Apple scab, one of the most important diseases of apple worldwide, is controlled by applying chemical fungicides at appropriate intervals during infection periods, but this fungus has developed resistance to some key fungicides. Past research on biological control of scab has focused on interrupting the overwintering stage of the fungus by applying microorganisms to fallen infected leaves on the orchard floor. Could microorganisms applied to growing leaves on trees prevent infection and development of symptoms?
Preliminary screening of 931 bacteria and yeasts isolated from soil and apple tissues collected from apple orchards in upstate New York show that many can significantly reduce the amount of apple scab developing on test apple seedlings. The isolates were first tested in petri plates for their ability to inhibit germination of apple scab spores. About a third of the isolates (92 successful ones and 268 that did not inhibit scab) were then tested for their ability to control scab on 3-week-old McIntosh apple seedlings in the greenhouse. The seedlings were sprayed with suspensions of the isolates, inoculated with apple scab spores after 24 hours, and rated for scab severity 2 weeks later.
Thirty-two of the 931 isolates prevented germination of apple scab spores in the plate tests, and another 135 inhibited growth of the spores' germ tubes. On the trees, 106 of the 365 isolates tested caused significant reduction in scab. However, only 21 suppressed scab by at least 30%. There was no correlation between the plate test results and ability to suppress disease on the leaves.
One bacterium, later identified as Pseudomonas syringae (isolate 508) prevented spore germination and effectively suppressed scab to a level comparable to that provided by the fungicide captan. This isolate provided greater suppression of apple scab than did other pathogenic isolates of P. syringae isolated from various crops. However, much additional research on isolate 508's ability to survive on apple leaves in the field and provide control in orchards is necessary before this microorganism can be developed for commercial use.
Although 508 was the most effective biocontrol agent identified, at least 4 other isolates showed promise despite the fact that none of them inhibited germination or growth of the apple scab fungus in plate tests. Testing will continue on these promising biocontrol agents for their effectiveness in suppressing apple scab.
Burr, T. J., M.C. Matteson, C.A. Smith, M. R. Corral-Garcia, and T-C. Huang. 1996. Effectiveness of bacteria and yeasts from apple orchards as biological control agents of apple scab. Biol. Control 6:151-157.
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