2 Maataloustieteellinen A ikakauskirja Vol. 62: 245—254, 1990 Yield reduction of spring barley in relation to disease development caused by Rhynchosporium secalis REIJO KARJALAINEN Department of Plant Pathology, University of Helsinki, SF-00710 Helsinki, Finland Abstract. Effects of barley scald caused by Rhynchosporium secalis on grain yield were studied in three spring barley cultivars under field conditions using artificial inoculation over three years. The disease stronglyreduced the green-leaf area duration compared with fungicide- treated leaves. At low infection level, R. secalis reduced the grain yield of barley by 3—5 %. Moderate and severe infection reduced the grain yields of susceptible cultivars by 10—12 %. Single-tiller analysis of yield components indicated that grain weight and ear weight were most affected, but the number of grains was only insignificantly reduced by the disease. Implica- tions of these results for controlling scald disease in Finland are discussed. Index words: Rhynchosporium secalis, spring barley, yield loss Introduction Rhynchosporium secalis (Oud.) Davis, the causal agent of leaf scald, is a serious patho- gen of barley in many parts of the world (Shipton et al. 1974). When environmental factors favour scald development, yield losses of up to 35—40 % have been reported for spring barley (Schaller 1951, Jenkins and Jemmett 1967, James et al. 1968). Rhyn- chosporium is heavily dependent on rain and high humidity. The spores are dispersed by rain (Polley 1971, Stedman 1980), and mycelia on dead leaves can produce new spores within24 h of wetting, but after drying there is no further sporulation until rewetting occurs (Skoropad 1962). Ryan and Clare (1975) have shown that, depending on the iso- late, the optimal temperature for germ tube production and growth as well as for maximal lesion development immediately following in- oculation, is 15—25°C. Leaf surface wetness for over 14hours appears to give optimal con- ditions for lesion and symptom development (Polley 1971, Ryan and Clare 1975). It is interesting, however, that in many barley growing areas R. secalis survives between cropping even when it is very hot and there is little rain (Mayfield and Clare 1985). For example, in Australia (Mayfield and Clare 1984) R. secalis in host debris survives sum- mer temperatures of up to 20°C above those 245 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=ArBZ-z5kS7kizBmg.R6dnSO2AanCo5Gi16mZRMA.WFgEaIYaI9qKsX1RcK1MB0ANCd8yhsQgB62g3MOA2jeuBDFDuUtyCfuSCb5bvpW7doi9aqMmiM44oJbXicd5672mQzp3i_KUYiw3N717CGN50Dr64LIeLCkzoWaXvrAGQ-ZHYnZG-LHJR1W8iTofClBKve1ROuud9MOgBO6HvQ that usually inhibit spore germination (Ryan and Clare 1975), and the pathogen is subse- quently capable of producing inoculum that infects barley. In Finland, I have found that R. secalis can easily survive and even grow in barley canopy in rainless and hot summer peri- ods, apparently because the canopy is wet at night, and low light intensity provides a favourable compensation environment for scald development. In the past twenty years, barley scald disease has become increasingly common in Finland. During thatperiod the barley area in Finland has remarkably increased, and barley cultiva- tion has become more specialized. The in- creased area and monoculture have at least partly contributed to the recent development of scald disease in Finland. In a disease survey carried out by Mäkelä (1974) in 1971—1973 30 % of the fields were infected by R. secalis. However, even though scald seems to be a very important pathogen of barley in cool and wet years, no information is available of its impact on the yield. The present study was designed to provide information of how R. secalis can influence yield and yield components of some common- ly grown barley cultivars in Finland. The sig- nificance of these results for disease control is discussed. Materials and methods Yield reduction in barley caused by scald disease