pass on to the next tile. The water entering the tiles finds its way to the soil through the openings between them and gradually works down the slope toward the next line of tiles. During rainy seasons the wells are closed down, the check gates and overflows all removed, and the irrigation system is made to serve as a means of draining the land. A plan of this system is shown in Fig. 30.

Temperature of Water for Celery Irrigation.As celery thrives best under cool conditions it is desirable to use water at a rather low temperature for irrigation. Warm, stagnant water, that has lain. for days exposed to the heat of the sun, should not be used for irrigating celery. Water that is pumped direct from wells and which has a natural temperature of from 50 to 60 degrees is preferable.

CHAPTER IX

Diseases of Celery

IN the production of a small quantity of celery for home use, plant diseases are not liable to cause serious difficulty, but on a large scale where many thousands of plants are grown together the chance of infection is greater. In the commercial production of celery it is necessary to observe every possible precaution to avoid loss from disease. Climatic conditions play an important part in the control of celery diseases. Owing to prevailing atmospheric conditions, which favor the spread and development of diseases, certain localities are not suited to the production of celery, although the soil and marketing facilities may be ideal. In regions where celery can ordinarily be grown without loss from diseases there will occasionally be a season during which a large portion of the crop will be affected. A warm, humid atmosphere with sultry nights is liable to develop diseases, but cool nights with clear atmosphere and bright sunshine during the day furnishes ideal conditions for holding diseases in check. The self-blanching varieties are more subject to the attacks of diseases than the stronger-growing green kinds.

Damping.-Immediately after the seedlings appear they are subject to attack from the disease known as "damping." which causes the stem and

root to decay near the surface of the soil. Damping may be attributed to any of the following fungi: Sclerotinia libertiana, Rhizoctonia, and Fusarium. The greatest liability of injury from this source is during the first two weeks after the seedlings appear. Plenty of light, care in watering, and thorough ventilation are the only satisfactory methods of preventing loss from damping. If the plant bed is in the open ground it may be necessary to shelter it from continuous rains, and a light dusting with dry Bordeaux mixture may prove beneficial. As a precaution against damping, seedlings that are in trays may be subwatered by setting each tray for a few minutes in a shallow trough, allowing the water to enter the bottom of the tray through the drainage holes and moistening the soil without wetting the surface.

Blight1. The disease of celery known as "blight" is caused by Cercospora apii, which attacks the leaves of the plants, appearing as grayish spots which turn brown or to a burned appearance within a few days. The commercial celery crop is greatly injured by the blight and the disease is liable to appear at any time after the plants are set in the open field. The blight generally appears first upon the outside leaves and rapidly spreads until the whole plant is infected. Warm days and nights with a high degree of moisture in the atmosphere are conditions suitable to the development of blight. Cool nights and a dry atmosphere will generally

1 U. S. Dept. of Agr., Farmers' Bul. No. 148.

check the attack, and the plants will sometimes outgrow the disease, but the product will not be firstclass. When the blight has become well established it is then too late for effective remedial measures,

FIG. 31-LEAF AFFECTED WITH CELERY BLIGHT (Cercospora apii) (Redrawn from Halsted, N. J. Expr. Sta. Special Bul. 1902)

and while its progress may be checked it is doubtful if it can be eradicated. If the plants are kept growing vigorously from the very start, and proper cultivation maintained throughout the growing season, there will be little danger from blight.

Another form of leaf blight, and which is commonly confused with the ordinary blight, is caused by the fungus known as Septoria petroselini1 var.

FIG. 32-CELERY LEAF AFFECTED WITH A BLIGHT

(Septoria petroselini var. apii)

(Redrawn from Halsted)

apii. In general appearance this disease is very similar to the ordinary blight, and the treatment is the same in either case.

Leaf Spot. This disease, while similar to the blight in its action, is somewhat different in general 1 N. Y. Expr. Sta. Bul. No. 51.