Blights and Other Ills of Celery

A. G. Newhall.

Celery is grown extensively as a truck crop on the muck lands and irrigated mineral soils of many States from Florida to Massachusetts, the Great Lakes States, and some of the Mountain and West Coast States. It is grown also as a market garden crop near many large population centers. It has an annual value of more than 50 million dollars.

In some districts celery has been grown intensively for nearly a century. The importation of much of our seed from Europe before 1920 and the free exchange of seeds and plants within the States at all times have meant that there are few if any celery diseases that we have not acquired. Appropriate control measures have been developed for many of them by Federal and State agencies and seed growers.

THE MOST WIDELY DISTRIBUTED and costly diseases of celery are the early and late blights, caused by the fungi Cercospora apii and Septoria apii var. graveolentis, and the rather minor bacterial leaf spot, caused by Pseudomonas apii.

Cercospora may be seed-borne, hence early blight occurs almost everywhere that celery can be grown. Because it requires hot weather for its most rapid development, the fungus is most troublesome on the early summer crops in the Northeastern and Great Lakes States. In Florida it may occur in the seedbeds in October and do some damage all winter, but the greatest losses occur on the late winter crop, which matures during the warm weather of March to May. It is checked by periods of cool weather below 40 F. Losses are due to stunting of growth, the necessity for heavy stripping of diseased stalks at harvest, and poorer keeping and market quality.

The fungus overwinters readily on debris from a previous crop. Early blight first appears on seedlings in the plant bed or on transplants in the field as small, pale-green or yellow spots a week after inoculation with the spores of Cercospora. The spots enlarge and often envelop much of the leaf. They turn brown to slate gray as the fungus fructifies on the lower surfaces of the leaves by pushing spore-bearing conidiophores through the stomatal openings. Spores are produced on the upper leaf surface also or on plant debris left on the ground. The spores, when abundant, give a delicate gray or pale-lavender sheen to the affected areas. Sunken, tan-colored, elongated spots may occur on the stalks just before harvest that require heavy trimming and loss of edible product.

L. J. Klotz, at the Michigan Agricultural Experiment Station, found the best temperature for growth and germination of spores of Cercospora apii to be about 70 F. Spores survived desiccation on dried leaves more than 170 days. They are well adapted to dissemination by air and can infect floral parts and grow into the seed coat. When the seed germinates, the fungus can attack the young cotyledons and from them pass to other leaflets. The life cycle takes 10 or 15 days. J. D. Wilson and I, working at the Ohio Agricultural Experiment Station, showed that the longer plants are left in the crowded seedbeds, the worse blight is apt to be later in the field.

An important leaf spot of carrot also is caused by a Cercospora, but it is nonpathogenic to celery as is the celery pathogen to carrot.

LATE BLIGHT, caused by the fungus Septoria apii var. graveolentis, can cause even more destruction than early blight in cool, wet seasons and on the later crop in the Northern States. It can attack any part of the plant above ground. As outer leaves and stalks turn dark and wither, the entire field may look scorched.

The fungus is seed-borne. It also overwinters on debris from a previous crop. It may get started in the seedbed where it forms small, circular, water-soaked spots on the leaves about one-sixteenth inch in diameter. In 10 or 20 days, the spots turn nearly black and become filled with many minute black dots, the fruiting bodies (pycnidia) of the fungus. Spores are formed in these closed, black, pear-shaped cups partly embedded in the plant. They are exuded during wet weather as gelatinous, snakelike tendrils, and require spattering raindrops rather than air currents for quick spread.

When celery is wet with dew, the clothes of workers can spread the fungus down the row. K. H. Lin, at the New York State College of Agriculture, found that the number of spores in a single pycnidium a structure no larger than the dot over this i varied between 1,448 and 5,493, with a mean for nine pycnidia of 3675. Dr. Wilson and I found more than 2,000 spots on untreated plants, with an average of 56 pycnidia per spot; potentially, therefore, half a billion spores can be produced on one plant.

Germinating Septoria spores can penetrate directly through the epidermis as well as through stomata and as readily on the upper surface of leaves as on the lower surface, although there are only about one-third as many stomata on the upper surface.

THE BACTERIAL LEAF SPOT is caused by a soil-inhabiting bacterium, Pseudomonas apii. In the Lake States and New Jersey it sometimes gives trouble on outer leaves in hot, humid weather. It makes small, circular, rusty-reddish-brown spots up to one-eighth inch in diameter; sometimes they have pale-yellow borders. The spots remain smaller than early blight spots and are a darker brown. They differ from septoria late blight in lacking the black pycnidia within.

