Suppression of Fireblight in Apple Shoots by Prohexadione-calcium following Experimental and Natural Inoculation

Received for publication 20 July 1998. Accepted for publication 1 Aug. 1998. Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty and does not imply approval to the exclusion of other products that may be suitable. We gratefully acknowledge the technical assistance of Allen Cochran II, William Royston, Jr., and Scott Kilmer. This project was partially supported by BASF Corp., Research Triangle Park, N.C. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1Associate Professor of Plant Pathology. 2Research Horticulturist. Professor of Horticulture. Trees. Vigorous, well-pruned trees, trained to a central leader, were used in these studies. At Winchester mature ‘Rome Beauty’/MM.106 trees were used in 1994 and ‘Golden Delicious’/M.7 trees in 1995 and 1997. At Kearneysville in 1995, 16-year-old ‘Law Rome’/MM.111 trees were used to study the effects of single and multiple applications of the growth regulator. Test materials and application. Prohexadione-Ca (3-oxido-4propionyl-5-oxo-3-cyclohexene-carboxylate) was applied as experimental formulations BAS 9054.2 10DF in 1994 and BAS 125 W 10DF in 1995 and 1997. Streptomycin sulfate (Agri-Mycin 17) was included or applied separately as indicated. A spray adjuvant, Regulaid (polyoxyethylenepolypro-poxypropanol and alkyl 2-ethoxyethanol dihydroxy propane) was included with all prohexadione-Ca treatments at 0.03% or 0.125% (v/v) as indicated. In 1994 treatments were applied dilute with a low pressure hand-wand sprayer. In 1995 and 1997, treatments were applied dilute to the point of runoff with a single nozzle handgun at 1380 KPa. A roller pump operating at 690 KPa was used to apply treatments at Kearneysville. Inoculation procedures. Inoculum for tests at Winchester was obtained by growing a culture of E. amylovora 1 or 2 d at 25 °C on nutrient yeast dextrose agar, and harvesting and suspending the bacteria in a buffer solution containing approximately 1 × 10 cells/mL (1994) or 1 × 10 cells/mL (1995 and 1997). Shoot tips were inoculated as indicated in the last leaf node with a No. 25 hypodermic syringe holding one droplet of a bacterial suspension. 1994 Winchester. A small-scale test was conducted with 14 selected, actively growing, shoots per treatment. All test shoots were distributed on four mature ‘Rome Beauty’ trees, using three or four shoots per treatment per tree. Full bloom occurred on 22 April. Prohexadione-Ca (375 mg·L) + Regulaid (0.125%, v/v), was applied to individual shoots with a small hand sprayer 2 May 1994. Shoots were measured before shoot tip inoculation 10 May. Fireblight incidence and extent of canker progression on inoculated shoots and length of noninoculated shoots were recorded 30 June. Mean shoot lengths were separated by Waller–Duncan k ratio t test (P ≤ 0.05). Frequencies of shoot infection were compared by using a Chi-square test. 1995 Winchester. An experiment was conducted on moderately vigorous ‘Golden Delicious’ trees using four single-tree replications in a randomized block design. All applications contained Regulaid 0.03% by volume. One set of trees was treated twice with prohexadioneCa (250 mg·L) at early bloom (20 Apr.) and full bloom (25 Apr.). Other sets were treated once at petal fall on 28 April with prohexadioneCa (250 mg·L), streptomycin (100 mg·L), or prohexadione-Ca (250 mg·L) + streptomycin (100 mg·L). Twenty treated shoots per tree were selected and flagged on 4 May, 10 for inoculation 5 May and 10 for inoculation 12 May (1 week and 2 weeks after application to the single-application trees). Fireblight incidence and extent of canker progression in inoculated shoots were assessed on excised shoots 20 June. Growth of shoots on non-inoculated trees was measured on 29 June. Noninoculated strikes were counted and removed 20 June. Fireblight, a disease caused by Erwinia amylovora (Burrill) Winslow et al., is an increasingly important disease in apple (Malus ×domestica Borkh.) production throughout the mid-Atlantic region of the United States and elsewhere. Factors related to its increase in prominence include increased planting of highly susceptible scion–rootstock combinations, the use of crabapple pollinizers within high-density orchards, and development of resistance to streptomycin by the fireblight bacterium in some eastern production areas (Jones and Sutton, 1996). Streptomycin resistance has not yet been proven in the mid-Atlantic region, but this concern, together with the increasing planting of highly susceptible scions and rootstocks, heightens the need to avoid cultural practices that increase tree susceptibility. In addition to chemical control, the program for overall management of fireblight includes proper tree nutrition and practices that promote early cessation of growth (Beer, 1990). In spite of long-term recognition that tree vigor is an important element in fireblight susceptibility (Van der Zwet and Keil, 1979), few reports have addressed the possibility that plant growth inhibitors could reduce susceptibility or suppress infection. Two reports discuss the potential effects of daminozide (Lombard et al., 1982) and other plant growth regulators (Deckers and Daemen, 1993) on shoot growth, secondary bloom and fireblight risk in pear (Pyrus communis L.). We are not aware of published reports with experimental data that confirm a positive or negative effect of plant growth regulators on fireblight development. Prohexadione-Ca is an experimental plant growth regulator currently under study for controlling vegetative shoot growth. The objective of this study was to evaluate the effects of prohexadioneCa and streptomycin, alone and in combination, on the shoot blight phase of fireblight. An abstract describing a portion of this work has been published (Byers and Yoder, 1997).