ONION Curing to Prevent Decay While in Storage

Mycelial neck rot (Botrytis byssoidea W.) is the most important storage decay of onions in the Middle West, where the crop is grown not only for table stock but also to a large degree for sets. The causal fungus overwinters in the soil or on refuse and subsists during the growing period of the crop primarily as a saprophyte upon old onion leaves or other dead organic material.  When the plant is mature the organism enters chiefly through the neck of the bulb, whence it proceeds down the succulent scales causing a semiwatery decay. Thus the disease is in general initiated in the field during the process of harvesting or curing. In extreme cases as much as half of the bulb may have rotted before it is placed in storage, but in the main the infection is in the incipient stage at this time and may not be recognized macroscopically.

A study of this disease which has extended over a period of some 10 seasons in southern Wisconsin and northern Illinois has brought out some important facts which have a direct bearing upon control.  These point to the conclusion that conditions which prevail during harvest and subsequent curing in a large measure determine the amount of decay which follows. Briefly, dry clear weather which tends to check the development of the fungus and at the same time hasten the drying of the neck tissues of the bulb tends to prevent infection while the reverse condition enhances infection. Thus, in certain years when favorable weather prevails at this critical time, the disease is negligible in storage, whereas in other seasons it may cause losses varying from 10 to 90 per cent.

Effective control has therefore been worked out through providing artificial means for sufficiently rapid curing of the bulbs, in spite of inclement weather. It has been found by experimentation that a surprisingly short treatment is sufficient to check the disease effectively. The procedure consists simply of forcing a current of warm dry air (first heated to 100° to 115° F. by drawing over a cast-iron stove or steam coils) through the onions in the slatted containers in which they are later stored. The period necessary will vary with the material at hand, but should be continued until the outer neck tissue is well dried. With onion sets this usually requires three to six hours. The bulbs may then be handled in the usual manner.

This control measure has been tested first in connection with white onion sets, where extreme susceptibility of the plant, the high value of the product, and the protracted storage period necessary all combine to warrant the extra cost involved. The results of 1923, 1924, and 1925 leave no doubt that the process is commercially practicable for this crop. The question of commercial adaptation to large onions is still unsettled. The greater succulency of the neck tissue makes a much longer treatment necessary, and the additional cost is enhanced by the fact that initial value per bushel of the large onion crop is ordinarily appreciably less than that of onion sets.