BREEDING FOR RESISTANCE to celery blight has been made possible by the use of blight-resistant foreign plants brought in by the Eastern States Farmers' Exchange and the Department of Agriculture. The first was a hollow-stalked Danish celery, which was crossed in 1937 with Golden Self Blanching and given to plant breeders at Cornell University, who crossed it to their new yellows-resistant variety, Cornell 19, in 1940. Progenies were later crossed with Cornell 6 to improve quality. Further testing and selecting in New York and Florida resulted in the release in 1951 of Emerson Pascal, the first celery variety fairly resistant to early and late blights and highly resistant to fusarium yellows.

In 1940 and 1941, G. R. Townsend, then a pathologist at the Florida Everglades Experiment Station, discovered Cercospora resistance in a number of plants grown from seed that was brought from Turkey by the Department of Agriculture. The plants resembled celeriac. Because he was unable to induce them to set seed, he shipped seven plants to Cornell, where R. A. Emerson, after two more years, finally induced two of them to flower. Emerson made reciprocal crosses between Cornell 19 and P. E. I. (for Plant Exploration and Introduction, Department of Agriculture) 115557. Workers began a program whereby progenies highly resistant to both blights were being selected from that cross in Florida during the winters and in New York during the summers. As celery is a biennial, progress has been slow, but facts of value to vegetable breeders have emerged: Blight resistance is due to more than one gene. Hollow stalk and green color are each dominant in the F₁ generation and are governed by a single factor, and hence are easily eliminated. Susceptibility to black heart and to magnesium deficiency in the soil are both hereditary factors that can be eliminated by breeding.

THE YELLOWS DISEASE was next in importance to the leaf spots in most of the Northern States until resistant varieties were discovered and developed in Michigan, New York, and California. Ray Nelson, G. H. Coons, and L. C. Cochran, at Michigan Agricultural Experiment Station, found there are three distinct diseases in the sense that three separate strains of Fusarium may be involved. Stunting of the plant, vascular discoloration, and crown and root rotting are common to all three, but leaf symptoms clearly distinguish two of them. Hot, dry weather is necessary to bring on a full expression of yellows. When the weather turns cool and wet, even infected plants may recover partly.

The first symptom, a lagging in the growth rate, is sometimes seen even in the seedbed. In the field, plants lose their glossy appearance and there is a yellowing of the outer leaves between the veins. Fusarium apii form 1 is responsible for those symptoms. When temperatures are high, the entire plant, if not more than half-grown, may turn yellow in a few days. Often leaves of an infected plant when about to turn yellow become brittle. When they are crushed in the hands, they crackle like dead twigs. That is particularly true of green varieties, which usually never turn yellow; that and stunting may be the only visible indication above ground of the disease in such varieties. Petioles of affected plants often develop a bitter taste.

In the second type of yellows the earliest symptoms are a downward curling of the young heart leaves. Then the veins lose color. In this type Fusarium apii var. pallidum or form 2 is at work, and the areas between the veins are the last to turn yellow. Stunting, root rotting, and browning of the vascular elements in the roots accompany above-ground symptoms in both instances.

The California variation of the yellows complex lacks symptoms of leaf yellowing, curling, and brittleness even in warm weather. It is primarily a. stunting, with secondary root rotting and primary vascular discoloration. The identity of the Fusarium that is responsible seems not to have been established. But it differs from the other two, which are more common causes of yellows in the Northeast.

In warm soil (77 to 85 F.), the symptoms below ground include brown discoloration and death of small secondary roots, vascular discoloration and dry rotting of the taproot and crown, and even splitting of the latter. Many plants die.

The fungi responsible for fusarium yellows have been isolated from vascular tissue of all parts of affected plants from lower roots to upper leaf stalks. Fusarium apii form 1 develops a pink to purple color on steamed rice, while F. apii var. pallidum or form 2 remains colorless. Some writers put both to-ether under the name F. oxysporum form apii, but not this one. The fungi live in the soil for many years even in the absence of celery.

Green varieties generally are much less susceptible. Many of them can be grown on infested land without fear of much loss. There is also a difference in susceptibility of different strains of the same variety as well as a difference in apparent behavior of one variety in different seasons. The most extensive testing of varieties has been done in Michigan by Ray Nelson and L. C. Cochran and in Ohio by J. D. Wilson.

The following lists of varieties resistant and susceptible to yellows are based partly on their findings:

Yellow Varieties More or less resistant: Michigan Golden, Michigan Green Gold, Cornell 19, Cornell 6, Morse's Masterpiece, Florida Golden, Tall Golden Pascal, Plume, Golden 99, Golden Pascal, Emerson Pascal, and some strains of Wonderful.