The disease is not seed-borne but it often gets its start in the seedbed. It is most troublesome on the crop maturing in August and September. It was the first bacterial plant disease to be controlled by dusting with a fungicide, in 1922.

CONTROL MEASURES for all three celery leaf spots are practically the same. In the absence of resistant varieties, growers have relied heavily upon the use of copper fungicides in the field ever since the work of B. D. Halsted in New Jersey in 1891. No fungicide has exceeded bordeaux mixture in effectiveness, but the residues it leaves and the inconvenience of preparing it (compared with the low-soluble coppers, such as basic copper sulfate, copper oxide, and copper oxichloride, and the organic fungicides) have led more and more growers to abandon bordeaux.

The use of disease-free seed and a 2- or 3-year rotation, to eliminate the two important sources of primary inoculum, have helped many growers. Because the Septoria fungus embedded in the seed coat usually dies within 2 years and the seed retains its vitality 3 to 6 years, many growers buy their seed in advance or ask for 2-year-old seed when it is available.

In Bermuda the blight problem was greatly reduced by microscopic examination of samples of all imported seed and rejection at port of entry of all seed lots showing pycnidia. A free examination service offered at Cornell University to New York farmers similarly aided growers.

Fresh seed can be treated in various ways to kill the pathogens that cause early and late blights: A dip in hot water for 30 minutes at 118 to 120 F.; a dip in formaldehyde solution (1 to 300) for 3 hours at room temperature, followed by a rinse; a preliminary soak in tepid water for 30 minutes, followed by a dip in mercury bichloride solution (1 to 1,000) for 5 minutes and by a 15-minute rinse. The hot-water treatment is the best.

Dusting or spraying seedbeds with a copper fungicide two to four times (first practiced by growers in New York and Ohio) is an economical and effective way of reducing and delaying the onset of all three blights. It has been widely used.

If treatment of seed and seedbeds does not eliminate the blights, lack of rotation might be to blame. Growers who cannot rotate crops can hold the diseases in check by spraying or dusting in the field.

Local practices respecting materials and methods of field spraying and dusting vary a great deal. They depend upon variations in climate, on chances of losses at different times of the year, on growers' preferences, and on the extent of local experimental testing of the newer fungicides.

Bacterial blight can be controlled with a dust of 20 parts copper and 80 parts lime. That was a standard treatment among muck growers in New York between 1924 and 1935. But its high lime content and the need to apply it when plants are wet has led to its gradual abandonment in favor of the low-soluble coppers. The modern trend is toward materials that do not clog nozzles very much and leave no unsightly residues. This change was made possible by general adoption of the seed and seedbed practices I mentioned, which give better control of blights in the seedbed and hence a reduced amount of blight in the field. The low-soluble coppers have given fair satisfaction where disease potential was not too high, although the rate at which they wash and weather off has made it wise to shorten the interval between applications to 5 or 6 days on many farms, even to 3 or 4 days in southern Florida.

Heavy fertilization, the use of mulches between rows, side dressing with nitrogen, and adequate irrigation to keep celery growing rapidly are enough in some cool seasons up north to give growers a satisfactory crop despite early blight. But those practices are not so reliable in a hot, dry summer, on the early crop maturing in August, or on the late crop in Florida, which matures in April when temperatures are rising.

Nabam has been widely used in the Everglades in Florida. Ferbam, ziram, and the coppers have proved more suitable to the upland soils of the Sanford area.

In California the most satisfactory and inexpensive program against late blight includes spraying with 3-3-50 bordeaux mixture (3 pounds copper sulfate, 3 pounds lime, 50 gallons of water); in some localities zineb and the fixed coppers are preferred even though they adhere less well to the plant.

New York growers are giving up copper lime dust for the low-soluble coppers. A few use Dithane in liquid form. In most years control is satisfactory, but in some seasons many have been urged to go back to bordeaux mixture or to an alternate schedule of bordeaux and ziram, zineb, or captan.

In Massachusetts growers prefer a low-soluble copper to bordeaux, but the organics, zineb and ziram, if applied more often, are finding favor from the standpoint of safety and effectiveness. Zineb is generally considered more effective than ziram against Septoria.

Growers in Colorado have started using zineb sprays. Ziram and tribasic copper sprays have been favored in Oregon.

Extensive tests of fungicides in Michigan culminated in the development of a yellow cuprocide-sulfur-talc dust, 7-30-63, which is easier to use than liquid bordeaux, possesses less lime than 20-80 copper-lime dust, is more lasting than other low-soluble copper dusts, and has better flowing and keeping qualities.