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African Crop Science Journal, Vol. 8. No. 3, pp. 213-222 TRANGRESSIVE SEGREGATION FOR RESISTANCE IN WHEAT TO SEPTORIA TRITICI BLOTCH P.F. ARAMA, J.E. PARLEVLIET1 and C.H. VAN SILFHOUT2 Kenya Agricultural Research Institute, National Plant Breeding
Research Centre, P.O. Njoro, Kenya (Received 5 March, 1998; accepted 14 March, 2000) Code Number: CS00023 INTRODUCTION Septoria tritici, the causal organism of septoria tritici blotch (STB), is at present the second most important wheat pathogen after yellow rust (Puccinia striiformis) in reducing yields in Kenya. The disease is also of major importance in the highlands of Ethiopia and Tanzania, in the coastal areas of the Mediterranean, in South America, in Australia and in Western Europe (Saari and Wilcoxon, 1974; Rajaram and Dubin, 1977). A crop failure due to blotch was reported from Kenya as early as the mid twenties by Burton (1927) and efforts were made in the sixties to identify sources of resistance (Saari and Wilcoxson, 1974). A Regional Disease and Insect Screening Nursery (RDISN) was subsequently established. Some of the best sources of resistance to S. tritici in the 1971-1972 nurseries were Kenya wheat lines selected in Ethiopia (Pinto, 1972) and submitted for regional testing in the RDISN. After this initial work in Kenya, little was done on STB over the next two decades. This was probably due to the importance of stem rust (Puccinia graminis) in the 1970’s and that of yellow rust (Puccinia striiformis) in the 1980’s (Danial, pers. comm.). To improve stem rust resistance and yielding capacity, early maturing semi-dwarf lines from the International Maize and Wheat Improvement Centre (CIMMYT), Mexico were introduced into the breeding programme. Because of the relationship between plant stature, maturity class and resistance to STB (Danon et al., 1982; Eyal et al., 1983; Arama et al., 1994) the more STB resistant germplasm was unconsciously discarded. Incidences of STB increased gradually but was initially unnoticed. Favourable weather conditions prevailing in Kenya in the 1985 and 1986 growing seasons led to severe STB epidemics and a renewed interest in STB resistance. Most of the high yielding wheat cultivars grown in Kenya today are quite susceptible to STB. Cultivation of resistant cultivars is the economically most feasible control measure for many wheat diseases. Undoubtedly, most breeders and plant pathologists want a form of resistance that keeps its effectiveness over time, and which is easy to transfer across genotypes, easy to identify in segregating progenies and effective under disease conducive conditions. However, germplasm resistant to STB is rather scarce, and little is known about the types of resistance and the mode of inheritance (Eyal, 1981). Conflicting reports are found in the literature regarding the nature of genetic resistance to STB. These range from simple Mendelian genetics to complex quantitative inheritance patterns. Mackie (1929) found, by analysing F2 populations, that a single recessive gene provided resistance in an unidentified cultivar. Single dominant genes for resistance have been reported to be present in Lerma‘50’ and P14 (Narvarez and Caldwell, 1957), Bulgaria 88 (Rillo and Caldwell, 1966), Veranopolis (Rosielle and Brown, 1979; Wilson, 1979), Carifen 12 (Lee and Gough, 1984), Vilmorin (Gough and Smith, 1985) and IAS20/#567.1 (Jlibene, 1990). Two to three dominant genes have been found to confer resistance in Thornbird (Jlibene, 1990). Wilson (1985), in evaluating 28 sources of STB resistance found that a single dominant gene was the most common type of genetic resistance. However, there were some exceptions including duplicate dominant, single incomplete dominant models, and for the cultivar Seabreeze, a two recessive gene model was suggested. In other studies Camacho-Casas et al. (1995) reported that additive and dominance effects were responsible for the resistance to STB in II50-18/VGDWF/3/PMF. Wilson (1985) proposed three different genes conferring resistance to STB. These were designated Slb1, Slb2 and Slb3 for the genes in Bulgaria 88, Veranopolis and Israel 493, respectively. Van Ginkel (1986) suggested that the search for single gene resistance to STB may be ineffective because of the presence of modifier genes, lack of discrete classes in segregating populations, evidence of transgressive segregation, disagreements on where to place the dividing border line for segregation between resistance and susceptibility and environmental influence. Failure to transfer satisfactory levels of resistance to septoria leaf blotch was also