Moderately susceptible: Wonderful, Golden Plume (some strains), Kilgore's Pride, Golden Prize, Kilgore's Pearl Special, Sneck's Florida Golden, Early Fortune, Superplane, and Paris Golden.

Very susceptible: Golden Self Blanching, Wonderful (some strains), Early Fortune (some strains), Meisch's Special, Hoover's Special, Gunson's Special, Golden Phenomenal, Golden 14, Golden Detroit, and Golden Plume (some strains).

Green Varieties Highly resistant: Curly Leaf Easy Blanching, Pride of the Market, Full-heart Easy Blanching, Winter King, Autumn King, Woodruff's Beauty, Sweetheart, Crispheart, Krispgreen, Holmes' Crisp, Earligreen, and Newark Market.

Moderately susceptible: Winter King, Utah, Fordhook, Columbia, Epicure, Pascal, Emperor, Newark Market, Crystal jumbo, and Winter Queen.

Very susceptible: White Plume, Houser, and Paragon.

Some varieties are classified as resistant at one time or by one worker and susceptible at another time or by another worker. T. C. Ryker, who grew plants in infested soils held at different temperatures at the University of Wisconsin, pointed out a possible reason for this discrepancy. He found some varieties or strains of a variety are resistant in soils up to a temperature of 79 F., above which they are susceptible. Early planting in the Northern States therefore enables a fairly susceptible variety to make good growth before soil temperatures become high enough to induce fungus infection. In wet, cool seasons infection may be delayed until the crop is made.

CONTROL OF CELERY YELLOWS has been accomplished in Michigan by the selection of individual plants that grew well in fields, where nearly everything else died of the disease, and propagating directly from them. That was done between 1919 and 1926 with selections from a field of Dwarf Golden Self Blanching. The variety released was named Michigan Golden. A similar process of field selection and multiplication from a field of Tall Golden Self Blanching resulted in the Michigan Golden Tall strain between 1930 and 1933. Both were yellow varieties, but in 1951 a green variety, Michigan State Green Gold, was released. It was the result of hybridizing the Downing Strain of Fordhook with the tall strain of Michigan Golden. It has a light-green stalk and is widely grown in Michigan and elsewhere.

At Cornell University in 1933 Swarn Singh crossed Utah, a moderately resistant green variety, and Golden Self Blanching, a popular susceptible yellow one. His work culminated in the development of two new varieties, Cornell 19 and Cornell 6, both self-blanching and highly resistant to the eastern strains of yellows. By crossing the blight-resistant strain described under early blight with each of these in succession, New York workers developed Emerson Pascal, a variety resistant to both diseases. It was put on the market in 1951.

PHOBIA ROOT ROT has occurred at times in the Northeast. The fungus that causes it, Phoma apiicola, can attack other members of the Umbelliferae, such as carrot, parsnip, parsley, and caraway, but not hemlock or dill. It can live over in the soil on plant debris and occasionally on the seed.

Its first appearance often is in the seedbed, where it causes stunting, yellowing of outer leaves, brown rotting of roots, and sometimes death of young plants. The fungus fruits on the roots and crown; spores from a diseased seedling thus can be spread at transplanting time to many other plants if they are pulled and soaked in water before they are set in the field. The fungus usually is confined to the crown, but sometimes it grows up into the outer leaf stalks far enough to turn them a dark bluish-green color and cause them to break over. Crown lesions take on a dark, brown, rough, scurfy appearance and frequently crack open in later stages. Plants may be killed in the field, although usually they linger on.

The fungus prefers a comparatively low temperature, 61 to 65 F., and requires oxygen and moisture for its most rapid development. Therefore it causes trouble principally on the spring crop and sometimes the late fall crop when temperatures and moisture are most favorable.

The pycnidia of the fungus, filled with very small one-celled spores, may be found partly embedded in the root lesions at any time. The fungus can occasionally become embedded in the seed coat, and it has been introduced by this means into new territories where it caused bad local seedling infections. Free spores once extruded from their pycnidia, however, cannot survive more than 30 days on the surface of seeds at room temperature.

No varieties are known to be resistant, although White Plume, Giant Pascal, and Easy Bleaching seem less susceptible. The fungus is not adapted to the climate of California, where most of our seed is grown. It has not been found in Florida, where much of our winter celery is grown. Losses can be kept down by treating infected seed with hot water, sterilizing infested seedbed soil, rotating crops, and destroying plants that show symptoms.