TH E APPLI CATI ON OF AN TAGON I STI C BACTERI A Pse u dom on a s flu or e sce n s
TO CON TROL SOFT ROT D I SEASE ( Er w in ia ca r ot ovor a ) ON POTATO CROPS
I RD A SAFN I , S.P
Fa k u lt a s Pe r t a n ia n Un ive r sit a s Su m a t e r a Ut a r a
1 . I n t r odu ct ion
Soft rot disease, w hich is caused by bact eria Erw inia carot ovora, is a com m on disease on fleshy v eget ables, including pot at oes, t om at oes, cabbages, cucum bers, carrot s, et c. This disease is one of t he delet erious plant diseases occurring bot h in t he field and in st orage. The bact eria is a soil- borne pat hogen, w hich can live and survive freely in t he soil for a long period of t im e. Most bact eria E.carot ovora infect t he t ubers and root syst em of pot at o crops. By secret ing t ox ic enzy m es, pect oly t ic, t he bact eria break dow n t he cell w all, t hen cause cell separat ion and soft en t he fleshy regions ( Sigee, 1993) . As a result , t he cells of pot at oes will collapse and die. This collapse, in part icular for t ubers and root syst em , causes w ilt or yellow discolorat ion and t uber decay can also occur in t he field and in st orage ( De Boer, 1997) . All of t he sym pt om s, t herefore, reduce t he v igor and y ields of pot at o crops. As a consequence, it causes a great loss in pot at o yields as w ell as t he econom ic losses because of t he bad qualit y and quant it ies of t he infect ed pot at oes.
Many cont rol st rat egies hav e been applied t o reduce t he int ensit y of disease, nam ely sanit at ion, using pat hogen- free seed t ubers, m odificat ion of t he env ironm ent , crop rot at ion, and pest icide ( Gudm est ad and Secor, 1993; De Boer, 1997; and Agrios, 1978) . How ever, t he result s hav e not been sat isfact ory because cont rolling soft rot disease is quit e difficult ( De Boer, 1997) . Alt hough m any farm ers st ill use pest icide in t he pot at o field, t his m et hod is not recom m ended any longer due t o it s im pact on env ironm ent and hum an life. The t ox ic residue of chem ical subst ances in t he pest icide can be deposit ed in t he ground and w ill exist for a long-t erm period. I long-t can conlong-t am inalong-t e long-t he ground w along-t er, w hich is also long-t he source of w along-t er for hum an and anim al surrounding. The poisonous subst ances can also be deposit ed in t he pot at o y ields, w hich m ight endanger people w ho consum e t hem .
Because of t hose reasons, biological cont rol is a good alt ernat ive m et hod. Com pared t o chem ical cont rol ( pest icide) w hich dest roys a range of m icro and m acro organism s, biological cont rol has m ore specific effect on t he pat hogen and has lim it ed im pact on t he env ironm ent ( Sigee, 1993) . A num ber of m icroorganism s hav e been used as biocont rol agent s, w hich can be ant agonist ic fungi or ant agonist ic bact eria, such as fungi Trichoderm a spp., bact eria Bacillus t huringiensis, bact eria
Agrobact erium radiobact er, bact eria Pseudom onas spp, et c.
Bact eria P. fluorescens has been successfully applied on m any t ypes of crops t o cont rol a w ide v ariet y of soil- borne pat hogens, including fungi, bact eria, v iruses, and nem at odes. According t o Defago and Keel ( 1995) , bact eria P. fluorescens can be considered as a biocont rol agent of t he fut ure t o inhibit t he grow t h of root diseases, w hich are caused by soil- borne pat hogens. They can significandy reduce t he infect ion of a great num ber of diseases, such as pot at o w ilt disease ( Pseudom onas
( Fusarium oxyporum ) , and fire blight disease on peach and apple ( Envzonia
am ylovora) , et c.
