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Get rid of onion diseases the easy way

Moses Njagi, a trader at Marikiti market in Nairobi repacks bulb onions into smaller quantities on August 25, 2021. [Denish Ochieng, Standard]

Neck Rot
Neck rot is a disease that affects onions after harvest during transportation or storage. The disease manifests itself through sunken spots and collapsed tissues around the neck of the onion bulb. Infected necks soon appear dried out, and a gray mold often occurs between the scales on the collapsed areas. Infection by the neck rot fungus often is followed by a watery soft rot of the bulb. Neck rot is controlled by proper post-harvest handling. Control of this disease is to store properly the harvested bulbs in a cool and dry environment with a temperature of 32°F. Field practices for prevention of neck rot include close plant spacing (12 plants per foot) and use nitrogen fertiliser in moderation. In addition, cut the tops close to the bulb during harvesting.

Botrytis Leaf light

This is a foliar disease common to onion growing areas. The disease causes leaf spotting and tip dieback, and can adversely affect the maturity and quality of the bulbs. Botrytis infection initially results in small, oval (1/4” in length), white spots on the leaves. These lesions often are surrounded by a halo of green water-soaked tissue. Leaf tissue within the spots eventually collapses and becomes tan coloured. Numerous lesions on a single leaf result in dieback of the entire onion top, giving severely affected fields a ‘blasted’ appearance.

Destruction of cull piles and rotation out of onions for at least 2-3 years are important to help prevent the buildup of inoculum and reduce the likelihood of severe epidemics. Because these cultural practices are only partially effective, and no blast-resistant varieties are available, onion growers must rely upon repeated application of protective fungicides for acceptable disease control.

Downy mildew

Downy mildew is manifested by a purple-brown mold occurring in irregular shaped patches on onion leaves. The mold is as a result of spores produced by the fungus in wet or humid weather. Severely affected leaves collapse and die in a few days. The downy mildew pathogen overwinters in infected crop residue and in soil as resistant spore-structures. It is also possible for the pathogen to be carried with the seed. During the growing season, the disease is spread by wind. The disease is controlled through crop rotation and the use of disease-free seed. Protective fungicides can provide a moderate degree of control. If downy mildew is already established in a field, a systemic fungicide can be used to eradicate existing infections.

Purple Blotch

Symptom for purple blotch first appear as small tan spots on leaves. The lesions become sunken and rapidly expand rapidly up and down the leaf. Individual lesions 1/4” - 3/4” in diameter frequently are surrounded by a band of purple tissue. Numerous leaf lesions contribute to collapse of the entire top. Wounds occurring at or shortly before harvest provide sites for Alternaria infection of onion bulbs. Wind and rain are responsible for dispersing spores to other plants and fields. Tops are more prone to infection as plants mature. Under favourable conditions, an unprotected crop can sustain a substantial amount of infection in a few weeks. Crop rotation is important to prevent pathogen populations from building up to high levels. Rotations out of onions for 2-3 years are recommended. Most commercial onion crops must be protected from purple blotch by using repeated applications of protective fungicides.

Onion Smut

Onion smut is a disease common to temperate growing regions, especially where onions are grown from seed. Most onions and related crops are susceptible to smut. Onion smut appear as Lesions with dark brown streaks running up and down the leaves. The streaks initially appear as long blisters on the leaf surface. As the lesions mature, they turn brown and contain a mass of dark powdery spores that give the tops a sooty appearance. Diseased leaves may bend or twist abnormally and usually are shed prematurely. Smut infected plants normally are stunted and produce bulbs highly prone to soft rot. Use of onion sets is recommended as they are rarely affected by the disease.

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