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NATURE OF THE LOSSES.
To the growing crop.-- The most important factor to be considered in this connection is the effect of the disease on the keeping quality of the bulbs. At the time of sorting, the neck-rotted bulbs are included with those attacked by the smut fungus, onion maggots, and all sprouted bulbs. This material is classed as "shrinkage by the onion grower. A careful analysis of this “shrinkage " into its component parts showed it to be about 60 per ct. neck rot in some instances. An examination of many of the sprouted bulbs showed that the neck-rot fungus may have been the initial or exciting cause of the sprouting in many cases, since some moisture is present after rotting takes place. In one storage house under observation the loss due to the neck-rot fungus, six weeks after the crop was placed in storage, was approximately one crate out of every five "shrinkage
” of about 20 per ct. In this particular house the loss was nearly complete by January 30.
Where the fungus attacks the bulbs thru the roots and the necks in the field the loss is due to a decrease in tonnage per acre because of a failure of the affected bulbs to produce size. The small-sized onions are classed as “seconds” and “picklers
and “picklers” and command a much lower price than the "primes."
Severe attacks of neck rot may follow frost injury to the crop early in the fall. Frosted onions are usually immature and bring considerable moisture into the storage house.
To the seed crop.- It is impossible to obtain accurate data as to the losses occurring in the seed crop since it is difficult to separate two factors which cause a failure of the seed crop, viz.: (1) Excessive moisture, which often prevents a setting of the seed heads; and (2) attacks of the neck-rot fungus, which also cause a failure of the flower heads to produce seed. Apparently, the two factors are inseparable, i. e., an attack of the fungus follows a period of excessive moisture. There is a so-called diseased condition brought about by excessive drought at the time of seed formation which is known to the onion seed growers as blast." This condition is to be differentiated from the blasting of the seed heads due to the neck-rot fungus.
A general failure of the onion seed crop in certain regions in Michigan in 1915, and in some sections of New York in 1913 is believed by the writer to be due to the attacks of this fungus. Clinton (1903, 1914) reported the Botrytis fungus as causing a yellowing of the leaves and a blasting of the onion flowers early in the season in Connecticut. Here it was reported as doing considerable injury to the flowers in moist seasons.
SIGNS OF THE DISEASE.
ON THE LEAVES.
Where infection has taken place on the leaves thru the germination of conidia one finds at first small white spots which later enlarge. The diseased spots are very frequently elongated in the same direction as the cells of the host plant. The spots may be surrounded by a yellowish area of tissue having a water-soaked appearance. This is to be distinguished from a so-called physiological disease of onion leaves known as White Tip. In this the tips of the leaves turn white, whereas the neck rot fungus causes a slight yellowing of the leaves. Sometimes infection may occur where the leaves are folded over or following severe injury by thrips. Shortly following infection, in any case, the tissue adjoining the point of infection takes on a water-soaked or boiled appearance, the green color of the leaf disappears and is replaced by a yellowish brown. Clinton (1903) reported that he found the fungus (an onion Botrytis) appearing as a parasite on the green leaves comparatively early in the .
ON THE NECKS.1 The disease occurs on the necks in the form of tufts of conidiophores bearing smoke-gray conidia (Plate XXIII, figs. 2, 3). The fungus may be readily found upon the necks of the “scallions," "bullnecks” or “thicknecks” which are put in storage. On these the fungus readily shows itself because the bulbs are not dry or mature when they go into the storage house. From the neck and outer leaves the fungus works down into the bulbs killing the tissue of the host in advance of the mycelium.
ON THE BULBS.
The neck rot is usually first brought to the notice of the onion grower by the appearance of roundish black bodies, or sclerotia,
1 The term neck as used in this paper is applied to that part of the onion plant just above the bulb, which resembles a stem, namely, the leaves which are enclosed in the sheath of the outermost leaf and extending well down to a point where the leaves become markedly thickened forming a bulb.
upon the onion bulbs in storage. The sclerotia are more frequently found at the necks of the bulbs occurring either singly or in the form of sclerotial crusts which may encircle the entire neck of the bulb (Plate XXIV, fig. 3). These sclerotial crusts at the necks may be pulled off rather easily, carrying with them the remains of the outer two or three bulb scales, or thickened bases of leaves, from which they are inseparable. These sclerotia may also be found upon the sides or bottoms of the bulbs, and often one finds a specimen with the entire side covered by a dense sclerotial crust (Plate XXIV, fig. 4).
