Proc. Fla. State Hort. Soc. 121:118–123. 2008.

Streptomycin Controls Canker on Sweet in and Reduces Risk of Copper Burn on in

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. 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 spp. citri, could minimize risk of phytotoxicity (burn) to the 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 . 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 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 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. Spray application environment. dates at São João do Cauia were 11 Oct., 9 Nov., 5 Dec., 5 Jan., and 6 Feb. Disease evaluation of branches at Paranavaí occurred Materials and Methods on 27 Sept., 26 Oct., 23 Nov., 28 Dec., 26 Jan., 26 Feb., 26 Mar, 25 April, and 24 May and at São João do Caiua occurred on 25 Brazil trials 2005–06 Sept., 25 Oct., 24 Nov., 27 Dec., 25 Jan., 26 Feb., 26 Mar, 24 Seven treatments were tested at two locations in Paraná April, and 23 May. Fruit drop was evaluated every 15 d starting State, Brazil: 1) streptomycin sulphate plus oxytetracycline (SS; on 9 Jan. at Paranavaí. No fruit were available for evaluation at Agrimicina [Pfizer, Brazil] - 15.0% streptomycin sulphate/1.5% São João do Cauia. Fruit were harvested and canker incidence oxytetracycline); 2) oxytetracycline (OT; Mycoshield [Nufarm] on fruit was evaluated on 31 May at Paranavaí. 20.0% a.i.); 3-6) 4 rates of copper hydroxide (CH; Kocide 2000 [Dupont]; 35% metallic copper): 1.0x, 0.5x, 0.25x; 0.125x of the Florida trial 2006 effective rate for canker control (Graham et al., 2006); 7) two Eight treatments were tested at one location in Felda, FL: 1) untreated controls (UTC-1 and UTC-2). Trials of the same design copper hydroxide (CH; Kocide 2000) at a 14-d spray interval; were conducted in the spring at Paranavaí and at Guairaçá with 2) CH at a 21-d interval; 3) Copper sulphate (CS; Cuprofix Ultra 2-year-old ‘Valencia’ and with 3-year-old ‘Pera’, respectively, [UPI-USA]; 30% metallic copper) at a 21-d interval; 4) Strepto- late and mid-season sweet oranges [Citrus sinensis (L.) Osbeck] mycin sulphate (SS; Firewall [Agrosource, Inc]; 17% a.i.) + CS on Rangpur lime rootstock (C. limonia Osb.). Tree spacing was at a 14-d interval; 5) CS + SS alternated with 0.5x CS at a 14-d 4.5 × 6.5 m (~350 to 400 trees/ha) and the eight treatments were interval; 6) CH alternated with pyraclostrobin (Headline [BASF]; arranged in a randomized block design with five blocks of five 23.6% a.i) at a 21-d interval; and 7) untreated control. The trials trees per treatment (five trees in a row, 25 trees per treatment). were conducted in the spring with 5-year-old ‘Marsh’ grapefruit Sprays were applied on six dates with a backpack sprayer trees, a scion highly susceptible to canker, on Cleopatra mandarin calibrated to deliver ~2 L of spray material per tree at incipient (C. reticulata Blanco) rootstock. Tree spacing was 4.6 x 7.6 m run-off from foliage. Spray application dates at Guairaçá were (~287 trees/ha). The trees were seep irrigated as opposed to a more 29 Sept., 27 Oct., 10 Nov., 8 Dec., 22 Dec., 5 Jan., 19 Jan., and standard irrigation practice in Florida using microsprinklers. The 2 Feb. Spray application dates at Paranavaí were 6 Oct., 31 Oct., seven treatments were arranged in a randomized block design with 15 Nov., 29 Nov., 13 Dec., 29 Dec., 12 Jan., 26 Jan., 6 Feb., four blocks of five trees per block (five trees in a row, 25 trees and 24 Feb. Disease evaluation of foliage was conducted on the per treatment). Spray volume was 1500 L/ha and was applied mature flush coincident with the time of evaluation. Incidence of with an airblast sprayer driven at 2.1 k/h. Applications began 22

