Control Measures for Citrus Bacterial Spot in Nurseries and Packinghouses

Citrus bacterial spot (CBS), caused by Xanthomonas campestris pv. citrumelo, produces unsightly blemishes on leaves and stems of nursery plants that may affect their mar ket value. In simulated nurseries, Swingle citrumelo and 'Ruby red' grapefruit infected with X. c. citrumelo were treated with combinations of defoliation and sprays of copper ammonium complex (CAC) to control CBS. Also, Swingle cit rumelo, budded with 'Ruby red' grapefruit, were treated likewise before and after unwrapping the buds. Removal of diseased leaves reduced disease to a low level but did not eradicate CBS on grapefruit or Swingle citrumelo. Bacteria from stem lesions on Swingle citrumelo infected new leaves. Biweekly CAC sprays reduced the recurrence of leaf infection but did not prevent new disease except on 'Ruby red' grape fruit budlings, where CAC-treated Swingle citrumelo rootstock stems were removed before unwrapping the bud. The effect of disinfestation of personnel and equipment on spread of X. c. citrumelo in a nursery was demonstrated by handling or hedging diseased Swingle citrumelo and following the trans mission of the disease to noninfested trees down the row. Rubbing wet, diseased leaves with gloves transmitted CBS to the end of 12 m rows of 44 plants. After contamination, sur face treatments of gloves with Gal lex 1027 and dilute chlorine bleach reduced the incidence of CBS on individual plants but not the transmission down the row. Hedging of diseased shoots did not transmit CBS. In fruit disinfestation studies, a spray application of sodium-orthophenylphenate for 30 sec on rotating brushes reduced the proportion of in fested 'Valencia' orange fruit from 25/25 to 1/25 and bacterial populations of X. c. pv. citrumelo from 1.7 X 103 cfu/cm2 to 0.1 cfu/cm2 fruit surface. Citrus bacterial spot (CBS), caused by Xanthomonas cam pestris pv. citrumelo Gabriel, has occurred in over 60 loca tions in Florida on 20 different cultivars, but is predomi nately found in nurseries and on the rootstock, Swingle citrumelo (Graham and Gottwald, 1990; Graham and Florida Agricultural Experiment Station Journal Series No. N-00477. This research was supported by USDA-ARS AAD/IFAS Cooperative Agreement No. 58-43YK-5-3 and 58-43YK-0-0008. We appreciate the expert technical assistance of T. D. Riley, M. A. Bruce, A. Miller, A. Dow, J. Bittle, and P. Bell. Proc. Fla. State Hort. Soc. 104: 1991. Gottwald, 1991). This hybrid cultivar (Poncirus trifoliata (L.) Raf. x Citrus paradisi Macf.) and the parent trifoliate orange are highly susceptible under nursery conditions (Graham and Gottwald, 1991; Graham et al., 1990); how ever, the incidence of CBS on these cultivars decreases when infected trees are transplanted to the grove (Gottwald and Graham, 1990; Gottwald et al., 1992). Citrus cultivars, such as grapefruit, are resistant to X. c. citrumelo based on artificial inoculations, but become infected to var ying degrees under nursery conditions depending on the aggressiveness of the strain and nursery practices. Mechan ical operations, such as leaf stripping, budding, sprouting, hedging and spraying create wounds on leaves and stems and opportunities for the bacteria to spread down nursery rows when plants are wet and bacteria are exuding from existing lesions (Egel et al., 1991; Gottwald and Graham, 1990; Timmer et al., 1991). Strains of X. c. citrumelo, even the least aggressive forms, are capable of infecting through wounds or stomatal openings when they are assisted by physical or water pressure on the leaf or stem surface (Gottwald and Graham, 1990; Gottwald et al., 1992; Graham and Gottwald, 1990; Graham and Gottwald, 1991). Only the most aggressive strains are capable of sig nificant spread by wind-blown rain and infection through stomata without wounding (Gottwald and Graham, 1990; Gottwald et al., 1992; Gottwald et al., 1988; Graham and Gottwald, 1990). The aggressive strains have occurred in frequently in nurseries, so in most cases, we are dealing with weakly to moderately aggressive that are spread by mechanical means (Gottwald and Graham, 1990; Graham and Gottwald, 1990). Since 1986, fruit shipped from groves within quaran tine areas for citrus canker, caused by X. campestris pv. citri, have been treated with chlorine or sodium-or thophenylphenate (SOPP) to disinfest the fruit in packing houses (Anonymous, 1987). In a previous test of quaran tine treatments for citrus fruit (Brown and Schubert, 1987), SOPP was the most effective disinfestant against another pathovar of X. campestris. Since CBS was discov ered in 1984, treatments were prescribed for fruit shipped from all citrus groves in Florida until the disease was dere gulated in 1990 by the USDA Animal Plant Health Inspec tion Service (APHIS) (Graham and Gottwald, 1991). In spite of the lifting of this state-wide quarantine, California and Arizona still require treatments for all fruit shipped to their markets (R. Gaskalla, personal communication). CBS continues to occur in Florida nurseries and causes unsightly blemishes on leaves and stems of nursery plants that may affect their market value (unpublished observa tions). Thus, means of eradication or control of CBS in the nursery are desirable. Treatments with defoliants and cop per bactericides for control of citrus canker (Muraro, 1989; Timmer, 1988) have been used for eradication of the ag gressive strain of X. c. citrumelo in Florida (Gottwald and Graham, 1990). Therefore, various combinations of defoli ation and sprays of a copper bactericide on rootstocks and scions were tested under simulated nursery conditions for control of CBS. Secondly, means of reducing mechanical spread of CBS by disinfestation of personnel and tools were investigated. Thirdly, since fruit treatments for citrus