Individual and combined effects of Enterobacter aerogenes and metalaxyl on apple tree growth and phytophthora crown and root rot symptom development

Apple ( Malus domestica) trees in four different commercial orchards located in the southern interior of British Columbia were treated with Enlerobacter aerogenes (strain B8), metalaxyl, or both over 2 consecutive yr. Phytophthora cactorum was isolated from the crown and root of trees showing premature senescence of leaves in late summer and early fall, symptoms characteristic of Phytophthora crown and root rot. These isolates killed 40–50% of inoculated apple seedlings in greenhouse experiments. Annual increase in trunk diameter, averaged over the test orchards and over 3 yr starting at the beginning of treatment, was 10% greater in trees treated with E. aerogenes than in control trees. The combination of E. aerogenes and metalaxyl, l g per tree, each applied once a year, showed a trend similar to E. aerogenes alone ( P = 0.07). This combination treatment also reduced expression of visual symptoms of crown and root infected trees. Leaf tissue analysis showed that in untreated trees, magnesium was at a lower concentration and iron at a higher concentration than in treated trees. Leaves of healthy trees without visual symptoms had a higher concentration of copper and a lower concentration of boron.