Proc. Fla. State Hort. Soc. 121: 2008. 119 Mar. and ended 6 Oct. Fruit disease was evaluated in October as canker and fruit drop were subjected to a one-way ANOVA. Means the total number of diseased fruit observed on each side of tree. were separated using Tukey’s multiple range test at P < 0.05. Severity of copper burn symptoms was rated on a scale from 0 = no symptoms to 5 = high incidence and severity of dark blemishes Results on the side of the fruit exposed to spray deposition. Brazil trials 2005–06 Florida trial 2007 GUAIRAÇÁ. The AUDC demonstrated that sprays at a 14-d The purpose of this trial was to assess the risk of season-long interval were effective at all rates of CH for reducing canker le- copper sprays of causing phytotoxicity on grapefruit. The trial sions on foliage of ‘Pera’ sweet orange compared to the two UTCs was conducted with ‘Marsh’ grapefruit on Cleopatra mandarin (Table 1). Although disease control was greater at the higher rates rootstock located in Ft. Pierce, FL. Tree spacing was 4.6 × 7.6 of CH, differences among rates were not significant. Streptomycin m and there were 36 trees per row. The trees were drip irrigated. (SS) sprayed at a 14-d interval was as effective for reducing the Each treatment was located in two tree rows (one double row AUDC as CH. Oxytetracycline (OT) did not significantly reduce bed) with one bed as a buffer between treatment rows. The the AUDC compared to the UTCs. treatments consisted of an increasing number of sprays added Incidence of canker on fruit was significantly reduced by the at 21-d intervals after the first three sprays in Apr.–May 2007. 1.0x and 0.5x rates of CH and substantially lower in the treatments There were 10 treatments representing 3 to 11 applications of CH with SS and 0.25x and 0.125x rates of CH (Table 1). Among (11.9 g/L, Nu-Cop 50WP) at 45 L/ha. Spray application dates treatments, over 78% of the fruit that dropped was affected by for the 11 treatments were: 1) 17 Apr., 9 May, 30 May; 2) add canker (data not shown). Fruit drop was significantly reduced 19 June; 3) add 10 July; 4) add 31 July; 5) add 21 Aug.; 6) add by the 1.0x and 0.5x rates of CH and SS, but not by OT or the 11 Sept.; 7) add 2 Oct.; 8) add 23 Oct.; 9) add 15 Nov.; 10) two lower rates of CH. Due to variability of the fruit production on untreated controls (UTC-1 and UTC-2). To minimize variations the young trees, none of the treatments significantly increased in fruit temperature, spray applications began at 7:00 AM and the yield, although all of the treatments had numerically higher ended no later than 9:00 AM. Evaluation of fruit was conducted fruit weight per tree than the UTCs. on 3 Dec. for 100 fruit on the outside of the tree canopy facing PARANAVAÍ. AUDC demonstrated that sprays at a 14-d interval the row middle of each treatment bed. Each fruit was inspected were effective at all rates of CH for reducing canker lesions on for symptoms of copper burn, citrus scab (caused by Elsinoe foliage of Valencia sweet orange compared to the two UTCs fawcetti) or melanose (caused by Diaporthe citri). The incidence (Table 1). Foliar disease control at the higher rates of CH was of copper phytotoxicity was expressed as the percentage of the significantly higher than that at the lowest rate. Streptomycin 