This essay w ill analyze w het her bact eria P. fluorescens can effect ively reduce t he infect ion of soft rot disease on pot at o crops. Firsdy, I w ill describe t he beneficial role of bact eria P. fluorescens as a pot ent ial biological cont rol agent . Aft er discussing t he case st udies of cont rolling several plant diseases w it h bact eria P. fluorescens, I w ill exam ine t he possibilit y of using bact eria P .fluorescens t o cont rol soft rot disease on pot at oes.
2 . Th e Be n e ficia l r ole of P. flu or e sce n s a s a pot e n t ia l biocon t r ol a ge n t
Bact eria P. fluorescens, w hich is also called rhizobact eria, is a soil- borne m icroorganism w hich lives perm anendy in t he soil. Generally, soil- borne ant agonist m icroorganism s can only infect soil- borne m icroorganism t oo. Since P.fluorescens is a soil- borne pat hogen and can infect t he parasit ic pat hogens, it has great pot ent ial t o becom e a biological cont rol agent . According t o Bak er and Cook in Jasm inarni ( 1996) , t he effect ive ant agonist ic pat hogens are soil- borne pat hogens w hich can cont rol soil borne- diseases. Therefore, t he ant agonist ic m icroorganism s can easily inhibit t he grow t h of parasit ic pat hogens t hat cause diseases because bot h of t hem live in t he sam e area and t he sam e environm ent al condit ion.
To becom e an effect ive biocont rol agent , each ant agonist ic pat hogen m ust fulfil t w o condit ions, sw viving in t he soil and adj ust ing t o t he field condit ions ( Sigee, 1993) . Under t hese condit ions, a biocont rol agent can com pet e w it h t he parasit ic pat hogen for a long period of t im e.
I n addit ion t o t hose reasons above, t here are fiv e charact erist ics w hich can be used as principles for bact eria P.fluorescens t o becom e a good alt ernat ive biological cont rol agent , i.e :
1. Colonizing t he root syst em of t he host plant s. 2. Producing t he secondary m et abolit es ( siderophores)
3. Com pet ing for nut rient s and space w it h ot her m icroorganism s 4. Easily found in t he ecosyst em
5. Not t oxic for a variet y of crops
The first charact erist ic is t he abilit y of P. fluorescens t o colonize t he root syst em of host plant s. By colonizing t he plant 's root s early before t he appearance of t he parasit ic pat hogens, t he parasit ic pat hogens alm ost hav e no opport unit y t o inv ade t he plant s and proliferat e in t he host plant s. As st at ed by Sim ons et al ( 1996) , t he v ery im port ant st ep t o cont rol t he soil- borne disease is t he abilit y of bact eria P.fluoresens t o est ablish it self in t he soil. The abilit y t o colonize is also relat ed t o t he bact eria's abilit y t o persist in t he soil because t he bact eria can prevent t he infect ion for an ext ended period. I t w ill not be effect ive if t he ant agonist ic pat hogens can only live in t he soil for a lim it ed t im e.
Consequendy, t here is less iron available in t he soil and it w ill affect t he proliferat ion of t he parasit ic pat hogens, and t hen t he pat hogens can not grow w ell and lose t heir vigor.
I t s abilit y t o com pet e for nut rient s and space w it h ot her m icroorganism s is t he t hird charact erist ic of P.fluorescens w hich m akes it a good alt ernat ive biocont rol agent . This capabilit y can occur because t he bact eria lives in t he sam e habit at w it h t he parasit ic m icroorganism s. Sigee ( 1993) st at ed t hat t o becom e effect ive, t he ant agonist ic pat hogen should live in " t he sam e m icrohabit at s and have sim ilar grow t h charact erist ic" . Under t he sam e habit at s, t he needs of nut rient s for m icroorganism s are also sim iliar. Thus, it is easy for t he bact eria t o com pet e for nut rient s w it h ot her pat hogens. Lik ew ise, t he bact eria also at t em pt s t o seize t he sit e t o live from ot her m icroorganism s. As a result , t he pat hogenic pat hogens lose t heir food and place t o live. This condit ion can lead t o reducing t he exist ing num ber of t he pat hogens in t he soil. As m ent ioned by Sigee ( 1993) , sit e com pet it ion can cause t he lim it at ion of life phase of t he pat hogens because m ost pat hogens, part icularly bact eria, m ult iply on t he plant surface.