was studied in three years, 1983, 1985, and 1986. Three cultivars, susceptible Hank- kija’s Pokko, Ida, and moderately resistant or less susceptible Birger were selected for field tests. Experiments were carried out in a ran- domized block design with eight replications, half of which were inoculated with R. seca- lis. In 1983, experiment I was designed to study the edge effects of plot on disease in- duced yield loss. Therefore, 20 m 2 plots were used, but only the central part of the plot was used for yield harvesting. In the other experi- ments, plot size was 10 m 2. Normal fertiliza- tion levels and herbicide treatments were used. All inoculated plots were surrounded by oat guard plots to prevent inoculum from spread- ing into control plots. Inoculation was carried out by spraying R. secalis spore suspension onto test plots. In most cases, mixtures of two or three isolates of R. secalis were used. The isolates were col- lected from various parts of barley area in Fin- land and cultured on lima bean agar at 16°C for 10—14 days for spore production. Spore suspension of about 106 spores/ml were used for inoculation. Inoculations were made late in the evening in order to ensure high humidity for disease development. In 1983, inoculations were start- ed at the beginning of flag leaf emergence, at a late stage of barley development. Occasional rains a few days after inoculations enhanced disease development so that only one inocu- lation was made. In 1985 and 1986, inocula- tions were started already at the three leaf stage, and three inoculations were made every fifth day. Disease development was monitored by counting 60 plants per plot and estimating dis- ease containing leaf area on three upper leaves. At the same time, green-leaf area du- ration was monitored on labelled tillers by visually estimating non-green leaf area because this trait was more accurate to estimate than green-leaf area. In some cases, fungicide treatments (Bayle- ton, Tilt) were included in the trials in order to evaluate the effects of fungicides to con- trol scald disease. Before harvest, the 60 labelled tillers per plot were collected and subjected to yield com- ponent analysis. Grain yields and thousand grain weights were analyzed from whole plot data. Analyses of variance were computed from yield data. Results Disease development Weather conditions during the growing sea- sons varied greatly (Table 1). In 1983 the 246 Table 1. Monthly mean temperatures and total rainfall in May-August in the years 1983, 1985 (Helsinki-Malmi airport), and 1986 (Helsinki, Kaisaniemi). Mean Rainfall temperature mm °C 1983 May 11.8 37.5 June 13.9 55.3 July 18.3 23.8 August 15.8 52.4 1985 May 9.4 60.5 June 13.7 66.3 July 16.1 77.5 August 16.3 95.4 1986 May 10.3 36.0 June 17.0 29.0 July 17.3 82.0 August 14.6 160.0 weather was dry, and particularly in July the rainfall was exceptionally low. However, at the same time the temperature was higher than in most years, and R. secalis infection progressed first moderately, but slowed down towards the end of the growing period (Fig. 1). In 1985 the weather was favourable for scald development. June and July were cool and rainy, and R. secalis progressed rapidly from the lower leaves to the upper parts of the canopy (Fig. 2). Summer 1986 was exceptional in many respects. It rained very little in June, moder- ately in July, but both June and July temper- atures were high compared with previous years. Low rainfall in Juneand the high tem- peratures in June and July apparently delayed the epidemic build-up of R. secalis, and sever- al inoculations were needed to induce disease outbreak (Fig. 3). In all experiments cv. Pokko was more se- verely infected by R. secalis than cv. Birger. Fungicide treatments reduced the disease pro- gress into the canopy (Fig. 1). Disease-induced green-leaf area destruc- tion was monitored simultaneously with dis- ease severity. The results indicate (Fig. 4) that leaf-area duration was inversely associated with disease progress. Birger appeared to pro- long leaf area duration under disease pressure Fig. I. Development of Rhynchosporium secalis on two upper leaves of cv. Pokko in 1983 under dry weather con- ditions. Fig. 2. Development of R. secalis on second leaves of cultivars Pokko and Birger in 1985 under favourable weather conditions for disease development. Fig. 3. Development of R. secalis on two upper leaves of cv. Pokko in 1986. 