Narcissus Basal Rot

W. D. McClellan.

Considerable concern was felt in England in 18go about a troublesome rot of narcissus bulbs. The affected plants were stunted, the tips of the leaves turned brown, the flowers developed imperfectly, and the base of the bulb was soft and rotten.

The cause of the rot was not determined. Some investigators blamed unfavorable conditions of soil or climate. One person thought it was due to a fungus. But little effort was made to investigate the trouble until the hot summer of 1911, when large numbers of bulbs rotted during storage in England and the Netherlands.

J. Jacob, in an article in The Garden in 1911, stated that the rot was due to the combined action of heat and Fusarium bulbigenum. That fungus, described in 1887, had been found on narcissus bulbs but had not been recognized as the pathogen. Later studies indicated that the fungus was nearly always present with nematodes, which also can cause a rot of daffodils. Johanna Westerdijk in the Netherlands in 1917 distinguished between the nematode disease and the bulb rot due to Fusarium.

Before 1924 about 77 million narcissus bulbs were imported annually into the United States, primarily for greenhouse forcing. The Secretary of Agriculture promulgated a quarantine against narcissus because of nematodes and bulb flies, to become effective July 15, 1926. Increasing numbers of bulbs were imported in 1924, 1925, and 1926 for use largely as planting stock to establish a domestic bulb supply. By 1926 the imports reached 142 million bulbs.

Along with the establishment of narcissus culture in the United States, basal rot became a serious commercial problem, for several reasons. At first many growers were unfamiliar with the culture of narcissus. The Hollanders who undertook production in this country were unfamiliar with the factors of climate and soil they encountered. The soil temperatures at harvest and planting seasons and the temperatures during summer storage and shipment along the eastern seaboard, where bulb production was started, were higher than the temperatures in England and the Netherlands. Most of the stock planted from the imports was made up of susceptible varieties, such as Golden Spur, Victoria, Spring Glory, Empress, and Emperor. The hot-water treatment of 110 F. for 2 1/2 hours, which was required for treatment of narcissus bulbs if nematodes were present in the stock, proved to be a way of spreading basal rot. Finally, basal rot was a new problem. Little was known about its cause, development, or control.

Losses due to basal rot often were heavy. For instance, one grower near Babylon, N. Y., had a stock of 70,000 Golden Spur bulbs in 1927. They were treated with hot water to control nematodes, and they seemed healthy when they were planted. The bulbs emerged the following spring and made good growth until shortly before digging time, when a period of hot, moist weather began. The high temperature continued through the digging period and the disease spread rapidly until fall. Half of the stock was so badly rotted that it had to be destroyed. Enough of the remaining bulbs were available to plant 808 flats of approximately 40 bulbs each for greenhouse forcing. The rot continued to spread so that only about 250 of the flats had healthy bulbs of any kind and none had more than three or four live bulbs. The entire planting was completely destroyed by basal rot in less than 18 months.

Because of the alarm felt by narcissus growers, the Department of Agriculture was directed in 1926 to study the disease the first specific directive that the Department had had to study a disease of ornamental plants.

Dr. Freeman Weiss began the study of the disease, and soon found it to be caused by the fungus Fusarium. That Fusarium had been assigned the name Fusarium bulbigenum, but American pathologists follow Snyder and Hansen's classification and call it F. oxysporum f. narcissi.

The basal rot Fusarium is highly specific for narcissus, although closely allied forms attack other ornamental and vegetable crops. The large trumpet types of narcissus, especially the bi-color and white trumpets, are most seriously affected. Some of the Poeticus varieties also are susceptible, but most of the Incomparabilis, Barrii, and Leedsii varieties are resistant or at least do not ordinarily rot under conditions in which the trumpet types rot so badly. The Polyanthus types, or paper whites, are also resistant but less hardy and are therefore limited to the warmer parts of the United States. Trumpet varieties with hard bulbs, such as King Alfred, are a little less susceptible to Fusarium than the soft-bulb types. The susceptible bi-colors have nearly disappeared from commercial culture because of basal rot.

Growers call the disease basal rot because the decay usually begins in the root plate or at the base of the scales, whence it spreads through the bulb. Basal rot is primarily a disease of narcissus bulbs during storage or transit, but in warm regions it may develop during the later stages of growth in the field and harm the bulbs before they are dug.

In early summer in warm areas, the tops of infected bulbs turn yellow and die down before the normal time of maturity. Bulbs only slightly affected at digging time often have many purplish roots, which die back from the tips towards the base of the bulb. penetration of the bulbs by the fungus is primarily through the young roots.

After infection has begun at the base of a scale, extension of the rot becomes rapid if the weather is warm. In partly resistant varieties such as Sir Watkin, the disease tends to advance in streaks and layers. The rotted tissue has a characteristic chocolate-brown or purplish-brown color, and the mycelium of the fungus appears as a weft of delicate white or pinkish-white webs between the scales. The tissue is softened. Later one can easily tell the affected bulbs by touch, but the rotted tissue remains somewhat dry and spongy. In a humid environment a small amount of white mycelium may be evident at the juncture of the scales and root plate, but otherwise there is little external evidence of the disease. It is hard therefore to eliminate all infected bulbs by sorting.