Hazards to Onions in Many Areas

J. C. Walker.

Onions are grown in every State.

In the South, onions are an early spring crop, mostly for immediate shipment, and Yellow Bermuda, Ex-cell, and Crystal Wax are the chief varieties.

In the North, the earliest crop is grown from sets produced the previous season, and the major part of the acreage, much of it on muck soils, is sown in early spring and harvested in September and October. Strains of Yellow Globe, Yellow Danvers, and Sweet Spanish predominate. White Globe and Red Globe also are planted.

Onion sets are grown mainly in northeastern Illinois and southeastern Wisconsin and near Greeley, Colo.

The chief set varieties are Ebenezer, Yellow Strassburg, White Portugal, and Red Wethersfield. All northern- grown varieties are suited to winter storage. Onion seed is grown chiefly in California, Idaho, and Oregon.

Garlic, chive, and shallot are grown from sets. Welsh onion and leek are grown from seed. Welsh onion, like shallot, produces many small bulblets and both are used chiefly as "green" onions.

While many diseases affect onion and its close relatives garlic, chive, shallot, Welsh onion, and leek a few stand out as potential hazards, particularly to onions, in many areas.

DOWNY MILDEW is most destructive in New York, Michigan, Louisiana,

California, and Oregon in an average year. In unusually cool, rainy seasons it is a major disease in other Midwestern States and once in a while in Texas and Colorado. It affects onion, Welsh onion, leek, shallot, chive, and garlic.

It usually appears in midseason as yellowish spots on the upper half of leaves of the onions. The fungus fruits on the surface of the spot as a bluish-gray, fuzzy mildew when humidity is high. The spots increase rapidly if moist weather continues. Spores produced on the surface are carried widely by air currents to cause new infections. The tops die back. The advance of the disease increases with high humidity and declines in dry spells. Plants are seldom killed, but growth of bulbs is reduced and the bulb tissue is inclined to be spongy and of poor keeping quality.

When expanding stems in the seed crop are infected, uneven, stunted growth follows. Spots on one side cause the stem to bend in the direction of that side. As the seed top grows heavier, weakened stems break over and seed is light in weight and poor in germination.

The causal fungus, Peronospora destructor, is spread by wind-borne, short-lived spores. In old leaves are formed winter spores, which remain viable in the soil until the next season. The fungus threads mycelium may also live in the bulbs and sets. From them infected plants may arise when they are used for the seed crop or for an early bulb crop.

Perennial onions may also become a source of summer spores in early spring. Summer spores are produced most abundantly during the night at about 55 F. They are spread during the day. As dew accumulates the following night, they germinate most rapidly at about the same temperature and penetrate the onion leaf or stem. Dew is required for germination and penetration. Windy weather favors spread of spores, but there is less dew then. That is why low muck areas with poor air drainage are the ones where onion mildew often appears first and causes greatest damage.

Downy mildew diseases usually can be controlled by protective sprays or dusts. Onion downy mildew has been an exception; many experiments have been conducted on it, but to little avail. At the Michigan Agricultural Experiment Station, Ray Nelson in 1951 reported encouraging results with a mixture of Dithane Z-78 and sulfur applied as a dust. We expect improvements in that line of control as other fungicides are developed.

Localities with dry summers have little to fear from mildew. The production of onion seed therefore has increased in Idaho and declined in California and Oregon.

A resistant variety, Calred, was developed by workers of the California Agricultural Experiment Station and the Department of Agriculture from a cross between a resistant strain of Italian Red and an Australian variety, Lord Howe Island. Its seed stalks are highly resistant and the leaves are moderately so. It is adapted to growing districts in California. We need to breed the resistant character into the more widely used varieties.

NECK ROT is one of the most serious of the storage rots of onion. It appears shortly after harvest as a softening of the scale tissue. It begins usually at the neck and occasionally at a wound elsewhere on the bulb. The affected tissue takes on a sunken, cooked appearance as it advances steadily down one or more of the bulb scales. There appear later on the decayed tissue grayish masses of fungus threads, which gradually form a compact mat on the surface. If conditions are even moderately moist, a gray, powdery mass builds up on the surface. Meanwhile the cooked-appearing zone advances while the fungus mass follows it down the scale. The scale gradually shrivels. If many scales are affected, the entire bulb dries down to a crisp mummy.

The causal fungi are three closely related species, of which Botrytis allii is the most widespread. The gray, powdery mass on the surface of the decaying scales consists of myriads of spores of the fungus, which are picked up readily by the lightest of air currents. They will live for some days or weeks, but not through the winter.

In the fungus mats there sometimes appear hard black bodies (sclerotia), about the size of a barley kernel. They are made up of finely woven fungus threads, which can survive freezing winter weather.

When cull onions are dumped from warehouses in the spring, the sclerotia give rise to spore masses in moist weather. The spores are carried to onion fields by air currents. They do not infect the growing plant, but when i they germinate they grow saprophytically, principally on the oldest leaves which are sloughed off as the plant develops. As the plant matures it becomes susceptible at the neck. If the tops are cut while still green, the wounded neck is ideal for penetration and infection by the fungus.

The saprophytic stage of the fungus builds up most effectively in cool, moist seasons. If such weather persists into the harvest period, spores are most abundant and infection is greatest. If the crop matures in dry, warm weather, the build-up of spores is reduced to a minimum and the disease which follows is negligible. That is why neck rot is not a major disease in areas where the crop matures in dry climate, such as the Rio Grande Valley, central California, Utah, and Idaho. In the more humid upper Midwest and Northeast, neck rot varies from season to season, depending on the climate just before and during harvest.

With those facts in mind, growers there can do much to reduce neck rot. They should allow the bulbs to mature well before being topped. Bruising in harvest should be avoided. If there is adequate ventilation in storage, the disease does not spread very much.

Artificial drying at harvest and during storage reduces somewhat the early advance of neck rot, and many growers use it as a standard procedure.

All varieties of onion are susceptible once penetration has taken place, but a great difference still exists between varieties. White varieties are most easily infected. They therefore need the most attention at harvest. Yellow and red varieties are more resistant, but one must be careful with them also, especially when weather favorable to neck rot prevails. The mild varieties of all colors are more susceptible than pungent varieties of corresponding colors. It is therefore important that such types as Sweet Spanish be allowed to mature well and be given the best possible airing. They should be protected from rain and dew during the curing process. The best storage conditions for onions include a temperature of 32 F. or slightly above and a relative humidity of about 65 percent.