247 better than the more susceptible Pokko (Figs 5,6). Fungicide treatment delayed the senes- cence of leaves (Fig. 4). In 1985, the disease reduced green-leaf area more rapidly than in years 1983 and 1986, which was in accor- dance with the rapid progress of scald under favourable weather (Fig. 2). Effect of R. secalis on grain yield Despite the unfavourable weather for scald development in 1983, R. secalis induced 10— 11 % (p<0.01—0.1) yield losses in Pokko in two experiments compared with the untreated control (Table 2). The plot size of experiment I was twice that of experiment 11, and only the central part of plot I was harvested and used for yield measurement. In this way the edge effects were considered to be reduced. However, there was only a small difference in yield loss between these two trials. In 1985, the weather was favourable for a rapid disease build-up, but R. secalis did not reduce the grain yield of Pokko more than 12 % (Table 3). In the same trial, R. secalis did not cause any reduction in the grain yield of Birger. That was interesting because the dis- ease was able to progress in Birger (Fig. 2). In Table 3, the results of Birger are based on two years’ combined data (1985—1986) be- cause the numberof replications in both years was small. In 1986, the weather was not suitable for Table 2. Effect of R. secalis on grain yield of spring barley cultivar Hankkija’s Pokko, in comparison with uninoculated controls in the year 1983. Treatment Grain yield kg/ha ratio kg/ha ratio Experiment I Experiment 2 Control 3753 100 4856 100 Rhynchosporium- inoculation 3365 90 4309 89 Significance * ** L.S.D. s% 332 329 *, ** significant at P<0.05, PcO.Ol levels respectively. Fig. 4. Green-leaf area duration on two upper leaves of cv. Pokko in 1983 under disease stress caused by R. secalis. Fig. 5. Green-leafarea duration on second leaves of cul- tivars Pokko and Birger under disease stress in 1985, Fig. 6. Green-leaf area duration on two upper leaves of cultivars Pokko and Birger under disease stress in 1986. 248 Table 3. Effects of R. secalis on grain yield of spring barley cultivars Hankkija’s Pokko and Birger, in com- parison with uninoculated controls in the year 1985. Treatment Grain yield kg/ha ratio Pokko Control 4340 100 Rhynchosporium-inoculalion 3807 88 Significance L.S.D.s? , 100Birger 4882 Rhynchosporium-inoculation 4822 99 Significance L.5.D.;% N.S. 758 N.S. = not significant Table 4. Effects of R. secalis on grain yields of spring barley cultivars Hankkija’s Pokko and Ida, in compari- son with uninoculated controls in the year 1986. Treatment Grain yield kg/ha ratio Hankkija 's Pokko Control 5156 100 Rhynchosporium-inocu\ation 4877 95 Significance N.S. L.S.D. 5„. 624 Ida Control 4223 100 Rhynchosporium-inocu\ation 4090 97 Significance N.S. L.S.D.5 „. 599 N.S. = not significant; * significant at P<0.05 level. rapid disease spread, and R. secalis reduced the grain yields of susceptible varieties Pok- ko and Ida only 5 and 3 %, respectively (Ta- ble 4). Effect of R. secalis on yield components Infection was severe in experimental fields in 1983 and 1986 only at a late stage of barley Table 5. Effects of R. secalis on 1000-grain weights of spring barleycultivars Hankkija’s Pokko, Ida, and Birger, in comparison with uninoculated controls in the year 1986. Treatment 1000-gw g ratio Hankkija’s Pokko Control 49.7 100 Rhynchosporium- inoculation 47.3 95 Significance * L.S.D.S* 2.1 Ida Control 54.1 100 Rhynchosporium- inoculation 