MANAGEMENT OF ZEBRA CHIP OF POTATO: A PATHOGEN-BASED APPROACH

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2 MANAGEMENT OF ZEBRA CHIP OF POTATO: A PATHOGEN-BASED APPROACH R. D. French-Monar, A. F. Patton III, and C. R. Dabney Department of Plant Pathology and Microbiology, Texas AgriLife Extension Service, Amarillo, TX 79106, USA Summary Zebra chip, a potato disease associated with the bacterium 'Candidatus Liberibacter solanacearum' and vectored by the potato psyllid, Bactericera cockerelli, is mainly managed through the use of insecticides. The focus of this study was to look at alternatives that would target the pathogen by using antibiotics or chemistries that could end up being bactericidal or by using chemistries that could trigger a plant defense response. Based on results fi om this year's study and previous trials, the potential exists for antibiotics, such as streptomycin sulfate, and plant activators, such as acibenzolar-s-methyl, to potentially play a role in the management of zebra chip. Introduction Zebra chip (ZC), a disease causing economic losses to the potato industry in the United States, Mexico, Central America, and New Zealand, was associated with the phloem-limited bacterium 'Candidatus Liberibacter solanacearum' (Abad et al., 2009; Liefting et al, 2008; Liefting et al., 2009). Recent research studies have documented that the potato psyllid, Bactericera cockerelli, is associated with zebra chip (Munyaneza et al., 2007; Hansen et al., 2008). Efforts to manage ZC have centered in controlling or managing this insect vector through the use of insecticides. Efforts are on the way to understand if planting date, tolerant varieties, repellents, and other alternative may be suitable additions or alternative to managing this disease. Rather than focusing on cultural practices, vector management, or plant resistance, two other options are to potentially prevent symptom development by targeting the pathogen and the ability for the potato plant to trigger its own defense responses or mechanisms against this pathogen. Targeting this phloem-limited bacterial pathogen could involve the use of antibiotics. In citrus greening (Huanglongbing), a disease caused by three other species of 'Ca. Liberibacter', leaf symptoms were reduced with tetracycline hydrochloride in several countries and relatively successful control was obtained with penicillin carbendazin (Abdullah et al, 2009). However, this was achieved by injection of antibiotics into the citrus trees and may require the use ofhigh capacity compressors. The use of plant activators, such as acibenzolar-s-methyl, have been documented to suppress viruses such as the Tomato spotted wilt virus, the oomycete Pythium ultimum on cucumber, and the bacterium Xanthomonas campestris pv. pruni on peach (Csinos eta!, 2001). 59