100 fruit with symptoms present. sulphate was as effective as the lowest rate of CH. Oxytetracycline did not affect AUDC compared to the UTCs. Statistical analysis Incidence of canker on fruit was not significantly reduced BRAZIL TRIALS. For each trial in each year, data for the area by any of the treatments in the young canker-susceptible ‘Pera’ under the disease curve (AUDC; calculated from seven or eight orange trees in Paranavaí (Table 1). Over 93% of the fruit that evaluations of foliar disease incidence), incidence of fruit with dropped was affected by canker (data not shown). Fruit drop due

Table 1. Spray treatment effects on foliar canker, diseased fruit, yield and percentage fruit drop due to canker for young Pera (Guairaçá) and (Paranavaí) in Paraná State Brazil, 2005–06. Product rate Diseased Yield per Fruit drop due Treatment (g/L) AUDC z fruit (%) tree (kg) to canker (%) Guairaçá Untreated control -1 --- 10.8 ax 49.8 a 135.8 a 15.1a Untreated control -2 --- 11.1 a 46.6 a 155.3 a 12.2 ab Copper hydroxide (CH) 0.125x 0.24 5.0 b 32.0 ab 167.4 a 7.6 abc CH 0.25x 0.48 5.3 b 31.2 ab 162.8 a 8.0 abc CH 0.5x 0.96 3.1 b 20.6 b 166.8 a 5.2 bc CH 1.0x 1.9 3.7 b 21.0 b 163.9 a 4.5 c Streptomycin sulphate (SS)y 2.4 2.8 b 25.8 ab 189.7 a 5.0 c Oxytetraceycline (OT) 2.0 6.6 a 35.8 ab 171.9 a 9.5 abc Paranavaí Untreated control -1 26.0 a 71.4 a 45.8 a 15.0 a Untreated control -2 25.6 a 69.4 a 45.7 a 20.5 a Copper hydroxide (CH) 0.125x 0.24 12.5 b 67.0 a 59.9 a 14.6 a CH 0.25x 0.48 10.7 cb 66.2 a 53.0 a 19.0 a CH 0.5x 0.96 6.9 c 67.0 a 56.2 a 13.7 a CH 1.0x 1.9 6.6 c 64.4 a 66.6 a 10.0 a SS 2.4 10.2 cb 64.8 a 43.2 a 15.9 a Oxytetracycline (OT) 2.0 25.7 a 76.5 a 40.7 a 17.5 a zArea under the disease curve on foliage y15.0% streptomycin and 1.5% oxytetracycline xMeans labeled with different letters indicate significant differences among treatments at P < 0.05 level according to Tukey’s test.

120 Proc. Fla. State Hort. Soc. 121: 2008. Table 2. Spray treatment effects on foliar canker, diseased fruit, yield and percentage fruit drop due to canker for young Valencia orange (Paranavaí) trees in Paraná State Brazil, 2006-2007 Product rate Diseased Yield per Fruit drop due Treatment (g/L) AUDC z fruit (%) tree (kg) to canker (%) Untreated control --- 18.9 ax 76.6 a 82.7 a 25.2 a Copper hydroxide (CH) 0.25x 0.48 9.6 b 45.0 bc 133.6 bc 15.4 ab CH 0.5x 0.96 5.9 b 56.2 abc 134.4 bc 13.4 b CH 1.0x 1.9 6.7 b 49.8 bc 147.0 c 10.3 b Streptomycin suphate (SS)y 2.4 10.8 b 66.6 ab 97.4 ab 18.6 ab CH 0.125x + SS 0.24 + 2.4 9.1 b 59.4 abc 106.5 abc 12.0 b CH 0.25x + SS 0.48 + 2.4 8.9 b 50.8 bc 127.0 abc 13.4 b CH 0.5x / SS alternation 0.96 / 2.4 8.3 b 63.0 abc 126.9 abc 13.5 b CH 1.0x / SS alternation 1.9 / 2.4 8.9 b 56.6 abc 113.9 abc 14.0 b zArea under the disease curve on foliage y15.0% streptomycin and 1.5% oxytetracycline xMeans labeled with different letters indicate significant differences among treatments at P < 0.05 level according to Tukey’s test. to canker infection of fruit was also not significantly reduced by with very low and poorly distributed canker on leaves at the the treatments. None of the treatments increased yield although beginning of the season. Due to high variability of disease inci- the CH, but not the SS and OT treatments, had numerically higher dence on fruit among replicate plots, fruit disease data were not fruit weight per tree than the UTCs. subjected to statistical analysis, but illustrate the incidence of copper burn in grapefruit and