Austin Hagan and J. Randy Akridge

Fireblight, which is caused by the bacterium Erwinia amylovora, is a common and often damaging disease in the nursery and landscape on crabapple, flowering pear, and other related member of the rose family. Little information is available concerning the effectiveness of bactericides such as Agrimycin 17, Kocide 101 77W, Phyton 27, and Aliette WDG in controlling this disease on crabapple or other woody ornamental members of the Rose family. An AAES study was conducted to assess the efficacy of registered and experimental bactericides for the control of fireblight in a simulated landscape planting of crabapple.

METHODS
Bare-root crabapple (Malus sp.) ‘Snowdrift’ were planted in March 1995 in a Benndale sandy loam at the Brewton Experiment Field. Before planting, soil fertility and pH were adjusted according to the recommendations of a soil assay. After planting, the beds were mulched with aged pine bark. Twice each spring, 50 pounds per acre of 16-4-8 fertilizer was broadcast down each row of trees. For preemergent weed control, a tank-mix of 1 pound per acre of Gallery 75DF and 2 quarts per acre of Surflan T/O was broadcast over the beds.

Plots were hand weeded and spot-treated with Roundup to control escape weeds. All treatments were applied to run-off using an ATV-mounted electric sprayer with a single hand-held nozzle. Kocide 101 77W was applied as a dormant spray just before bud break in early March. All other treatments were applied weekly from mid-March until mid-May. Fireblight severity was rated on May 19, 1997 and May 9, 1998 on a scale of 1 to 5 with 1 = no disease, 2 = one or a few diseased branch tips, 3 = numerous diseased branch tips with a few major branches killed, 4 = major portion of tree killed, and 5 = tree killed.

RESULTS
None of the treatments evaluated prevented the development of fireblight on crabapple. Of the six treatments, only Agrimycin 17 reduced the severity of fireblight in 1997 and 1998 as compared with the unsprayed control (see table). Damage on the Agrimycin 17-treated trees was limited to the blighting of scattered bloom clusters and shoot tips. In 1997, the dormant treatment of Kocide 101 77W failed to reduce the level of blighting below that on the unsprayed control. In 1998, Kocide 101 provided fireblight control equal to that given by Agrimycin 17. In 1997 and 1998, damage to both the unsprayed controls and the crabapples treated with Fluazinam 500F, Aliette WDG, and Phyton 27 was similar with symptoms that included considerable blighting of the flower clusters and shoot tips as well as the dieback of some lateral and scaffold limbs.

While none of the bactericides gave complete protection, Agrimycin 17 provided the most effective and consistent control of fireblight on crabapple. A dormant spray of Kocide 101 77W was as effective in controlling fireblight as Agrimycin 17 in one year while the other registered bactericides, Aliette WDG and Phyton 27, failed to reduce the disease severity in this study.