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FIREBLIGHT SUSCEPTIBILITY OF APPLE ROOTSTOCKS: AN EXAMINATION OF TEST METHODS
Although blossoms are the major avenue of infection in the field, screening tests rely heavily’ on the use of young shoots because they are easier to produce and handle in large numbers and such shoots were used in these tests.
Throughout our experiments we have used a single highly virulent strain of Erwinia amylovora.
Shoots of a variety of rootstocks were inoculated via undamaged folded leaves in the shoot tip, via cut just-unfolding leaves or petioles of such leaves, or by needle inoculation of the stem just above such a leaf.
All tests were made in a greenhouse with cooling units keeping the maximum temperature below 24°C. The minimum temperature was usually between 10 and 15°C. Plants were shaded from direct sunlight.
Prior to testing, stocks were brought on in an uncooled greenhouse and the pre-inoculation conditions (temperature and light) were more weather dependent.
The effect of pre-inoculation growth conditions was examined by holding plants in growth rooms at two day/night temperature regimes (15/15°C and 21/15°C) with a day length (with full light) of 16 h.
The results showed that a highly susceptible rootstock such as M26 was readily infected under all test conditions and that infection could also be obtained when older leaves or mature twigs were inoculated, though symptom expression was delayed.
With less susceptible rootstocks, only young stem inoculation was followed by progressive infection.
Some less susceptible stocks (e.g.
M25 and MM109), when grown under the 21/15°C regime before inoculation, were more severely infected than those grown under the 15/15°C regime.