Anot her charact erist ic is t hat bact eria P.fI uorescens can be easily found in t he env ironm ent , such as in t he w at er, air, soil, food, and plant s. Besides being able t o suppress t he grow t h of t he pat hogens nat urally, t he bact eria can also be m ade art ificially t o becom e a com m ercial product . There are m any bact eria P.fluorescens w hich are av ailable in t he m arket and sold as biocont rol product s. For inst ance, Blight ban AS06, Conquer, and Vict us are w idely sold com m ercially in t he Unit ed St at es ( Unit ed St at es Depart m ent of Agricult ure, 1999) . I n I ndonesia, m any t y pes com m ercial product s of P.fluorescens are av ailable, such as B29, B39, A506, and WI ( Suw ant o et al, 1996; ] asm inarni, 1996; Efri, 1994) .
The last charact er is t hat t he bact eria is not pat hogenic or t ox ic t o a v ariet y of crops. Kerr ( 1980) w ho conduct ed a t est t o dist inguish w het her bact eria
P.fluorescens w as pat hogenic or not , proved t hat t he bact eria gav e a negat iv e
react ion t o pot at o and t obacco crops, w hich m eans t hat bact eria P.fluorescens is not pat hogenic t o pot at o crops. This charact erist ic is also v ery essent ial for t he parasit ic pat hogens. I f t he ant agonist ic bact eria can also funct ion as a pat hogenic m icroorganism , t he plant w ill suffer m ore severely.
All of t he specific charact erist ics of bact eria P.fluorescens suggest t hat t he bact eria hav e a great pot ent ial for becom ing a biocont rol agent t o reduce t he num ber of diseases w hich are infect ed by soil- borne pat hogens. The follow ing paragraph w ill show several successful cont rol m et hods using bact eria P.fluorescens as a biocont rol agent .
3 . Ca se st u die s of con t r ollin g se v e r a l pla n t dise a se s w it h ba ct e r ia P.flu or e sce n s
A great num ber of plant diseases hav e been successfully cont rolled by bact eria P.fluorescens. All of t he exam ples I am going t o discuss w ill becom e m y basis for using t he bact eria t o cont rol soft rot disease w hich is caused by
E.carot ovora. The first exam ple is pot at o w ilt disease, w hich is caused by bact eria P.solanacearum . This parasit ic pat hogen, bact eria P.solanacearum , is a soil- borne
P.fluorescens st rain B29 and WI on pot at o plant s can inhibit t he dev elopm ent of
bact eria P.solanacearum and t he pot at o y ields from plant s t reat ed w it h P.fluorescens w ere not infect ed. This described before t hat an experim ent proved t hat
P.fluorescens is not t ox ic t o pot at o crops. She also observed t hat w hen harvest ing,
P.fluorescens st ill exist ed around t he t ubers of pot at oes and could probably be used for t he nex t plant ing. I t m eans t hat P.fluorescens can exist in t he soil for an ext ended period, w hich is one of t he specific charact erist ics of P.fluorescens as a biocont rol agent . During t he colonizing of t he pot at o root , P.fluorescens m ight secret e sidophores and t hen can suppress t he grow t h of t he pat hogen
P.solanacearum .
