April, 1997

Fire Blight (FB) has been a perennial curse of my apples, ever since I moved to the country l9 years ago. It never hit in suburbia where ca. 600 grafted cuitivars flourished on a half-acre lot. My present planting is irregularly contoured and has ca. 500 positions, spaced 15' x 20' with a remainder of about 200 cultivars, two to eighteen years old, on a variety of rootstocks and interstems.

In 1968, France's director of agricultural research denounced FB as "without doubt the most terrible disease of pip fruits..."(10) First described in the 1780's as killing cultivated apples, pears, quinces, and related wild plants in the Hudson River region, FB has now spread to over 30 countries around the globe. America's first pomologist, William Coxe, observed in 1817 "that species of blight which is sometimes called fire blight frequently destroys trees in the fullest apparent vigour and health,

in a few hours, turning the leaves suddenly brown, as if they had passed through a hot flame, and causing a morbid matter to exude from the pores of the bark, of a black ferruginous appearance.... "(3) He blamed this devastation on an innate fixed lifespan of cultivars, a theory popularized by T. A. Knight in England. When, after many years, FB suddenly struck his 'Seckle' pear trees, Coxe believed that theory confirmed.

No other fruit tree plague has spawned so much speculation. Not only cultivar senility, also "frozen sap," "electrical charges," "sun scald," "tree borers," and "sap fermentation" had been blamed before an Illinois professor, T J. Burrill, identified it as a bacterial plant disease - the first recognized as such.

I first realized the havoc FB can cause at a NAFEX Meeting at Purdue when we were shown rows of FB-stricken 'Jonathan' trees beside rows of clean 'Delicious'. The FB-prone cultivar 'Laxton's Superb', long used as a pollinator in England, developed into a virulent source of infection until its propagation was forbidden there by law, and all existing nursery trees were destroyed or worked to less infectious cultivars.

FB infections are named after their locations. Blossom, leaf, shoot, twig, limb, trunk, collar, rootstock, interstem, and fruit FB are caused by the same bacterium, Erwinia amylovora, named after a distinguished USDA microbiologist. Its epithet means "starch-devouring." Some of FB's intergeneric relatives kill giant cacti, garrotte grafted trees with galls, cause potato blackleg, soft-rot tomatoes, and wilt corn. Other relatives attack humans: Klebsiella cause fatal pneumonia; Escherichia, Salmonella,

and Shigella live in or may invade our intestines to cause food poisoning, dysentery, or typhoid fever. A rabbit injected with FB solution usually dies.(10) Put some FB bacteria in milk and they will first curdle, then fully digest it.

Fifteen thousand FB bacteria lined up in a row measure about one inch. In a congenial ambiance, each will divide and divide and divide into about 70,000 within twenty-four hours with a diseasing pace of up to two inches in tree tissue. Live bacteria, oozing out from infected tissues, are distributed by anything that touches them: wind can carry nests of entangled bacteria many miles away. Rain, birds, people with tools, but especially insects are other carriers. Honey bees can cause mass infections, but, fortunately, FB bacteria will not overwinter in a beehive. They may stay alive for three days in the body of the green aphid which, with other aphids, ants, house and other flies, earwigs, white apple leafhoppers, and shothole borers are known vectors. Sudden outbreaks during blossom time may be brought on by heavy rain or heavy dew which dilutes the nectar sugars sufficiently for the bacteria to thrive, multiply, and be carried to neighbouring blossoms by pollinators or winds. After heavy storms, especially hail, they penetrate the foliage through soaked lenticels, hail scrapings, or wind bruises. Young fruits become mummified black miniatures that hang on into winter. Most invaders, however, have bad luck: barred by natural resistance, killed by sprays, washed away, landing on unresponsive surfaces and losing their virility there.

The first illustration shows a FB bacterium magnified ca. 20,000 times. The others date back to a pre-War Cornell Extension Bulletin(6), where the artist facilitates amusingly our grasp of FB's lifestyle!

The traditional fight against FB is two-pronged: spray the blossoms and remove infected woody parts. Streptomycin is the now-favoured pesticide against FB of apples, just as it is an antidote to FB's pathogenic relatives in our guts. After resistant FB strains emerged in California, other antibiotics, copper formulations, plant extracts, and various "new approach" compounds have been used or tested. There is hope still.

