Black Mold Onion Symptoms and Treatment

What is the black mold on onions? What causes black mold on onions? Let’s find out about black mold onion symptoms and treatment guidelines!

Black Mold Onion Definition

Black mold onion meaning – What is black mold onion?

This disease is most common in areas where crops are grown in the subtropics and tropics, where high temperatures help it grow. Black mold can cause problems in the field, but most damage happens when the food is stored. Concerns about black mold in temperate places like the UK in the 1980s have been linked to bad storage conditions and high temperatures.

Black Mold Onion Causes

What causes black mold onion? What is the most common cause of black mold onion?

Black mold is caused by a fungus called Aspergillus niger. The pathogen looks dark brown to black in culture. The fungus makes a conidiophore that is easy to recognize. It has an upright, unbranched stalk, a single swollen spherical vesicle, prophialides and phialides that cover the surface of the vesicle, and chains of spherical, black, spiny conidia that are 2–5 m in diameter. Conidia are dry and easily spread in the air.

Black Mold Onion Symptoms

Signs and symptoms of black mold onion: What are some symptoms of black mold onion?

This disease causes ugly, black, dusty fungal growth on and between the bulb scales. In severe cases, the entire surface of the bulb may turn black, and bulb rot may follow. Secondary bacteria usually cause soft rot.

Black Mold Onion Disease Cycle

The onion seed spreads the pathogen. In addition, the fungus is common in field soils and is a saprophyte that grows on dead plant leaves. Aspergillus niger starts to grow on onion leaves that are about to die, then moves to the neck of the bulb and finally grows into the bulb itself. Bulb infection in the field is usually linked to damage to the fleshy scales, such as when growth splits.

The spores on infected leaves are spread during harvesting, giving the pathogen another way to spread within the crop and nearby fields. The best temperatures for growth are between 28º C and 34º C, and the minimum temperature for spore germination is 17º C. For spores to start growing, the humidity needs to be higher than 80%. For infection to happen, the onion surface has to be wet for 6–12 hours.

Black Mold Onion Treatment Guidelines

Treatment of black mold onion – What is the best treatment for black mold onion?

Use seed that does not have a lot of the pathogen. If a clean seed is unavailable, broad-spectrum treatments like thiram are effective as seed treatments. Applying foliar fungicides near the end of the crop can reduce black mold. Harvest the crop when it is dry and try not to damage the neck tissue of the bulbs.

Keep the temperature and relative humidity at the right levels during storage. When using high temperatures to dry out onions after harvest, keep an eye on the storage conditions. Temperatures shouldn’t go above 30–32º C. Relative humidity should stay below 80% for as long as possible. A. niger can’t grow when stored at low temperatures for a long time. Still, the fungus can grow at temperatures above 15º C.

I hope you understand black mold onion symptoms and treatment guidelines.