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Streptomycin Controls Citrus Canker on Sweet Orange in Brazil and Reduces Risk of Copper Burn on Grapefruit in Florida

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Streptomycin Controls Citrus Canker on Sweet Orange in Brazil and Reduces Risk of Copper Burn on Grapefruit in Florida Proc. Fla. State Hort. Soc. 121:118–123. 2008. Streptomycin Controls Citrus Canker on Sweet Orange in Brazil and Reduces Risk of Copper Burn on Grapefruit in Florida JAMES H. GRAHAM1*, RUI P. LEITE, JR.2, HENRY D. YONCE3, AND M. MYERS1 1University of Florida, IFAS, Citrus Research and Education Center, Lake Alfred, FL 33850 2Instituto Agronômico do Paraná, Londrina, Paraná, Brazil 3KAC Agricultural Research Inc., Deland, FL 32720 Additional index words. Xanthomonas axonopodis pv. citri, canker epidemic, spray trials in Florida and in Paraná, Brazil Reduced rates and frequency of copper as a bactericide to control citrus canker, caused by Xanthomonas citri spp. citri, could minimize risk of phytotoxicity (burn) to the fruit rind, development of bacterial resistance to copper, and reduce copper accumulation in the soil environment. Field trials in Brazil demonstrated that rates of 0.125x, 0.25x, and 0.5x copper hydroxide (CH), when applied at a 14-day interval, were as effective as the full rate (1.0x) of CH for control- ling canker on foliage and fruit. Lower rates of CH also reduced canker-induced fruit drop of moderately susceptible sweet orange cultivars. Streptomycin sulphate (SS, Agrimicina, 15% streptomycin + 1.5% oxytetracycline) was as effective as intermediate rates of CH, whereas oxytetracycline (OT, Mycoshield) was ineffective. At 21-day interval of application, SS combined with reduced rates of CH reduced fruit disease incidence and SS combined or alternated with CH significantly reduced fruit drop. In Florida on white grapefruit, CH and copper sulphate (CS) applied at 14- or 21-day intervals controlled canker on fruit, but caused considerable copper burn to the fruit rind. Alternation of SS (Firewall, 17% streptomycin) with a reduced rate of CS provided a similar reduction in diseased fruit with less copper burn than the full rate of CH or CS. In the 2006 and 2007 Florida trials, copper burn on grapefruit occurred in July. These incidents of phytotoxicity were preceded by below average rainfall from January to June leading to drought conditions by mid-summer. Asiatic citrus canker is caused by the bacterial pathogen per reduces bacterial populations on leaf surfaces, but multiple Xanthomonas citri sbsp. citri (Xcc; syn. X. axonopodis pv. citri). applications are needed to achieve adequate control on susceptible Canker is a serious disease of most commercial citrus cultivars citrus hosts (Graham and Leite, 2004; Leite et al., 1987; Stall et and some citrus relatives (Graham et al., 2004). The pathogen al., 1980, 1982b). Protective activity of copper is diminished by causes distinctive necrotic, erumpent lesions on leaves, stems, wind-blown rain that introduces bacteria directly into stomata and fruit. Severe infections can cause defoliation, blemished fruit, (Gottwald and Graham, 1992; Gottwald and Timmer, 1995). premature fruit drop, twig dieback, and general tree decline. The Therefore, copper is used in combination with windbreaks in establishment of canker in Florida, the end of the eradication southern Brazil and northeast Argentina with subtropical climate program in Jan. 2006, and the interim rule on 1 Aug. 2006 quar- similar to Florida (Behlau et al., 2008; Leite, 1990; Leite and antined the entire state of Florida for citrus canker (http://www. Mohan, 1990; Muraro et al., 2002). Disadvantages of long-term thefederalregister.com/d.p/2006-08-01-E6-12314). Presently, use of copper bactericides include induced copper-resistance in shipping of all citrus produced in Florida to citrus-producing Xanthomonad populations (Marco and Stall, 1983; Rinaldi and states is prohibited whether the fruit shows symptoms or not. Leite, 2000) and accumulation of copper in citrus soils with po- New regulations were approved in 2007 that allow “asymptomatic tential phytotoxic and adverse environmental effects (Alva et al., fruit” to be shipped to other states regardless of canker presence 1995). However, other contact bactericides are not as effective in the grove (http://edocket.access.gpo.gov/2007/pdf/E7-9898. as copper because they lack sufficient residual activity to protect pdf). The experience from other citrus producing countries with leaf and fruit surfaces for extended periods (Graham et al., 2006; endemic canker indicates if the incidence of infected fruit in a Leite et al., 1987; Rinaldi and Leite, 2000). block is greater than 2% to 5%, it is hazardous to harvest fruit Another important aspect of integrated management of canker from that block for fresh market (Ritenour et al., 2007). Therefore, involves citrus leaf miner control (CLM; Phyllocnistis citrella). reduction of canker incidence in the grove is an essential first step CLM larvae feed by tunneling beneath the epidermal cell layer in