53.6 99 Significance N.S. l.O Birger Control 59.8 100 Rhynchosporium-inoculation 59.7 100 Significance N.S. 6.8 N.S. = not significant; * significant at p<0.05 level. development, and thus gram weight was the yield component principally affected by the disease. Measurements of thousand grain weights from whole plots in 1986 revealed sig- nificant reduction only in the grain weight of Pokko (Table 5), while the susceptible culti- var Ida suffered only slightly. The grain weight of Birger was not affected at all. Single tillers were taken from the plots for more detailed analyses. The yield component data (Table 6) showed that disease induced sig- nificant reductions in the grain weight of Pok- ko. Grain numbers/ear and ear weights were also reduced but non-significantly. Infection reduced all yield components of Ida, but grain weight and ear weight were significantly reduced (Table 6). The results thus suggest that the grain weight data above, based on samples from whole plots, might be more liable to sample errors than the databased on single tillers. 249 Table 6. Effect of R. secalis on yield components of spring barley cultivars Hankkija’s Pokko and Ida in com- parison with uninoculated controls in the year 1986. Treatment grains/ ear weight 1000-gw ear g g ratio Hankkija ’sPokko Control 34.8 1.31 37.3 Rhynchosporium- inoculation 32.9 1.17 35.0 Significance N.S. N.S. ** L.S.D. 5„. 4.4 0.19 2.7 Ida Control 16.4 0.83 50.3 Rhynchosporium- inoculation 15.8 0.75 47.1 Significance N.S. ** *** O.B 0.06 1.5 N.S. = not significant; **, *** significant at PcO.Ol, P< 0.001 levels respectively. Discussion The Finnish climate is favourable for scald disease development. In most years the grow- ing season is cool and wet, and later in sum- mer dew provides humidity inside barley cano- py for many hours, which ensures scald sur- vival even in rainless periods. Finnish farm- ing practice also favours the development of scald disease because barley fields are often small and surrounded by forests which keep them humid. Autumns are often wet, which makes effective ploughing difficult with the consequence of abundant debris remaining on soil. R. secalis can overwinter in plant debris (Polley 1971) and spread into new barley growth in the next season. This and infected seed are apparently the main sources of its in- oculum in Finland. There are some clear in- dications that the increasing use of CCC to prevent lodging promotes the incidence of R. secalis (Shipton et al. 1974), and in south- ern Finland farmers have increasingly used CCC in recent years. All these factors and the fact that most currently grownbarley cultivars are susceptible to R. secalis (Karjalainen, unpublished) have contributed to the wide- scale incidence of scald disease in Finland in the past years. The results reported here show a yield reduction of 5—12 °7o in the susceptible culti- var Pokko in relation to untreated control plots. A similar type of experiment was car- ried out by Skoropad (1960), who compared inoculated and fungicide treated plots. He found 10—12 % yield losses due to induced field epidemics caused by R. secalis. In Cali- fornia, Schaller (1963) reported crop losses of up to 35 % using isogenic lines. It is in- teresting to note that even during the severe epidemic of 1985, R. secalis induced yield losses of only 12 %, whereas under similar dis- ease conditions Septoria nodorum can reduce the yield of susceptible wheats by 20 % (Kar- jalainen 1985). This might be due to the longer growing time of spring wheat in Fin- land compared to barley, and thus S. nodo- rum has a longer time to affect grain filling of wheat than R. secalis of barley. Our cultivars appear to respond differen- tially to the disease. Birger, which was also in- fected but not to such extent as Pokko and Ida, did not seem to suffer any yield losses. It remains to be seen whether this is due to tolerance effect or not. In a previous glass- house study