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Fruit Varieties Journal
(Fruit Var J)

American Pomological Society

Volume 30 Number 1 Article 4 Pages: 8-9
Year 1976 Month 1
Title: Greenhouse Screening of Pear Seedling for Fire Blight Resistance
Authors: T. van der Zwet and W.R. Zook
During the past 5 years, many pear seedlings have been screened for fire blight resistance, in order to reduce losses in the field due to natural infection. The method mvolves a combination of .growing large numbers of seedlings in the greenhouse, the application of a rapid inoculation device, and the use of a simple humidity chamber to ensure optimum blight infection. Seed from controlled pollinations is collected in September, stratified for about 3 months at 5C, and planted in jiffy trays (containing 5.5 cm pots in strips of 12) in early January in the greenhouse. With these trays, about 360 plants can be grown on each m2 of greenhouse bench. After about 100 days, the plants will have an average height of 20-40 cm and are inoculated wIth the blight pathogen. The inoculation device consists of two aluminum bars connected near one end with a hinge and separated in the center by a spring. Near the open end of one bar is a small circular well containing a florist pin holder and the opposite bar contains a small sponge. This well is connected toa bottle of inoculum by means of a plastic hose fitted with a regulating valve. For adequate flow of the inouculum, the bottle is carried above shoulder height on a backpack made of aluminium tubing and rubber hose. Tips of succulent terminals of the seedlings are pinched twince between the pins and the sponge, thus releasing the inoculum onto injured plant tissue. The sponge is kept dripping wet to provide suffieient inoculum for infection. One culture of a virulent strain of Erwinia amylovora, grown on nutrient-yeast dextrose agar slants for 24 hours at 26C, is used to prepare the inoculum, which contains about 5 x 107 cells ml.
To maintain optimum environmental conditions for blight development for large lots of seedlings, a tent-like structure of 6 ml plastic sheeting over a wood and wire frame is used in an portion of the greenhouse. The enclosed areas contained approximately 38 m2 of bench space which is adequate for about 13,000 seedlings. Following inoculation, the open tent side is closed with plastic, two humidifiers plus wet floor provide a RH of 85-100% within the structure, and the temperature is maintained at 21-27C.
After about one month, blight has taken its toll and resistant seedlings (0-25% of seedling blighted) can be selected and moved to nursery or field plantings. With our screening technique, one liter of inoculum is sufficient to inoculate 300 plants and about 4,500 plants can be inoculated rapidly and uniformly per day.


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