reduction of that risk by alternat- Brazil trials 2006–07 ing copper sulphate (CS) with SS (Firewall) or pyraclostrobin PARANAVAÍ. CH sprays at 21-d interval were effective for re- (Headline). In July conditions were unusually hot and dry and, ducing canker lesions on foliage of ‘Valencia’ compared to the in this event, all treatments caused low to moderate copper burn two UTCs (Table 2). Streptomycin sulphate was numerically (Table 3). Alternating CS with SS or CS+SS was as effective less effective than CH for reducing AUDC. SS combined with for reducing the number of fruit with canker as CS or CH with 0.25x and 0.125x CH or alternated with 0.5x and 1.0x CH was reduced copper burn (Table 3). Phytotoxicity from CS alternation numerically as effective as the lowest rate of CH and more ef- with SS was similar to that for the treatment alternating CH with fective than SS alone. pyraclostrobin. This fungicide was ineffective as a bactericide, The 0.25x and 1.0x rates of CH significantly reduced fruit but was not phytotoxic to fruit. Copper burn was lowest in this disease incidence and at the 0.5 and 1.0x rates reduced fruit drop treatment wherein CH was applied every 42 d. Thus, reducing due to canker. Streptomycin suphate alone did not reduce fruit the frequency of copper application accounted for the lower disease. SS combined 0.25x CH significantly reduced fruit disease levels of copper burn among these treatments. Copper burn was incidence and SS combined or alternated with CH significantly estimated to have occurred in July after a period of severe drought reduced fruit drop. Only the CH treatments significantly increased in the Felda location through June (Table 4). Monthly rainfall for yield but all of the treatments had numerically higher fruit weight each month from January through July (except February) was per tree than the UTC. far below the average monthly rainfall between 1998 and 2007 SÃO JOÃO DO CAIUA. Areas under the foliar disease curves obtained from the nearby Florida Automated Weather Network (AUDC) demonstrated that spray treatments at 21-d interval were (FAWN) station at Southwest Florida Research and Education ineffective for reducing canker lesions on foliage of ‘Pera’ orange Center (SWFREC) in Immokalee. compared to the two UTCs (data not shown). Due to sparse fruit production, disease levels on fruit could not be estimated. Florida trial 2007 In 2007, no scab or melanose was observed on ‘Marsh’ Florida trial 2006 grapefruit due to extreme dryness early in the season (Table 4). The 2006 trial was conducted in a ‘Marsh’ grapefruit grove Symptoms of copper burn did not occur until the fifth copper

Table 3. No. of canker diseased ‘Marsh’ grapefruit and phytotoxicity to the fruit rind from sprays of copper formulations alone or alternated with streptomycin sulphate (SS; Firewall) or the fungicide pyraclostrobin (Headline) Spray No. of spray No. canker Fruit copper Product interval applications infected phytotoxicity Treatment rate (g/L) (days) per season fruit per tree rating (0–5)z Untreated control ------20.3 0.0 Copper hydroxide (CH) 1.5 21 10 3.5 1.43 CH 1.5 14 15 5.3 1.64 Cuprofix Ultra (CS) 1.3 21 10 1.8 1.11 CS / SS alternation 1.3/1.1 14 15 3.3 0.59 CS+SS / CS alternation 1.3 + 1.12/1.3 14 15 4.5 1.28 CH / pyraclostrobin alternation 1.5/0.70 21 10 20.0 0.43 zSeverity of copper burn symptoms was rated on a scale from 0 = no symptoms to 5 = high incidence of dark blemishes on the side of the fruit exposed to spray deposition.