Anot her exam ple is t om at o w ilt disease, w hich is caused by fungi Fusarium
oxysporum and t obacco w ilt disease, w hich is caused by Pseudom onas solanacearum . Alt hough pat hogen F.oxy sporum is a fungy pat hogen, it is included in
soil- borne pat hogen groups. Besides t hat , t he host plant s, t om at o and t obacco plant s, belong t o fam ily of solanaceae. Sim ilarly, pot at o crops include in fam ily of Solanaceae. According t o Efri ( 1994) , P.fluorescens can inhibit t he dev elopm ent of t om at o w ilt 71.7% com pared t o 0% of plant s w it hout applicat ion of t he bact eria. I n t he laborat ory t est , she observed t he appearance of em pt y zone in t he pet ridish t hat cont ained F.oxy sporum and P.fluorescens. The em pt y zone proved t hat t here w as a com pet it ion bet w een t he parasit ic pat hogen and t he ant agonist ic pat hogen for iron. As w as described in t he previous sect ion, P.fluorescens has a high affinit y t o bind iron. The bact eria t ak es t he iron and bind it t o it s necessit y , so t he fungi
F.oxy sporum lacks t he iron for it s proliferat ion and t hus t he dev elopm ent of t he
disease is suppressed. Lik ew ise, t he inhibit ion of t obacco w ilt disease occurs due t o siderophore com pet it ion. I n t he laborat ory experim ent , Arw iy ant o et al ( 1996) found a large inhibit ing zone in t he pet ridish w here P.fluorescens com pet ed w it h
P.solanacearum . I t m eans t hat bact eria P.fluorescens invades and suppresses t he
proliferat ion of t he bact eria.
The successful applicat ion of P.fluorescens has also been observed in soybean pust ule disease ( Xant hom onas cam pest ns) and apple and peach w ilt disease ( Erw inia
am ylovora) . Sim ilar t o t hose reasons above, bact eria X.cam pest ris and bact eria E.am ylovora are soil- borne pat hogens, w hich are t he sam e as bact eria E.carot ovora.
According t o Suw ant o ( 1996) , P.fluorescens st rain B29 and B39 can inhibit t he developm ent of pust ule disease because of t he com pet it ion for nut rient s bet w een bact eria P.solanacearum and bact eria P.fluorescens. Sim ilarly , t he dev elopm ent of w ilt disease on apple and peach plant s can also be reduced due t o t he com pet it ion bet w een bact eria P.fluorescens and bact eria Erw inia am y lovora ( St ellj es and Senft , 1998) . This is because of t he nat ural behav ior of living organism s w hich com pet e for food in order t o survive. This condit ion occurs in t he ecosyst em - land, w at er and air. The parasit ic pat hogen lacks t he nut rient s for t heir grow t h and m ult iplicat ion. Thus, t heir capabilit y t o invade t he host plant w eakens.
4 . Th e possibilit y of a pply in g P.flu or e sce n s a ga in st soft r ot dise a se
Aft er analyzing all of t he case st udies above, I suggest t hat bact eria
P.fluorescens can cont rol soft rot disease, caused by bact eria E. carot ovora, on
pot at o crops. The m ain reason is because of t he charact erist ics of bact eria
P.fluorescens, w hich have been m ent ioned above, nam ely it colonizes t he root
syst em of t he host plant , producing t he m et abolit e subst ances, com pet it ion for nut rient s and space, is easily found in t he env ironm ent and it is not pat hogenic t o m any host plant s. Using t hese charact erist ics, bact eria P.fluorescens could be able t o cont rol bact eria E.carot ovora w hich causes pot at o soft rot disease. I t had been proven by t he researchers t hat t hose charact erist ics play im port ant roles in cont rolling diseases.
Anot her im port ant reason is t hat bact eria E.carot ovora are included in soil-borne pat hogen groups. The pat hogens in t he experim ent conduct ed, nam ely
P.solanacearum , F.oxysporum , E.am ylovora, and E.carot ovora, are also soil- borne
pat hogens. Since m ost soil- borne pat hogens can be cont rolled by P.fluorescens, soft rot disease w hich is caused by bact eria E.carot ovora m ay also be able t o be cont rolled by bact eria P.fluorescens. This is also support ed by Defago and Keel ( 1995) say ing t hat bact eria P.fluorescens, soil- borne pat hogens, can effect ively suppress t he grow t h of soil- borne pat hogens, bact eria, fungi, or nem at odes. Moreover, t he host plant s of som e of t he experim ent s conduct ed w ere from t he fam ily of Solanaceae, w hich is t he sam e as pot at o crops. Consequent ly, t hey could also be successful t o cont rol soft rot disease on pot at o crops since it is a sim ilar bact eria and sim ilar fam ily host plant . The bact eria w ill easily adj ust t o t he environm ent .
