the fungicide by acting as a sticker, spreader and penetrant.
- Timely control of the disease is important as it prevents/reduces the losses attributed to the disease infection.
