CHAPTER IX.

MILDEW OF ONIONS.

Peronospora Schleideniana, Ung.

ONE of the best known fungus pests of Onions is the dreaded onion mildew caused by the fungus named Peronospora Schleideniana, Ung. As its name indicates, it is a close ally of the fungi of clover mildew, Peronospora trifoliorum, D.By., and P. exigua, W.Sm., already described, and of the fungus of the potato disease, Peronospora infestans, Mont.

Peronospora Schleideniana, Ung., is named in honour of the botanist Schleiden. The fungus is illustrated, enlarged 200 diameters, at Fig. 15; it is shown as growing from the base of a leaf near the collar of the bulb; at this position there is but little leaf-green or chlorophyll in the cells of the leaves. The spawn or mycelium of the fungus ramifies amongst the loose cells of the leaf and sets up decomposition in its progress. At Fig. 15, A, the threads of the fungus may be seen emerging through an organ of transpiration into the air. The minute openings, "mouths," organs of transpiration or stomata, occur abundantly on most plants, generally on the under surface of the leaves, but also in various other positions. The general habit of the different members of the genus Peronospora is to grow within the leaves and stems, and send their fruiting branches through the stomata into the air.

This habit is fatal to the growth of the host plant, for the spawn not only causes putrefaction of the inner cells of the leaf and stem by contact, but the fertile threads choke up the organs of transpiration and prevent

Peronospora Schleideniana, Ung. Enlarged 200 diameters; resting-spores, 400 diameters; single spore, 1000 diameters.

the evaporation of water in the form of vapour from the plant attacked. So potent are the different members of the genus for evil that their spawn threads are capable of pushing aside the plant-cells of the plants attacked either from without or within. The mycelium of some species has also the power of piercing through the cell-walls and traversing the interior of a plant, not by creeping between the cells, but by breaking down the cell-walls in its progress. Putrefaction attends the whole progress of growth of the invading Peronospora. The complete treelike habit of the fungus of onion mildew is shown in Fig. 15. It will be noticed that the fungus repeatedly branches and rebranches, and at the ends of all the minor branchlets the ovate spores or acrospores termed conidia are borne. These are shown at B, and a single conidium is farther enlarged to 1000 diameters at C. The conidia are pale gray or pale lavender in colour, and are very large in comparison with the conidia belonging to other species of the genus Peronospora. At the time of germination the spores usually burst at the side. At Fig. 16 part of the Peronospora is enlarged to 400 diameters, so that the different illustrations of the genus in this work may be presented uniform in size. The difference in size between Peronospora Schleideniana, Ung., Fig. 16, and P. exigua, W.Sm., Fig. 2, is very great. From the irregular mass of protoplasm exuded at the time of germination mycelial threads and fruiting branches quickly arise. The fungi of clover mildew are transparent and almost colourless in all their parts, whereas the onion fungus is more or less tinted with a pale reddish-gray, a brownish, or a dull violet hue throughout. This is especially noticeable in the comparatively large conidia. The Rev. J. E. Vize, of Forden, Welshpool, has found the resting-spores of this species in decaying patches upon onions, where the Peronospora in its conidiumbearing state previously existed. The oogonium, or unimpregnated oospore, or resting-spore, is shown at D (Fig. 15),

FIG. 16.-MILDEW OF ONIONS.

Peronospora Schleideniana, Ung. Enlarged 400 diameters.

and mature fertilised oospores or resting-spores at E and F, enlarged 400 diameters. These bodies carry on the life of the fungus in a hibernating state during the winter

months. Their mode of formation is described under Peronospora infestans, Mont.

Peronospora Schleideniana, Ung., is specially common on the garden onion, Allium Cepa, L., and on the Rock, Welsh onion, or stone leek, named Allium fistulosum, L. The latter plant does not form a true bulb, but is sown to form small green onions for spring salads. The fungus attacks numerous other species of Allium, and is even suspected on lilies and amaryllids. It is sometimes extremely common and virulent, attacking the onions early in the season, and so preventing the bulbs from reaching perfection. The chief point of attack is the leaves in an early stage of growth, and to such an extent is the work of destruction sometimes carried on that the entire onion plant may be seen covered with one mass of whitish-gray semi-transparent bloom. In bad cases the onions are left as one offensive mass of putrescence.

One form of mildew is caused by the attack of a Fusisporium named F. atrovirens, Berk. This fungus is not unlike F. Solani, Mart., already described, but the spores are grayish-green or greenish-black in colour and more decidedly curved. The disease begins with the exhibition of grayish gelatinous spots, which speedily become confluent. The Fusisporium often accompanies the Peronospora. Both mildews materially affect the crop of seed. Practical growers say mildews are favoured by an extremely dry season, as in the mildew of peas and roses, and also by a wet and cold season, as well as by bad cultivation. Deep trenching is generally advocated for the prevention of onion mildew.

A good plan for the avoidance of mildew in onions is to sow the seeds in autumn; by following this plan the onions are able to make good strong growths before the appearance of the mildew in the following spring. This is perhaps the only reliable plan for obtaining sound onions of a large size in districts subject to attacks of mildew. Autumn sowing is advocated by onion growers.

E