producing blemish-free fruit for packing. forming galleries on immature leaf flushes. Wounding of the cuticle There are no highly effective canker disease suppression strate- by CLM feeding results in the direct exposure of the leaf tissue gies for the most susceptible cultivars such as grapefruit when to infection by Xcc, resulting in massive increases in bacterial these varieties are grown in wet, subtropical areas like Florida inoculum, and greater spread and severity of canker (Gottwald et (Kuhara, 1978; Leite and Mohan, 1990; Stall et al., 1982a). Cop- al., 2007). Thus, control of CLM on the most canker susceptible varieties is essential (Stein et al., 2007). To attain a sustainable level of canker control for fresh fruit *Corresponding author; email: jhgraham@ufl.edu; phone: (863) 956-1151 production, Florida growers must utilize all methods available. 118 Proc. Fla. State Hort. Soc. 121: 2008. Foremost, windbreaks will be required to reduce the incidence foliar lesions and percentage defoliation induced by canker was and severity of canker on fruit. Windbreaks are most important estimated on eight branches per tree (four branches on each side for grapefruit and protection may be needed for blocks as small of the tree) of the three middle trees in each plot. Evaluations as 5–10 acres each. Until windbreaks are more widely adopted, at Paranavaí occurred on 6 Oct., 8 Nov., 7 Dec., 4 Jan., 9 Feb., more effective copper applications are needed. Research in Paraná 8 Mar, 5 April, 10 May, 8 June, and 5 July and at Guairaçá on State, Brazil, with moderately susceptible oranges demonstrated 25 Sept., 26 Oct., 28 Nov., 28 Dec., 25 Jan., 23 Feb, 28 Mar, 27 that copper sprayed at 14- and 21-d intervals was more effective April, 25 May, 28 June, 27 July, and 24 Aug. Premature fruit than 28-d intervals for reducing incidence of canker on fruit, abscission and the cause of fruit drop was assessed to measure premature fruit drop, and may increase yields (Graham and Leite, crop loss. Fruit drop was evaluated every 15 d beginning on 19 2004; Belhau et al., unpublished). Reliance solely on copper to Jan. at Guairaçá and on 10 Jan. at Paranavaí. The percentage control canker presents several challenges. Up to 12 copper ap- fruit drop with canker lesions was calculated as the number of plications will be made from the time of spring leaf flush to the fruit dropped due to canker divided by the total fruit dropped point of full fruit expansion if sprayed at 21-d intervals throughout due to all causes. Fruit were harvested and canker incidence on the summer months (Graham et al., 2008). Season-long applica- fruit on the tree was evaluated on 30 Aug. in Guairaçá and on 4 tion of copper fungicides to grapefruit, especially under hot, dry July at Paranavaí. Incidence of diseased fruit was assessed on periods, can lead to phytotoxicity to the fruit rind (Albrigo et al., 100 fruit per middle three trees in each plot. Yield per tree was 1996). Copper phytotoxicity results when a “burn” causes rind calculated as the sum of the weight of fruit dropped due to canker tissues to develop corky lesions, a symptom sometimes resulting (based on the average weight of the fruit from all plots) plus the in a blemish called “star melanose.” On grapefruit, these blem- weight of fruit dropped due to other causes plus the total weight ishes may make the fruit unacceptable for the fresh market, and of fruit left on the tree. instead, the fruit must be diverted to much less profitable processed juice. Since there are no EPA-registered bactericides for canker Brazil trials 2006–07 other than copper, reliance on copper as the sole bactericide may Nine treatments were tested at two locations: 1) streptomycin also lead to resistance within Xcc populations. Availability of an sulphate plus oxytetracycline (SS; Agrimicina); 2–4) 3 rates of alternative bactericide to integrate into a copper-based program copper hydroxide (CH; Kocide 2000; 35% metallic copper): 1.0x, will not only help manage the risk of copper burn, but also re- 0.5x, 0.25x of the effective rate for canker control (Graham et duce the risk of resistance development within Xcc populations al., 2006); 5+6) 2 rates of CH plus SS: 0.25x CH+SS and 0.125x and maintain the long-term usefulness of copper to the Florida CH+ SS; 7+8) 1.0x CH or 0.5x CH alternated with SS (first ap- grapefruit industry. plication was CH); and 9) untreated control (UTC). Trials of the The purpose of these trials was to establish if streptomycin same design were conducted in the spring at Paranavaí and at has activity as a bactericide for control of canker similar to that São João do Caiua with 3-year-old ‘Valencia’ and with 3-year-old of copper. Streptomycin was tested in combination or alternation ‘Pera’, respectively, late and mid-season season sweet oranges on with copper to evaluate its use for reducing phytotoxicity and Rangpur lime rootstock. Spray application dates at Paranavaí were reducing the amount of copper applied per season in the grove 10 Oct., 1 Nov., 30 Nov., 28 Dec., and 30 Jan.
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