their response to inoculation with the bacterium which causes walnut blight. Tested germplasm was selectedj for its unusual

Transcription

1 WALNUT BLIGHT: SUSCEPTIBILITY OF GERMPLASM AND THE RETENTION OF OVERWINTERING PATHOGN POPULATIONS Keith Woeste, Gle McGrnhn, Roy Yri nd M.N. Schroth ABSTRACT Aspects of the susceptibility of English wlnu~ to infection by Xnthomons cmpestris pv. juglndis, the terium which cuses wlnut blight, were investigted. Leves of 15 mture English wlnut trees were inoculted with the terium which cuses wlnut blight, nd fter 35 dys ech cultivr ws rted for infection severity. During the following dormnt seson, the buds ssocited with the infected folige of ech cultivr were smpled nd nlyzed to determine their frequency nd level of infesttion. Brnches with modertely infected folige hd higher frequency of bud infesttion thn brnches which were lightly or severely infected. Leves of 15 wlnut cultivrs, including 11 introductions not previously evluted for blight susceptibility, s well s 4 stndrds (included for comprison) were evluted in the field for their response to inocultion with the terium which cuses wlnut blight. Tested germplsm ws selectedj for its unusul genetic kground (reltive to cultivrs cu~rently grown in ) nd potentil contribution to bre~ding progrm for blight resistnce. Germplsm ws rted for lef infection 35 dys fter inocultion. Significnt differences in lesion size nd number were found, including n extreme rection which my be disese-voiding. A degree dy model ws fit to the increse in blight lesion size nd number. Methods for inoculting wlnut tissues in vitro with Xnthomons cmpestris pv. juglndis were tested. OBJECTIVE The objective of these experiments ws to compre the response of wide rnge of wlnut germplsm to Xnthomons cmpestris pv. juglndis inocultion, to develop in vitro inocultion techniques which will speed the screening of germplsm for blight susceptibility, nd to ssess the reltionship between severity of X c juglndis lef, infection nd subsequent overwintering popultions of the pthogen. This reserch will contribute to the development of new options for blight control. PROCEDURE Reltionship between severity of X c juglndis lef infection nd

2 subsequent overwintering popultions of the pthogen Inoculted nd control English wlnut brnches which were rted for blight susceptibility in 1989 were hrvested seprtely ccording to completely rndomized design, plced in plstic bgs nd stored t 10 C for 1 to 3 dys. Four pprently helthy buds from ech brnch were selected for evlution. When possible, two ctkin nd two vegettive/pistillte buds were chosen. Buds with necrotic res were only chosen if no helthy-ppering buds were vilble. All four buds from ech brnch were triturted with mortr nd pestle in 10 ml 0.01 M phosphte buffer. After triturtion, 1.0 ml of the suspension ws drwn off nd serilly. diluted. X-g, julndis colonies from the buds were isolted nd identified by spreding 0.1 ml of the serilly diluted bud suspension onto semi-selective medium. In vitro screening methods Mture lef nd nut nd green stem tissues from Sunlnd, PI , Hrtley nd wingnut (resistnt control) were surfce sterilized nd plced on wter gr or wet filter pper. Tretments (i.e., cultivrs, gr or filter pper medium, tissue type inoculted, inocultion method, inocultion vs. control) were completely rndomized. Inocultion ws with wter suspensions of X c juglndis t 105 nd 10" c.f.u. delivered with syringe into susceptible tissue which hd been punctured or scrtched with sterile needle. Evlution of germplsm for susceptibility Most of the tested ccessions originted in Europe or Asi, but N. Americn mteril with n unusul genetic kground ws lso included (Tble 1), s well s cultivrs with n estblished response. Inocultions were performed using methods described in previous reports. RESULTS AND CONCLUSIONS Reltionship between severity of X c juglndis lef infection nd subsequent overwintering popultions of the pthogen Anlysis of dormnt bud smples from inoculted brnches reveled tht Chndler nd PI hd significntly (R < 0.05) smller overwintering popultions of X c juglndis thn ll other cultivrs tested. When the lef-lesion rtings of ll cultivrs were pooled, brnches with lef-lesion rtings lower thn 17.9 or higher thn 28 were significntly less likely to ber infested buds thn brnches with lef-lesion rtings between 17.9 nd 28 (R < 0.001), nd brnches with lef-lesion rtings from 17.9 to 23.9 were more likely to ber infested buds thn brnches with lef-lesion rtings greter thn 23.9 (R < 0.039) (see Fig. 1). There is strong evidence tht the rpid senescence nd bscission of severely infected leves nd nuts my serve s n effective disese voidnce mechnism. By shedding severely infected tissues, infected plnts cn remove the pthogen from the cnopy before it hs chnce to spred. Necrotic or prtilly necrotic buds (ssocited with brnches with severely infected folige nd/or lrge, girdling stem cnkers) my lso reduce overwintering X c juglndis popultions if such buds mke poor hbitts for the pthogen

