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
Seed from controlled pollinations is
collected in September, stratified for about 3 months at 5°C, and planted
in jiffy trays (containing 5.5 cm pots
in strips of 12) in early January in
With these trays, about 360 plants can be grown on
each m2 of greenhouse bench.
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 26°C, 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 21-27°C.
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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