Occasionally, a bulb will be found which shows the presence of the sclerotia between the scales. The bulb may be very soft, contain considerable liquid and give off an offensive odor. In such cases it is believed that soft-rot bacteria follow the invasion of the fungus as secondary rot producers. As a rule but very little moisture is formed following the attacks of the fungus. It is because of this fact that this disease has been called dry rot and dry neck rot by
Some bulbs, especially those from the dry locations in the storage houses, will show a smoke-gray covering of the conidia and conidiophores of the fungus in the region of the neck. This form of the fungus is very frequently found on the flat white onions shipped to the northern states from the southern onion growing regions. Other bulbs may show the presence of both the sclerotial and the conidial stage of the fungus at the same time. Upon cutting into the diseased bulbs one frequently finds the spaces between the scales filled with a mat of mycelium varying in color from a dirty white to a dark brown. On cutting open bulbs which show only a few tufts of conidiophores at the neck, the bulb scales are nearly always found to be softened and discolored in the region of the neck and may show a mat of mycelium present.
ON THE ROOTS.
The fungus has been found causing a serious root rot of the bulbs in the field. The condition brought about by the root rotting is very noticeable later when the bulbs are put in storage where the fungus appears in the form of either sclerotia or tufts of conidiophores at the bottom end of the bulb, and also by the fact that the bulbs are considerably smaller than the average run of stock. When the sclerotia are formed at the bottom end of the bulbs they are frequently in the form of a ring or crust surrounding the base of the onion. Data obtained from storage inspections show that there was more root or bottom infection during the year 1915 than in any one of the previous three seasons. The ratio of neck to bottom infections as shown in some cases in 1915 was about as 13 to 8.
ON THE SEED HEADS.
The onion Botrytis has been found fruiting on the dead leaves, scapose stems, and the base of the seed stalks in a number of onion seed fields visited. In several instances the growers complained that the seed heads had been blasted. A careful examination of these heads showed that invariably the seed stalk, or seed head, or both, bad been attacked by the neck-rot fungus. The heads showed the coarse, branching mycelium of the fungus upon and in the dead floral parts. Isolations showed it to be the onion Botrytis. A species of Cladosporium was found quite frequently upon the dead seed heads but inoculation experiments, reported elsewhere in this paper, showed the Botrytis to be the cause of the blighting or blasting of the seed heads attacked. At the base of the seed stalks where the fungus attacked the leaves the disease appeared to be more prominent where the leaf sheath clasped the stem and on old remnants of leaves adhering to the stem or seed stalk. Fig. 1, plate XXVI, shows two diseased and one healthy, normal seed heads. The head partly open shows the fungus fruiting just beneath the split in the membrane enclosing the flowers. The diseased seed head which is open shows the fungus fruiting upon the flower pedicels. Fig. 2, plate XXVI, shows a seed head which has been attacked by the neck-rot fungus (blasted). The black fungus bodies on the stalk are due to the presence of a species of Pleospora which appears after the seed stalks are dead.
INFECTION OF DRY BULBS BY DUSTING WITH DRY SPORES
During the work of going over hundreds of crates of red and yellow onions stored in various storage houses and in which the Botrytis fungus was constantly associated with the disease, in but very few instances was evidence obtained that the fungus passed from onion to onion except where the bulbs had been wounded either during harvesting or storing, or by being bruised while screening. The dry skins or scales surrounding the healthy bulbs appear
to be a barrier to infection. However, it must be conceded that infection of healthy bulbs may take place where they are covered by a mass of wet, sprouting and rotting specimens. In order to test experimentally the possibility of infection thru dry skins, large sound onions were cleaned aseptically, by carefully removing the outer dry scales with sterile forceps, then placed on crystallizing dishes in deep preservation jars kept moist by the use of wet cotton in the bottom of the jars. Some of the bulbs had one of the outer dry skins intact while on others these were removed, exposing the fresh fleshy scales. These onions were divided into two lots; on some of each lot spores in sterile distilled water were placed upon the fleshy and the dry scales, while on others the dry spores were dusted upon the scales.
the scales. Suitable numbers of each lot were left untreated as checks. Upon examination later it was found that none of the dry spores placed upon the dry, firm, fleshy scales had germinated; but in the cases in which the spores were suspended in drops of water, germination had taken place on both the dry skins and on the fleshy scales. Infection of the tissue followed on only a few of the fleshy scales. The extent of infection of the fleshy scales was especially marked where a small wound was present. The mycelium penetrated thru the fleshy scales discoloring the tissue and killing the cells. Harter (1916) reports that in his study of the storage rots of economic aroids it was found necessary to apply sterile water once or twice to the tubers and corms after inoculation with the fungi studied, but that no further applications of water were necessary after decay had once started. It was plainly evident from these experiments that drops of water, or sufficient moisture, must be present to bring about germination of the spores, and later, infection of the bulbs.
INFECTION OF GROWING PLANTS BY MEANS OF DRY SPORES.
All attempts to bring about infection of normal green and uninjured leaves and necks of onion plants by dusting dry spores over them were unsuccessful except when small drops of water were supplied, and the plants covered with bell jars for 36 hours. On very sunshiny days when only a small amount of moisture was supplied infection did not follow. In other words, infection from spores was successful only when the surrounding conditions were very