3 In vitro screening methods significnt differences were found in lef-lesion width mong English wlnuts, with 'Sinensis 5' proving more resistnt thn 'Hrtley' or 'Sunlnd'. However, lthough sttisticlly significnt differences in lesion width were detected for the lef inocultions, nd the procedures we used seemed to relibly infect the tissues (the pthogen ws consistently recovered from inoculted leves t the time of scoring); but the lesions tht developed were not typicl blight lesions nd they rrely expnded much beyond the re which hd been wounded by the inocultion procedure. In this respect, the inocultion methods were not stisfctory. Evlution of germplsm for susceptibility The results of the germplsm inocultions re found in Tble 2. The highest possible lef score t 35 dys ws 50. A wide rnge of scores ws observed. There does not seem to be ny correltion between lefing dte nd susceptibility. Most interesting were the most nd lest susceptible cultivrs. Sinensis 5 rted below 20 for the third yer in row. Chndler hd somewht higher rtings thn in previous yers. PI (lso erly), ws the most susceptible cultivr previously tested, but Abbotbd ws mrkedly more susceptible thn even PI in Nerly ll the inoculted leves of Abbotbd turned yellow nd bscised by 21 d.p.i.. Rpid lef bscission my be useful disese voidnce response under (dry) conditions. When ll the dt were pooled, there ws (highly significnt) liner correltion between degree dys ccumulted fter the ppernce of lesions nd lef lesion rting t 35 dys fter inocultion (R=0.417, R2=0.173). The regression eqution from the pooled dt for predicting the lef lesion rting ws X = Rting, where X is the number of degree dys ccumulted since lesions ppered. 207

4 Tble 1. Germplsm inoculted with Xnthomons cmpestris pv. juqlndis, its origin, lef lesion score 35 dys fter inocultion (d.p.i.), stndrd regression coefficient of lef lesion scores on degree dys ~ stndrd error, R2 vlue for regression of lef lesion score on degree dys, significnce of regression coefficient, predicted lef lesion score 197 dy degrees fter inocultion + stndrd error. Germplsm Origin Lef score t 35 d.p.i. stndrd regression coeff. + SE R2 PredictedZ significnce lef score of regress. t 197 decoefficient gree dys ~ SE IV o ex> Plcenti Sinensis 5 Bdjoz Conwy Myette Conch Chndler Shrkey Mnregin Alsoszentivni Olmo-70 Idho PI Olmo-20 Eurek Abbotbd Jpn Spin Frnce Chile Chin Hungry Crpthin Pkistn Y b b b b b b b c zpredictions were not mde when the regression coefficient ws not significnt t the 10% level. YDifferent letters indicte significnt difference t 5% level ccording to Tukey's test.

5 10 0 w 90 llj l.l 80 Z 70 0 ::> 60 ill l.l w -.J 30 n.. :2 «20 10 t--i t CONTROL h-} BRANCH RATINGS Fig. 1 Percent of smples of overwintering buds of Juqlns regi infested by blight from brnches with differing brnch rtings. During the growing seson, folige of 35 brnches on ech of 10 ~. regi cultivrs ws inoculted with spry suspensions of Xnthomons cmpestris pv. juglndis or wter only. Ech of six leves on ech inoculted brnch ws rted for lesion number nd size with 0 to 5 disese severity scle, nd the rting of ll leves on brnch were summed to give brnch rting. Smples of four buds from ech of the 175 inoculted nd 161 control brnches were tken during the dormnt seson. verticl brs re I stndrd error