The second mode of attack calls for removal of the infected part well below the lesion site, up to 12" for limbs. For sanitizing the cut area, diluted laundry bleach is handy; my unpatented FBS-12 paint [see below] is tree-friendlier. The life of a limb or trunk with a localized lesion (often started by a water sprout) may be extended by excising it generously, and soaking the wound in embalming fluid such as thymol or formaldehyde. Painting the infected area with the "New York" mix (6) is one of the recommended alternatives. When I'm in doubt as to whether sick-looking bark expresses FB or other canker, I use Gerald Walker's test (14): "Take a sliver of wood tissue from around the margin of the canker and place it in a green apple. Incubate for a few

days at room temperature. If the fruit turns brown but remains firm, it is collar rot. If it turns soft and juicy, another type of rot is present. But if the apple turns brown and a sticky ooze comes out of the skin surface, it is Fire Blight."

Professor Anderson's book on fruit crop diseases (1) was current and useful to me when I started dabbling with fruits in a suburban backyard. Of FB he wrote: "In the regions where farm and city home orchards are distributed throughout the country it is not possible to eradicate the sources of the primary inoculum." This became clear to me later when my country orchard had me first snipping off FB-darkened crooked shoots, later cutting twigs and branches, eventually cauterizing limb lesions with a blow torch until they boiled. In some years I sprayed with Bordeaux, but an ingrained aversion to splashing antibiotics around kept me from using streptomycin, short-lived as it is said to be. One day I found myself cutting what turned out to be healthy branches - their flaky bark had been blackened by Sooty Blotch spores, or tar-based pruning paste, or both, washed down from higher branches. The colourful FBS-12 paste with bactericidal - but, alas, no systemic - impact, avoided similar mistakes.

Keeping an ear to the FB battleground and believing that, at present, "there is no eradication or absolute cure for Fire Blight...."(10), I made up my mind to lessen the constant aggravation and grubbed out every tree with FB in its wood.

Fire Blight Euthanasias and Autopsies

Alkmene

Idared

Mutsu/Crispin

Amasia

Ilzer Rose

Northern Spy

Ananas Reinette

James Grieve

Old Pearmain

Aromatic Russet

Julyred

Ontario

A. W. Barnes

Kandil Sinap

Pink Pearl

Baron Wolseley

Karin Schneider

R'tte Grise Parmentier

Blair

Karmijn de Sonnaville

R'tte du Canada

Cardinal Bea

Kalco

St. Cecilia

Chenango Strawberry

Kidd's Orange Simirenko

Sandow

Drap d'Or

King of the Pippins

Spokane Beauty

Elton Beauty

Linda

Suntan

Empire

Lindel

Tannenkrueger

Gala

Lyman's Large Summer

Tumanga

George Cave

Minister Hammerstein

Wayne

Graham's Jubilee

Monarch

Wealthy

Greensleeves

Moscow Pear

Worcester Pearmain

Golden Nugget

Mrs. Phillimore

Zabergau

Grise Pontoise

There will be more victims in my orchard, cultivars as well as replacements of already-grubbed trees.

The degree of resistance to FB can only be defined in relation to one's own planting. In current horticultural lingo, FB resistance may be expressed by a dozen or so unstandardized terms, some reflecting subjective views analogous to a glass half full or half empty. Recently introduced cultivars with "free" in their names are termed "moderately resistant to FB," or FB just remains unmentioned. The PRI program(8) has been most successful in breeding cultivars that miraculously resist scab, mildew, and cedar-apple rust, but only six out of 38 final selections are classified as "highly resistant" to FB: 'Priscilla', 'William's Pride', 'Co-Op 27, 'Co-Op 29', 'Co-Op 30', and 'Co-Op 34'. In the follow-up series, 'CRR1T73' was termed a "disease-free tree," suggesting FB immunity(?). Mature trees on seedling, MM106, or MM111 rootstocks, which never had FB infected wood in my planting, are:

