CROWN GALL INCIDENCE IN PLANT NURSERIES OF ALGERIA, CHARACTERISTICS OF AGROBACTERIUM-TUMEFACIENS STRAINS, AND BIOLOGICAL-CONTROL OF STRAINS SENSITIVE AND RESISTANT TO AGROCIN 84

Crown gall was observed in 99% of the plant nurseries surveyed in the major fruit tree production region of Algeria. Among 11 tree species surveyed, those with highest disease incidence were peach (5.39%), almond (3.43%), cherry (1.50%), apple (1.47%), and olive (1.30%). None of the quince and fig trees inspected had crown gall. Significant differences in the frequency of galled plants were correlated with the rootstock used by nurserymen for peach, cherry, apple and pear trees. Analysis of 60 galled samples, which yielded 154 Agrobacterium tumefaciens strains, revealed that biovar 2 and agrocin 84-sensitive strains were predominant among local agrobacteria. The ratio of agrocin-sensitive to agrocin-resistant was more than 3 times higher for biovar 2 (6:1) than for biovar 1 (1.67:1) strains; thus, suggesting that biovar 2 strains are most likely to harbor a nopaline/agrocinopine A-type Ti-plasmid. Biological control of crown gall was achieved in a field experiment in which peach 'Missour' seedlings were dipped first in a suspension of A radiobacter K84 and subsequently in a suspension of a local agrocin-sensitive strain. In the same experiment, K84 reduced the incidence of galling by 94% in seedlings inoculated with a local agrocin-resistant A tumefaciens strain. These data indicate that K84 could be used in Algeria despite the presence of agrocin- resistant strains.