Rowling and Jones (1976) ob- served some evidence of tolerance after R. secalis infection. In our study the yield loss of barley was ap- parently mainly due to disease-induced green- leaf area destruction. Thus the total photosyn- thetic area was reduced with the consequence that dry matter accumulation into grains was limited. This is in accordance with our obser- vations, which showed that the 1000-grain weight was the main yield component reduced due to infection. In 1983, inoculation was made during flag leaf emergence, and it is known (Teng and Gaunt 1980) that such late inoculation principally reduces grain weight because the other yield components have al- ready been formed. However, in 1986 inocu- 250 lation was made immediately at the three-leaf stage and repeated three times. In this experi- ment all yield components of susceptible Pok- ko and Ida were affected, but most of all the 1000-grain weight. Our results confirm some previous glass- house and field trials (Rowling and Jones 1976, Jackson and Webster 1981) where R. secalis infection mainly lowered grain weight and the number of grains per ear. There is some evidence (Gallagher et al. 1975) that although photosynthesis post-anthesis gener- ally provides the majority of the dry matter in the barley grain, under extreme conditions considerable part of the final grain weight can also be provided by retranslocation. Therefore early infection can also influence the final grain weight. It is evident that the yield loss caused by R. secalis is not accounted for merely by the reduced amount of green-leaf area dura- tion. R. secalis is a necrotrophic pathogen in- fluencing the diseased barley plant in sever- al ways. Abnormal stomatal behaviour oc- curs after the fungus has penetrated into leaves (Ayres 1972, Branchard and Laffray 1987). Increased transpiration of scald infect- ed leaves has also been observed (Ayres and Jones 1975). In addition, R. secalis excretes toxic molecules, such as Rhyncosporoside- type compounds (Auriol et al. 1978), which have been found to affect the loss of turgor in young plants. Recently, Martin (1986) showed that photosynthesis in barley leaves was reduced mainly at about the time of scald symptom appearance, but reduction was not observed before visible symptoms. The above experiments show that the scald disease of barley can reduce grain yield by up to 3 —12 % under moderate to severe infec- tion conditions. Yield reduction is mainly due to lowered grain weight, which also implies that scald apparently reduces the hectolitre weight. There is large variation in the inci- dence of scald disease between different years, regions, and fields. The occurrence of scald is not uniform even within a single field, and the spatial distribution of disease units in a pathosystem is the most important factor in- fluencing field estimations of disease and yield loss (Teno 1983). Therefore, it is extremely difficult to predict how much scald can total- ly reduce Finnish barley yields because exten- sive field surveys of disease incidence would be required to address this question. James et al. (1968) developed a yield loss disease severity relationship equation, which based on the observation that there is a linear relation- ship between yield and the percentage of lami- nar area affected by leaf blotch on the flag and second leaves at growth stage 11.1. How- ever, more recently it was demonstrated (Khan and D’Antuono 1985) that third leaf disease amount correlates best with observed yield loss. Recent advances in using remote sensing as- sessment method and their implications (Nut- ter and Cunfer 1988) that they may predict yield losses caused by R. secalis relatively ac- curately might give an easier and more effec- tive way to evaluate the national losses of bar- ley crop induced by scald disease. It seems evi- dent that for the past two decades, and even before that (Mäkelä 1974), scald has been the major leaf disease