Onion Downy Mildew

Onion downy mildew is a plant disease triggered by the Oomycete organism Peronospora destructor, exhibiting initial infection in onion leaves and subsequently affecting the bulbs. This condition tends to be more severe during cool and wet seasons as well as in areas with high moisture levels. If left unmanaged, it leads to significant reductions in crop yields.

Disease Cycle

The fungus-like organism responsible for onion downy mildew has a rapid reproduction cycle, completing it in approximately 11-15 days, which generates spores for infecting neighboring onion plants.

To disperse its spores, this mildew extends spore-bearing stalks through the stomata, the breathing pores on the surface of onion leaves. Although the leaves may appear green and normal, a velvety, purplish-gray growth indicative of the mildew can be observed. Each piece of leaf tissue only produces one round of spores, causing the tissue to collapse and perish, resulting in the characteristic blasted appearance in fields affected by mildew.

The spores can be transported by rain splashes and wind over considerable distances.

Sustained periods of moisture on the leaves are necessary for spore production and infection, making severe outbreaks of downy mildew more likely during wet periods.

While airborne spores have a short lifespan, the fungus can also generate a second type of spore (resting spores) within the infected plant tissues. These resting spores possess greater resilience and are released into the soil as the diseased material decomposes. They are capable of surviving for extended periods within the soil.

The fungus can remain dormant within affected bulbs and continue producing spores, thus perpetuating the spread of the disease.

Symptoms

The disease manifests as irregularly shaped areas, ranging from pale-green to yellowish or brownish, on the leaves or seed stalks of affected plants. These areas can have an oval to cylindrical form and may exhibit layers of alternating yellow and green tissue.

As the mildew develops, the infected leaves become constricted, leading to their eventual collapse. The dead leaf tissue quickly becomes colonized by a dark-colored purple blotch, which obscures the presence of the downy mildew.

In specific areas of the field, defined by distinct boundaries, dead leaf tips can be observed.

The bulb tissue, particularly the neck region, may undergo a spongy transformation, compromising its storage quality.

Management Strategy

Chemical method

The chemical approach involves the utilization of fungicides and is widely practiced as a reliable method. To prevent the development of resistance in the fungus, it is recommended to alternate the use of fungicides with different active ingredients throughout the growing season.

The following fungicides are suitable for both preventing and eliminating the Downy mildew disease:

  • FORTRESS GOLD 720WP: Add 40g per 20 liters of solution.
  • GEARLOCK TURBO 250WP: Add 25g per 20 liters of solution.
  • KATERINA 720SC: Add 50ml per 20 liters of solution.
  • PROPELLER 722SL: Add 50ml per 20 liters of solution.
  • SACRIFIDO 125EC: Add 20ml per 20 liters of solution.
  • TOWER 720WP: Add 50g per 20 liters of solution.
  • TRINITY GOLD 425WP: Add 50g per 20 liters of solution.
  • COMRADE 450SC: Add 20ml per 20 liters of solution.
  • PYRAMID 700WP: Add 50g per 20 liters of solution.
  • ABSOLUTE 375SC: Add 10ml per 20 liters of solution.
  • CADILAC 800WP: Add 50g per 20 liters of solution (for preventive purposes).

Non-chemical control

  • Avoiding overcrowding of crops, as well as damp and sheltered growing conditions.
  • Maintaining proper weed control to ensure adequate airflow throughout the crop.
  • Refraining from using overhead irrigation methods to keep the foliage dry.
  • Promptly removing and disposing of infected plants, without composting them.
  • Ensuring that no bulbs remain in the soil from one year to the next, as the fungus can remain dormant in infected bulbs and produce spores in spring, which can spread the disease to newly planted crops.
  • Avoiding the planting of any onion sets that appear soft, moldy, or otherwise suspicious.
  • Implementing crop rotations with non-host plants.

Additional notes:

  • When spraying, it is advisable to mix the fungicide with INTEGRA at a ratio of 3ml per 20 liters of solution. This additive enhances the effectiveness of the fungicide by acting as a sticker, spreader, and penetrant.
  • Timely control of the disease is crucial as it helps prevent or reduce the losses associated with the infection.

 

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