3 For this study, the use of antibiotics, plant activators, and potentially bactericidal compounds were evaluated on potatoes against ZC and/or the causal agent, Ca. Liberibacter solanacearum in field plot experiments. Materials and Methods Field Site. Field experiments for 2010 were conducted from 30 March until 24 August near Springlake, Texas, on a commercial potato production farm. This site had a previous history with both potato psyllids and Ca. Liberibacter solanacearum. The soil was Tivoli fine sand. A total of 15 treatments were arranged in a randomized complete block design and were replicated 4 times. Each replicate consisted of2-row plots of nine potato plants per row, for a total of 18 plants per replicate. Potato seed tubers 'Russet N orkotah 223' were used for this study. Potatoes were planted on 30 Mar, vines were killed on 16 Aug, and potato crop was harvested on 24 Aug. Field Treatments. For this study, the treatment numbers and treatments were: 1) Untreated, 2) Firewall (Streptomycin Sulfate), 3) Prophyt (Potassium phosphate), 4) K-Phite (Potassium salts of phosphorous acid), 5) Keyplex KPX-B2 (Yeast extract hydrolysate), 6)Phostrol (Phosphite), 7)Actigard (Acibenzolar-s-methyl), 8) Firewall plus SAver (Salicylic Acid), 9)Firewall plus SAver, 10) K-Phite plus Actigard, 11) K-Phite plus Firewall, 12) K-Phite plus SAver, 13)KPX-B2, 14) KPX-B2 plus Actigard, and 15) Renew (Phosphate plus Phosphite). Chemistries for this trial were applied as foliar sprays on 28 May, 11 June, 22 June, 2 July, and 15 July. All chemistries were applied with a backpack C0 2 sprayer at 35 psi using a 3 and/or 4 nozzle aluminum spray boom. Fungicide, herbicide, and insecticide applications were applied by the producer as needed and consistent with commercial production practices. Fungicides used include: Bravo, Dithane, Quadris, and Super Tin. Herbicides included Sencor, Roundup, and Treflan. Insecticides used include: Movento, Rimon, Admire Pro, Fulfill, Agrimek, Tracer, Venom, and Abacus. Assays for detection of zebra chip or Ca. Liberibacter solanacearum were conducted by visual ratings of cut, raw tubers at harvest and by tuber frying. The presence of the pathogen was confirmed by PCR (Li et al, 2009). Results and Discussion. Field plots had lower temperatures than usual at planting and throughout April, which delayed plant establishment and higher than usual rainfall in the third week of April, third week of May, and the first week of July. Plots received a moderately severe hail in May while higher than normal temperatures were recorded for June. Psyllid populations were high. All these factors may have contributed to low yields in comparison to previous years. One replication had lower yields than the other replicates and may have been partially due to factors such as location in the field and soil parameters such as compaction and drainage. By the end of the season, all plants exhibited psyllid damage (ie. yellowing of tissue, leaf curling, inflated nodes, and/or purpling). Yield. There was no significant difference between treatments and the control (untreated) in terms oftotal yield, individual yield for tubers weighing 4-6 oz., 6-10 oz., oz., under 4 60

4 oz., and over 18 oz. (data not shown). However, the removal of one replicate allowed for some significant differences between some chemistries and the untreated control (Table 1 ). For total yield of all tubers harvested, only the application of Firewall gave higher yields than the untreated control while treatments with K-Phite plus Actigard, K-Phite plus Firewall, K Phite plus SAver, and KPX-B2 plus SAver, ended up with significantly lower yields (Table 1). For total yield of 4-18 oz. tubers (U.S. No. 1), only the application of Firewall and Firewall plus Actigard gave higher significant yields, while KPX-B2 plus SAver, Prophyt, and Phostrol plus Firewall, ended up with significantly lower yields (Table 1). For tubers weighing less than 4 oz., only the treatment with Prophyt ended up with significantly higher yields, while K-Phite plus Firewall, K-Phite +SAver, and KPX-B2 + SAver, ended up with significant lower yields (Table 1). For 4-6 oz. tubers, only the application offirewall plus Actigard ended up with significantly higher yields while K-Phite plus Firewall and KPX-B2 ended up with significant lower yields (Table 1 ). For 6-10 oz. tubers, the application of Firewall, Firewall plus SAver, and Firewall plus Actigard, ended up with significantly higher yields while K-Phite plus Actigard, K-Phite plus SAver, KPX-B@ plus SAver, KPX-B2 plus Actigard, Prophyt, K-Phite, and Phostrol plus Firewall, had significantly lower yields (Table 1 ). For oz. tubers, there was no significant difference amongst treatments. Unlike similar trials conducted in 2009 (data not shown), this year there were no tubers larger than 18 oz. for any treatments. Although trials conducted this year had good psyllid pressure and Zebra chip was present in the plots (data not shown), other factors such as initial low temperatures, higher rainfalls in three months, high temperatures in June, and hail, affected potato production and may not have allowed for effects of the chemical applications for this trial to be properly accounted for. In trials conducted in 2009 with some similar chemistries, applications of streptomycin sulfate gave significantly higher yields for total U.S. No. 1 tubers (4-18 oz.) while the treatment with Actigard was the only one that had tubers weighing over 18 oz. Based on data for the three replications, Firewall allowed for significantly higher yields for total tuber yields and for total yields of 4-18 oz tubers. By itself, or in combination with Actigard, SAver, or K-Phite, Firewall also provided for significant higher yields of other tuber weight categories. Chemistries for this study were applied as foliar sprays and would be an ideal method for application of chemistries in the field especially if they can be translocated to the phloem. Unlike antibiotics and fungicides that can be applied to trees as injections, this is not an option for potatoes. Experimentation with substrates that can facilitate chemical availability to the phloem needs to be conducted for foliar sprays and also for potential applications at the root or crown level. Future trials will attempt to achieve this and replicating treatments at least six times will allow for better comparison and statistical analyses. Although it may be difficult to label an antibiotic for foliar applications in potato field production systems, streptomycin sulfate and other antibiotics are used as seed treatments, but could further be labeled for a single foliar or soil application at plant establishment.

