Austin Hagan and Randy Akridge

Fireblight, which is caused by the bacterium Erwinia amylovora, is a widespread and often damaging disease in both nurseries and landscape plantings of many of the ornamental shrubs and trees in the apple sub-family (Pomodidae). While the most effective way to control this disease is to plant resistant varieties, when chemical controls are necessary AAES research has shown that Agrimycin provides the most effective control.

Several days of mild, wet, cloudy weather during and shortly after bloom favors rapid development of fireblight. In Alabama, certain cultivars of crabapple, flowering pear, Indian hawthorn, cotoneaster, and loquat are the most common targets of this disease. Other ornamental hosts of this disease include photinia, flowering quince, hawthorn, and pyracantha. After several years of attack by this disease, shrubs and trees often become badly disfigured and may sometimes be killed by repeated outbreaks of fireblight.

Although the establishment of fireblight-resistant cultivars is the preferred method of controlling this disease, fireblight-susceptible trees and shrubs are common in many Alabama landscapes. Also, shrubs and trees that are susceptible to this disease continue to be marketed by retail outlets across the state. As a result, chemical treatments may be needed to maintain the health and beauty of shrubs and trees that are vulnerable to fireblight.

Currently, Agrimycin 17, Kocide 101, Phyton 27, and Aliette WDG are labeled for the control of fireblight on shrubs and trees. Little information is available, however, on the effectiveness of these products against fireblight in either a nursery or landscape setting. An AAES study was conducted to assess the efficacy of these bactericides and the experimental product fluazinam for the control of fireblight in a simulated landscape planting of crabapple.

In March 1995, Snowdrift bare-root crabapple cultivars were planted on 10-foot centers in rows 10 feet apart in a Benndale sandy loam at the Brewton Experiment Field in Escambia County. Before planting, soil fertility and pH were adjusted according to the recommendations of a soil fertility test. At planting, a drip irrigation system was installed and the trees were watered as needed. After planting, the beds were mulched with pine bark.

Twice each spring, a 16-4-8 fertilizer was broadcast down each row of trees at a 50-pound-per-acre rate. For preemergent weed control, a tank-mix of one pound per acre of Gallery and two quarts per acre of Surflan was broadcast over the mulched area. The beds also were hand weeded and spot-treated with Roundup to control escape weeds. All bactericide treatments were applied to run-off using an ATV-mounted electric sprayer with a single hand held nozzle. Each year, a dormant application of Kocide 101 77W was made in early March just before bud break. One plot was not treated to serve as a control. All other treatments were applied at weekly intervals from mid-March until mid-May. Fireblight severity was rated on May 29,1996, May 19, 1997, and May 9, 1998, on a 1-to-5 scale.

Crabapple treated with 0.9 pound Agri Strep in May 1997.

Control plant shows extensive shoot dieback due to fireblight.

Although none of the bactericides prevented the development of fireblight on crabapple, substantial differences in disease control were noted among the products tested (see the table). Of the five bactericides, only Agrimycin 17 reduced the disease severity in all three years as compared with the nonsprayed control. Damage on the Agrimycin 17-treated crabapples was limited to the blighting of a few bloom clusters and shoot tips. In two of three years, the dormant application of Kocide 101 also reduced the severity of fireblight to levels well below those noted on the nontreated controls.

Bactericides Compared for Control of Fireblight on Snowdrift Crabapple
Treatment	Application rate per 100 gal. of spray volume	Fireblight severity1
		1996	1997	1998
Agrimycin	0.5 lb.	1.2	1.5	1.5
Fluazinam 500F	12.0 fl. oz.	2.7	2.4	2.3
Aliette WDG	2.0 lb.	2.8	2.7	2.9
Phyton 27	12.5 fl. oz.	1.9	2.8	2.5
Kocide 101 77W2	12 lb.	1.6	2.3	1.8
Nonsprayed control	--	2.3	2.6	2.4
1 Fireblight severity was rated on the following scale: 1 = no disease, 2 = one to a few flower clusters or shoots diseased, 3 = numerous blighted shoots with a few large branches killed, 4 = major portion of tree killed, and 5 = tree dead. 2 Kocide101 was applied as a dormant spray just before bud break.

Disease severity ratings for the Phyton 27-treated trees were below those recorded for the nonsprayed control in 1996 but not in 1997 or 1998 (see table). In all three years, the registered product Aliette WDG and the experimental material Fluazinam 500F failed to reduce the severity of fireblight as compared with the nontreated control (see table). In fact, fireblight levels were slightly higher for the Aliette WDG-treated trees than for the nonsprayed crabapples. Overall, trees sprayed with Aliette WDG, Phyton 27, and Fluazinam 500F, as well as the nonsprayed controls suffered considerable blighting of numerous flower clusters and shoot tips and also dieback of some larger (scaffold) limbs.

In summary, Aliette WDG, Phyton 27, and Fluazinam 500F failed to control fireblight on crabapple. Agrimycin 17 consistently gave the best control of fireblight. Damage from fireblight on Agrimycin-treated trees was considerably less than that seen on the nonsprayed controls. The single dormant application of Kocide 101 77W, which proved nearly as effective against fireblight as were weekly sprays of Agrimycin 17, may be the most practical control option for many homeowners and landscape contractors. However, Kocide 101 77W is highly phytotoxic to the foliage of crabapple, particularly at the rates used in this study, and must be applied before the trees begin to leaf-out in early spring.