Black Oxford Lady Sudeley Priscilla Blenheim Orange Laking Quinte

Borsdorfer Maigold Sinta

Crower #30 Mantet Summerred

Breakey Nova Easygro Swaar

Egremont Russet

Indeed, Blenheim Orange has been termed "immune" to FB by Crawford(4), the only such in his survey of ca. 2,700 cultivars in England. For years, the above cultivars had been exposed and invaded until the blighters were naturally stopped. In a controlled experiment (2) this occurred when a lesion had reached 16% of shoot length in 'Nova Easygro' and 81% in'Priscilla. "Choosing resistant fruit varieties, rootstocks, and interstems must be the first priority in preventing or controlling Fire Blight" is a prominent heading in (13), written by a distinguished hands-on USDA researcher. Yet, why is there only mild enthusiasm for this approach in the apple industry? A recent international conference on disease resistance heard claims that FB resistant cultivars grow too large for economic production. At a proud "open house," the Dutch Exp. Sta. at Wilhelminadorp introduced seventy-five different red mutations of 'Jonagold' which is elsewhere classified as "very susceptible"(4) and "least resistant"(13)

and has long since been discarded here.

Immense and admirable research of the FB organism has been in process for over a century, sifted and usefully presented in (12) and (13). Plant pathologists know about its lifestyle and span, procreation and invasion habits, food preferences, sensitivities to poisons, light, pH, temperature, and competitors. But has there ever been an attempt made to insert FB immunity into an apple? It would improve some rootstocks, especially M26 and Ottawa 3 as interstems, and make better winners of table cultivars such as 'Braeburn', 'Empire', 'Jonathan', 'Jonagold', 'Gala', 'Idared', 'Merton Beauty', 'Wayne', and my pink-fleshed 'Pink Lady' (renamed 'Pink Princess', Pomona, April 1998, ed.)

One reads about "genome improvement" in Dutch elm, potato, tomato, vegetables and even tobacco. If any pomological geneticist (at Purdue or elsewhere) is ready for the challenge, can he/she get any non-supply-industry-sponsored funding? Would not the membership of NAFEX want to become just such a "corporate" sponsor? Apple scab may leave a spot on an apple but Fire Blight leaves no apple. That apple can be preserved if we follow up on the "Fixing Bad Genes" method (5, p76). "Today antibiotics help, but they are no cure. The only real hope involves manipulating cells and genes."

Two useful compounds:

1. FBS 12 pruning paint

200 ml White glue (Polyvinylacetate latex)

200 ml Tangletrap, brush-on formula

200 ml Interior latex flat enamel

50 ml Iodine solution, first-aid type

50 ml 50% Potassium permanganate solution, aqueous

10 ml (optional) Paint coloring from tube.

Mix in food blender until homogeneous. Keep in squeeze bottle.

2. FB lesion-arresting solution. NY type (6)

100 g Cobalt nitrate

50 ml Glycerine

100 ml Oil of Wintergreen

50 ml Acetic Acid

800 ml Ethyl alcohol, denatured

Pour liquids together while stirring. Add cobalt nitrate and stir until dissolved. Store in tightly-stoppered bottle.

References

(1) Anderson, H. W. Diseases of Fruit Crops. NY, 1956.

(2) Bonn, W. G. and D. C. Elfving. "Response of apple and crabapple cultivars ...to Fire Blight, 1992" [Ontario]. In Biol. & Cult. Tests f. Control of Plt. Dis.8.

(3) Coxe, W. A View of the Cultivation of Fruit Trees. Rockton, Ont. 1817,1976.

(4) Crawford, M. Directory of Apple Cultivars. Agroforestry Research Trust. Totnes, UK. 1994.

(5) Drlica, K.A. Double-edged Sword. Reading, MA, l994.

(6) Hildebrand, E.M. Fire Blight and its Control. Cornell Ext.Bull. 405. 1939.

(7) Integrated pest management for apples and pears. U. of California. Publ. 3340. 1991.

(8) Janik, J.Ca. Disease-resistant apple selections and releases from the PRI program. Purdue University. 1994.

(9) Jones, D.H. Fire Blight and its eradication. OAC Bull. 342. Guelph, Ont. 1929.

(10) Ride, M. Bacterioses des arbres fruitiers. Paris. 1968.

(11) Rosen, H.R. The life history of the Fire Blight pathogen. U. of Arkansas. Bull. 244. 1929.

(12) Van der Zwet, T. and H.L. Keil. Fire Blight. A bacterial disease of rosaceous plants. USDA Ag. Hbk. 510. 1979.

(13) Van der Zwet, T. and S.V. Beer, Fire Blight - its nature, prevention, and control. USDA Ag. info Bull. 632. 1995

(14) Walker, G.M. "Workshop unveils diagnosis and treatment of Blight." In The Grower, Guelph, Ontario. May 1995.