How ever, t o be effect ively used as a biocont rol agent , som e considerat ions should be t aken int o account . I nit ially, select ing st rains of P.fluorescens is t he m ost im port ant considerat ion. Each st rain has different capabilit y, in part icular for colonizing t he root syst em . This is also support ed by Sim ons et al ( 1996) w ho found t hat various st rains P.fluorescens have different abilit y t o colonize pot at o root syst em . For inst ance, st rain WCS307 and WCS315 are poor colonizers of pot at o rot , WCS374 and WCS379 are m oderat e colonizers, and WCS365 is efficient colonizer. I n addit ion, select ing P.fluorescens st rains is also im port ant t o let erm ine w het her t he subst ances secret ed are t ox ic or not for bact eria E.carot ovora and ot her beneficial m icroorganism s in t he soil since som e of P.fluorescens st rains could be pat hogenic t o som e m icroorganism s.
Besides t hat , environm ent al condit ions should be carefully considered since bact eria P.fluorescens, as living organism s, are influenced by environm ent al fact ors surrounding t hem , such as t em perat ure, m oist ure, soil acidit y, soil at m osphere, et c. Therefore, t hey m ay not be consist ent in cont rolling t he pat hogens. As m ent ioned by Efri ( 1994) , t he abilit y of P.fluorescens for suppressing t he dev elopm ent of t om at o w ilt disease w as only effect ive in low er soil acidit y ( PH 5.7) . Likew ise, t he alt erat ion of oxygen and carbon dioxide level in t he soil, w here P. fluorescens w ere applied t o cont rol P.solanacearum on t om at o crops influenced t he effect iveness of bact eria
P.fluorescens ( Kim and Mishagi, 1996) .
support ed by Jasm inarni ( 1996) w ho st at es t hat t he applicat ion of P.fluorescens aft er t he proliferat ion of t he parasit ic pat hogen on pot at o w ilt disease could not reduce t he dev elopm ent of t he disease.
Finally, t he quant it y or dosage of P.fluorescens can also affect t he ibilit y of
P.fluorescens t o cont rol t he disease effect ively . The am ount of dosage det erm ines
t he effect iveness of P.fluorescens t o suppress pat hogen E.carot ovora in each pot at o t ubers. I n an experim ent , Jasm inarni ( 1996) not ed iliat t here w as a different int ensit y of disease on pot at o crops infect ed by w ilt disease, w hich w as giv en by different dosage of P.fluorescens. The plant s t hat w ere giv en only a sm all dosage of
P.fluorescens show ed a severe infect ion. This is because w it h t he sm all am ount of
ant agonist ic pailiogen, ilie bact eria could not com pet e w it h t he larger populat ion of t he parasit ic pat hogen in t he soil.
5 . Con clu sion
Bact eria P.fluorescens is w idely know n as a good biological cont rol agent for soil- borne diseases- diseases in w hich t he pat hogens live pem lanent ly in t he soil. Aft er exam ining t he successful experim ent s w it h oilier soil- borne diseases, it can be concluded t hat P.fluorescens m ay also be able t o inhibit t he grow t h of bact eria
E.carot ovora, w hich cause soft rot disease. First ly, t his is because it has several
specific charact erist ics, nam ely colonizat ion of host plant s' root syst em , secret es m et abolit e subst ances ( siderophore) , has abilit y t o com pet e w it h nut rient s and space, is easily found in t he ecosyst em , and it is not t oxic for a variet y of crops. Secondly, bact eria E.carot ovora is included in soil- borne pat hogens t o w hich t he bact eria P.fluorescens can be effect ively applied. I n addit ion, t o being effect ively used, som e considerat ions should be observed, including select ion of st rains, env ironm ent al condit ions, t im e of applicat ion, and t he quant it y of P.fluorescens.
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