Proc. Fla. State Hort. Soc. 121: 2008. 121 Table 4. Monthly rainfall (mm) at the Felda and Ft. Pierce grove locations. Month Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Felda 2006 5.1 109.2 0.0 0.0 50.8 96.5 233.7 383.5 114.3 40.5 5.1 Immokallee avgz 36.1 45.3 65.0 40.9 79.8 219.1 188.5 197.0 181.2 78.4 44.3 Ft. Pierce 2007 15.2 15.2 22.7 55.9 76.2 144.8 256.5 160 228.6 175.3 25.4 Ft. Pierce avgz 32.7 50.9 48.0 64.6 71.1 163.9 132.7 159.4 180.1 116.6 64.2 zAverage monthly rainfall, 1998–2007, obtained from the Florida Automated Weather Network at SWFREC and IRREC. spray application on 10 July (Fig. 1). Incidence of fruit symptoms established efficacy of 14-d intervals for control of canker on foli- increased to approximately 30% after the sixth spray on 31 July. age and fruit comparable to intermediate to full rates of CH. In Subsequent sprays did not substantially increase the incidence 2006–07, streptomycin sprays at 21-d intervals when alternated except in the rows that received nine sprays. Incidence of copper or combined with reduced rates of CH were also as effective as toxicity coincided with the end of a drought (Table 4). Monthly intermediate CH rates. Streptomycin in combination or alternated rainfall until July was substantially below the average monthly with CH was more effective than streptomycin alone. Use of rainfall from 1998 to 2007 obtained from the nearby FAWN streptomycin in this mode may not only reduce rate and frequency station at Indian River Research and Education Center (IRREC) of copper applications but may also contribute to management of in Ft. Pierce. the risk for development of bacterial resistance to either copper or streptomycin. Discussion The 2006 trial conducted with ‘Marsh’ grapefruit demonstrated the utility of reducing rate and frequency of copper with strepto- Since windbreaks are not currently used in Florida, the only mycin (Firewall) to maintain disease control with commensurate management tool available to limit fresh fruit blemishes is bacte- control of canker on fruit. Conditions in summer 2006 were hot ricidal copper sprays. Our trials in Paraná State, Brazil previously and dry, resulting in copper burn on grapefruit. Alternating a demonstrated that copper sprayed at 21-d intervals is consistently 0.5x rate of CS with streptomycin was effective for control of effective for canker control on moderately susceptible orange canker on fruit with reduced phytotoxicity to fruit. Copper burn varieties (Graham et al., 2006). In Florida, for highly suscep- in this streptomycin treatment was similar to that of alternating tible grapefruit, copper sprays at 21-d intervals are necessary copper with the fungicide pyraclostrobin which was ineffective from the spring flush in March to the time when fruit are fully as a bactericide but not phytotoxic to fruit. Thus, the concept of expanded (Graham et al., 2008). As many as 14 applications of substituting streptomycin for some copper applications during copper are required to protect fruit during this period. Copper is the high risk period for copper burn was established. already widely employed for fungal disease control on Florida In the 2006 and 2007 trials, the risk of copper burn posed by grapefruit. It is well known that applications of copper, when season long copper spray programs was confirmed to be coin- the temperatures are high and conditions are dry, causes copper cident with a period in July after below average rainfall from burn on the fruit rind (Albrigo et al., 1993). These factors affect January to June (Table 4). In addition, water deficit in the trees grapefruit marketability by reducing pack-out. due to drought may have been exacerbated by the presumed The 2005–06 trials with streptomycin (Agrimicina) in Paraná suboptimal irrigation methods used in each grove situation, i.e., seep in Felda and drip in Ft. Pierce. In each 60 case, incidents of copper burn occurred after rainfall increased in July. During this period, trees would have been recovering from drought 50 and the rate of fruit growth increasing rapidly. Under these conditions the expanding rind may have become vulnerable to copper burn. Based 40 on these observations, the recommendation would be to manage irrigation to avoid water 30 deficit in the tree and fruit to the extent possible. Optimizing water management in the east coast and southwest flatwoods, however, has become 20 increasingly difficult due to diminishing avail- ability and quality of water sources. Trials from Brazil and Florida demonstrate 10 the efficacy of streptomycin for reducing risk of phytotoxicity to fruit while maintaining control of canker disease. We propose that 0 UTC UTC 3 4 5 6 7 8 9 10 11 two or three applications of streptomycin be targeted to the window of risk for copper burn Number of copper applications during the season. Use of streptomycin should be limited to minimize the risk of resistance of Fig. 1. Number of copper applications at 21-d intervals from April to November and incidence of Xcc to streptomycin and to maintain a sufficient copper phytotoxicity to fruit of Marsh grapefruit trees at Ft. Pierce, FL in 2007 (UTC = untreated preharvest interval for prevention of residues in control). harvested fruit.

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