of barley of significant economic importance in Finland. The most serious losses have occurred in wet summers when humidity has provided ideal build-up conditions for the disease. An effective and the economically cheapest form of crop protection is to use disease resis- tant cultivars (Doodson 1981). There are several barley cultivars and lines possessing major genes for scald disease resistance (Webster et al. 1980), which can be incorpo- rated into well-adapted barley backgrounds. However, breeding scald-resistant barleys in Finland is still at an early stage, and all culti- vars currently grown in Finland are fairly sus- ceptible to the disease (Karjalainen, unpub- lished). Chemical control of leaf spot diseases of barley is relatively new in Finland, and two fungicides (Bayleton and Tilt) have been used 251 252 to some extent. The present study shows that fungicide treatments clearly prolong the green- leaf area duration with the consequence of in- creased grain weight compared to the untreat- ed controls. Yield increase due to fungicide treatment in this experiment varied between 5—20 % compared to untreated controls. The current barley prices in Finland imply that the benefit due to fungicide control should be about 250 kg/ha or more (Kurtto 1986). The present study suggests that fungicide treatment is profitable in cool and wet years, when scald reduces yield by s—lo5 —10 % or more, but it is unlikely to be profitable in dry, rain- less summers. The rational use of fungicides to control scald disease requires a disease monitoring system which allows the right tim- ing of control. Simple forecasting techniques used e.g. for Seploria nodorum monitoring (Tyldesley and Thompson 1980) might prove out to be valuable aids for predicting the op- timal time for the control of scald disease be- cause scald is strongly dependent on temper- ature, the frequency and amount of rain. References Aufhammer, W. von, Kubler, E. & StOtzel, H. 1984. Effekte der Sortenmischung auf die Ertragsbildung von Gerstenbeständen. J. Agronomy & Crop Science 153: 385—397. Auriol, P., Strobel, G., Beltran, P.J. & Gray, G. 1978. Rhynchosporoside, a host-selective toxin produced by Rhyncosporium secalis, causal agent of scald disease of barley. Proc. Natl. Acad. Sci. USA 75: 4339—4343. Ayres, P.G. 1972. Abnormal behaviour of stomata in barley leaves infected with Rhynchosporium secalis (Oudem.) J. J. Davis. J, Exp. Bot. 23: 683—691. & Jones, P. 1975. Increased transpiration and the accumulation of root absorbed 86Rb in barley leaves infected by Rhynchosporium secalis (leaf blotch). Physiol. 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Disease progress of non-specialised fungal pathogens in in- traspecific mixed stands of cereal cultivars. I. Models. Ann. Appi. Biol. 98: 187—198. Jenkins, J.E.E. & Jemmett, J.L. 1967. Barley leaf blotch. NAAS O. Rev. 75: 127—132. Karjalainen, R. 1985. Host-pathogen interaction be- tween spring wheat and Seploria nodorum with refer- ence to resistance breeding. J. Agric. Sci. Finl. 57: 1—66, 1987. Effect of cereal cultivar mixtures on the epi- demic development of necrotrophic leaf pathogens. Växtskyddsrapporter, Jordbruk 21 —28. & Hiivola, S-L, 1987. Performance of cultivar mix- tures under northern growing conditions in Finland. Barley Genetics V, ed. Yasuda, S. & Konishi, T. p. 719—725. Sanyo Press, Okayama. Khan, T.N. & D’Antuono, M.F. 1985. Relationship be- tween scald (Rhynchosporium secalis) and losses in grain yield of barley in western Australia. Aust. J. Agric. Res. 36: 655—661. Kurtto, J, 1986. Milloin viljojen lehtilaikkutautien tor- junta kannattaa. Koetoim. ja Käyt. 43, 24.6.1986. Martin, P.J. 1986. Gaseous exchange studies of barley leaves infected with Rhynchosporium secalis (Oudem) J, J. Davis. Physiol, Mol. Plant Pathol. 