5 Table 1 Total yield, total yield of US. No.1, under -1 ounce, over 18 oz, and culls!no.2 potatoes for three replicates of 15 Treatments on 'Russet Norkotah 223' potatoes grown near Springlake, Texas Treatment No. and Treatment Total Yield CwtJA Total Yield U.S. No. I Cwt. Per Acre IO oz oz IO-I8 oz Over I8 oz Under 4 oz. Culls/ No.2 I-U ntreated 2-Firewall (Fwl) 3-Prophyt 4-K-Phite (KPh) 5-KPX-B2 (KB) 6-Phostrol + Fw1 7-Actigard (Act) 8-Fwl + SAver 9-Fwl + Act IO-KPh + Act 11-KPh + Fwl I2-KPh +SAver 13-KB + SAver 14-KB + Act IS-Renew I I66.8 I I I0.7 I48.8 I I20.3 I I40.7 I82.7 I03.5 I25.5 I46.8 I I52.0 I79.2 I16.9 1I8.7 II6.2 9 II I II II9.7 I I I I0.5 I I I I Average L.S.D. (.05) Il ns ns ns

6 The judicious use of insecticides in combination with plant tolerance or resistance, and other practices such as the ones that may come out of this and future studies, management of Zebra chip could be attained using an integrated control approach for disease management. References Abad, J. A., Bandla, M., French-Monar, R. D., Liefting, L. W., and Clover, G. R. G First report of the detection of 'Candidatus Liberibacter' species in zebra chip disease-infected potato plants in the United States. Plant Dis. 93:108. Abdullah, T. L., Shokrollah, H., Sijam, K., and Abdullah, S.N.A Control of Huanglongbing (HLB) disease with reference to its occurrence in Malaysia. Afr. J. Biotechnol. 8: Csinos, A. S., Pappu, H. R., McPherson, R. M., and Stephenson, M.G Management of Tomato spotted wilt virus in flue-cured tobacco with acibenzolar-s-methyl and imidacloprid. Plant Dis. 85: Hansen, A. K., Trumble, J. T., Stouthamer, R., and Paine, T. D A new Huanglongbing (HLB) species, "Candidatus Liberibacter psyllaurous", found to infect tomato and potato, is vectored by the psyllid Bactericera cockerelli (Sulc). Appl. Environ. Microbial. 74: Li, W., Abad, J.A., French-Monar, R.D., Rascoe, J., Wen, A., Gudmestad, N.C., Secor, G.A., Lee, I-M., Duan, Y., and Levy, L Multiplex real-time PCR for detection, identification and quantification of' Candidatus Liberibacter solanacearum' in potato plants with zebra chip. Journal ofmicrobiol. Methods 78: Liefting, L. W., Weir, B. S., Pennycook, S. R., and Clover, G. R. G 'Candidatus Liberibacter solanacearum', a liberibacter associated with plants in the family Solanaceae. Int. J. Syst. Evol. Microbial. 59: Munyaneza, J. E., Goolsby, J. A., Crosslin, J. M., and Upton, J. E Further evidence that zebra chip potato disease in the Lower Rio Grande Valley of Texas is associated with Bactericera cockerelli. Subtrop. Plant Sci. 59:30-37.