Onion downy mildew - treatments, identify and control

Onion downy mildew, Peronospora destructor is considered the most dangerous disease that can completely compromise the crops of chives, onions, or seed crops. The disease was discovered in 1841 in England. It has spread rapidly in all cultivating countries.

Symptoms. The disease manifests itself on leaves, flowering stems, flower stalks, and bulbs in all stages of development. On the leaves there are oval-elongated spots of light green color at first, then yellow. In wet weather, the spots are covered with a gray-violet fuzz. Over time, the tissues near the spots will necrotize and the leaves rot. In case of strong attacks, the mycelium of the fungus passes from the leaves to the bulb, being located in its apical part. Attacked bulbs do not grow, remain small, soften and rot early. In the section, through the affected bulbs you can see the gray color of the tunic. In warehouses with high temperatures and humidity, the infected bulbs are quickly and completely destroyed.

On flowering stems, the disease is manifested by symptoms similar to those on the leaves, except that the spots are larger and more obvious. Frequently the flower stalks break near the spots at the slightest breeze.

The pathogen is Peronospora destructor. Its transmission from one year to another is done in two ways: through resistance spores that remain in plant debris and through infected bulbs. Primary infections in the spring are ensured by the germination of resistance spores. And, during the growing season, the spores that appear on the mycelium are the ones that spread the fungus. They appear at temperatures between 3-27°C, the germination of spores occurs in atmospheric humidity conditions for at least 10 hours. The spores are easily carried by the wind and will give new infections on onions, but also on other plants such as garlic, leeks, shallot.

Prevention and control. It is recommended to apply crop rotation. Onion must return to the same field after 3-4 years. It is also recommended to plant material (bulbs, chives) and use seeds from healthy crops, harvest the bulbs in dry weather, and expose them to the sun for 8-10 days before storage.

During the growing season to control onion downy mildew treatments with specific fungicides are made.