28: 3—14. Mayfield, A.H. & Clare, B.G. 1984, Survival over sum- mer of Rhynchosporium secalis (barley leaf blotch). Trans. Br. Mycol. Soc. 60: 273—282. & Clare, B.G. 1985. Effects of high temperatures on scald lesion development and sporulation in barley in- fected with Rhynchosporium secalis. Aust. J. Agric. Res. 36: 197—200. McDonald, 8.A., Allard, R.W. & Webster, R.K. 1988. Responses of two-, three-, and four-component bar- ley mixtures to a variable pathogenpopulation. Crop. Sci. 28: 447—452. Mundt, C.C. & Leonard, K.J. 1986. Analysis of factors affecting disease increase and spread in mixtures of immune and susceptible plants in computer-simulated epidemics. Phytopathol. 76: 832—840. Mäkelä, K. 1974. Occurrence ofRhynchosporium seca- lis (Oud.) J. J. Davis on spring barley and winter rye in Finland. J. 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Agronomy J. 43: 183—188. 1963. The effect of mildewand scald infection on yield and quality of barley. Agron. J. 43: 183. Shipton, W.A., Boyd, W.J.R. & Ali, S.M. 1974. Scald of barley. Rev. Plant Pathol. 53: 839—861. Skoropad, W.P. 1960. Barley scald in the Prairie provinces of Canada. Comraonw. Phytopath. News 6: 25—27. 1962.Effect of alternate wetting and drying on sporu- lation and survival of Rhynchosporium secalis. Phytopathol. 52: 752. Stedman, O.J. 1980. Observations on the production and dispersalof spores, and infection by Rhynchosporium secalis. Ann. Appi. Biol. 95: 163—175. Teno, P.S. 1983. Estimating and interpreting disease in- tensity and loss in commercial fields. Phytopathol. 73: 1587—1590. & Gaunt, R.E. 1980—81. Modelling systems of dis- ease and yield loss in cereals. Agricultural Systems 6: 131 154. Tyldesley, J.B. & Thompson, N. 1980. Forecasting Sep- toria nodorum on winter wheat in England and Wales. Plant Pathol. 29: 9—20. Webster, R.K., Jackson, L.F. & Schaller, C.W. 1980. Sources of resistance in barley to Rhynchosporium secalis. Plant Dis. 64: 88—90. Wolfe, M.S. 1985. The current status and prospects of multiline cultivars and variety mixtures for disease re- sistance. Ann. Rev. Phytopathol. 23: 251 —273. Ms received October 6, 1989 SELOSTUS Rengaslaikkutaudin vaikutus ohran sadontuottoon Reijo Karjalainen Kasvipatologian laitos Helsingin yliopisto, 00710 Helsinki Tässä tutkimuksessa selvitettiin rerngaslaikkutaudin (Rhynchosporium secalis) vaikutusta kahden alttiin (Rok- ko, Ida) ja yhden kohtalaisesti tautia kestävän (Birger) ohralajikkeen satoon ja eräisiin satokomponentteihin. Kokeet tehtiin Helsingin yliopiston Viikin koetilalla vuo- sina 1983, 1985 ja 1986. Olosuhteet taudin leviämiselle vaihtelivat vuosittain. Taudin kehitystä seurattiin lehti- analyyseillä arvioimalla ylimpien lehtien tautisuusindek- si ja vihreän lehtialan kesto. Sadonkorjuun jälkeen las- kettiin jyväsato ja 1000-jyvän paino. Lisäksi määritettiin Rokon ja Idan satokomponentit. Rengaslaikku on viileiden ja sateisten kesien tauti, ja siksi vuosina 1983 ja 1986, jolloin oli lämmintä ja kui- vaa, se levisi kasvustossa hitaasti. Vuonna 1983 tauti alensi kahdessa kokeessa Rokon jyväsatoa 10—11 % ja vuon- na 1986Rokon satoa 5 % ja Idan 3 %. Vuonna 1985 kas- vukausi oli sateinen ja viileä, ja tauti levisi aikaisin ohra- kasvuston yläosiin ja aiheutti Rokolle 12 % satotappion. Tauti alensi huomattavasti yksilöpuitujen tähkien jyvän painoa ja koko tähkän painoa muttei merkittävästi jy- vien lukumäärää tähkissä. Rengaslaikkutauti lyhensi voimakkaasti ohran lehtialan kestoa, jolloin yhteyttävä kokonaispinta-ala pieneni ja jy- vän kuiva-aineen muodostus väheni. Tämän seuraukse- na jyvien koko pieneni. Tilt-käsittely hidasti tehokkaasti taudin kehitystä ja piti lehdet vihreänä pidempään. 253 Lippulehden kehittymisen jälkeen tehty Tilt-käsittely paransi eri kokeissa jyväsatoa 5—20 %. Tulokset viittaa- vat siihen, että taudin torjunta kannattaa sadevuosina, jolloin patogeenipopulaationkehitys voidaan pysäyttää oikein ajoitetulla torjuntakäsittelyllä ja siten estää leh- tien ennenaikainen kuihtuminen. Sen sijaan poutavuosi- na taudin kehitys kasvustossa on hidasta, eikä torjunta useimmiten kannata. 254