Inhibition of fireblight disease by injection of D-galactose or L-fucose into susceptible apple plants

Abstract

The colonization of 1-year-old apple plants (cv. M26) by Erwinia amylovora (Burrill) Winslow was arrested by the administration of D-galactose or L-fucose solution to the plants. Treatment started 24 h before inoculation of shoot tips with bacteria, and was continued for 8–14 days. Solutions were administered via three debladed leaf midribs adjacent to the site of inoculation, or by gravity injection into the woody stem. Inoculated plants taking up 0·6% w v D-galactose solution via the petioles (mean rate per plant 3·8 ml day −1 for 8 days) showed inhibition of disease. Similar uptake of water, 0·6% w v sucrose, D-mannose or sorbitol had no effect. Treatment with 0·6% w v D-galactose was not effective when uptake per plant was less than approximately 2 ml day −1, at which rate 0·6% w v L-fucose inhibited the disease. Stem-injection of 1·2% w v D-galactose (mean rate per plant 12 ml day −1 for 4 days) inhibited the disease but caused abscission of the lower mature leaves and immature leaf curl. Stem-injection of 1·2% w v D-galactose (mean 10·4 ml per plant) for 24 h followed by continuous stem-injection of 0·6% w v D-galactose (mean rate per plant 5·7 ml day −1 for 5 days) also inhibited the disease and did not cause visible leaf reactions. Simultaneous stem and petiole injection of 0·6% w v L-fucose (mean total rate per plant 4·9 ml day −1 for 4 days) similarly inhibited the disease without causing visible leaf damage. Treated plants showing disease inhibition developed significantly more shoots from lateral buds than did untreated inoculated plants. Very little or no bacterial ooze was exuded by inoculated plants successfully treated with D-galactose of L-fucose, but D-galactose, D-glucose and L-fucose allowed the synthesis of extracellular polysaccharide in bacterial cultures containing sorbitol, a host-plant carbohydrate. Heavily capsulated cells similar to those found in infected plants were produced in cultures containing D-galactose. Bacteria grown in the presence of L-fucose were only moderately capsulated. The outer surface component lipopolysaccharide produced by E. amylovora contains D-fucose. It is suggested that D-galactose and L-fucose caused changes in the outer surface components of bacteria in treated plants, and that such changes inhibited the disease by interfering with reactions at the host-pathogen interface.