noted by Eyal et al. (1987) who suggested the presence of modifying genes that affect the expression of dominant genes for resistance. In durum wheat, Van Ginkel and Scharen (1987) reported that resistance to Septoria tritici blotch was explained by models involving additive and dominant gene effects and that the additive gene effects were more important than dominant gene effects. Epistatic gene effects were of minimal importance. The objective of this experiment was to study the quantitative inheritance of adult plant resistance of bread wheat to S. tritici. MATERIALS AND METHODS Fifty seven bread wheat cultivars were evaluated for their resistance to natural inoculum of the S. tritici populations in Njoro (2160 m) and Timau (2640 m) in 1988 and 1989. After correcting the disease severity for maturity and tallness, 14 cultivars, which ranged from low to high levels of resistance to STB, were selected (Table 1). A single ear of each cultivar was harvested and planted in a 2 m row. At the heading stage some ears in each row were bagged individually to ensure complete selfing. This was repeated in 1990 and 1991. Cultivars were grouped as early and late maturing. There were seven early maturing and six late maturing entries. The cultivar 343 (TRAP#1*2//ERP‘S’/RUSO), a medium maturing cultivar, was included in both groups (Table 1).
The entries were planted four times at an interval of 10 days beginning 23 September 1991. Half-diallel crosses were made within each group. F1 progenies and F2 generations realised were planted widely spaced in 2 rows, 4 m long in 1992. Some crosses were discarded in F3 because they were found to have been mixed up. Only 36 crosses were harvested in the F3. From each cross about 100 ears were randomly and individually harvested. The ears were planted n rows at a wide spacing to raise F4 populations. This was repeated to raise the F5. Three plants from each row were harvested and threshed separately. In 1995 the seeds of three plants from an F5 line were planted in three rows to make F6 plots. Rows were planted 1.5 m long, 20 cm apart. The two parents of the cross were planted in 3 rows after every 20 plots. Because of infrequent rainfall in the months of June and July, irrigation was provided twice a week. Inoculum preparation and inoculations. The isolate IPO93001 (previously sampled from Njoro) was selected for the inoculations. An infected leaf segment was attached to a glass slide and placed in a petri dish fitted on the bottom with filter paper saturated with sterile water. The petri dish cover was replaced to provide a moist environment for 4 hrs. It was then transferred to a laminar-flow clean air cabinet bench. Oozing pycnidia were located under the stereoscopic microscope. With the help of a fine-pointed needle, sterilised in a flame and cooled briefly, a single cirrus was picked and transferred to Yeast-Malt Agar (YMA) (Eyal et al., 1987) medium in a petri dish. The growing colony was spread on the medium and sub-cultured in fresh media for multiplication. The cultures were inoculated into 1 L Erlenmeyer flasks containing 1 L Yeast sucrose liquid medium (Eyal et al., 1987). Cultures were shaken on a Lab-Line Orbit Shaker at 150 RPM for five days. The shaker was turned off overnight to allow the spores to settle down. The liquid medium was carefully decanted after which spores were re-suspended in distilled water and filtered through a cloth filter. The spore concentration was determined and adjusted to 1x106 spores/ml. Tween 20 surfactant was added into the inoculum just before inoculation. A CP15 knapsack sprayer was used for inoculations. The experiment was inoculated four times at an interval of seven days beginning 30 days after planting. The heading date for each plot was noted and marked. Plants in plots that had similar heading dates were observed at the same time. Five main tillers were sampled at random from the middle row of each plot. Percentage necrosis and percentage pycnidia coverage were visually assessed on the two uppermost leaves. As the irrigation was not uniform on all the plots the disease spread was not expected to be uniform. Indeed the disease severity (DS) as measured on the same genotypes (parents) did show clear variations. The mean percentage pycnidia coverage of the two leaves after logit transformation, was therefore corrected for within site variation based on 2-d- polynomials and fitted with the SAS PROC GENMOD method (SAS., 1985) using the DS observed on the manyfold replicated parents in the 36 crosses. The frequency distribution of the progenies was calculated and grouped at 10% DS interval classes. RESULTS Testing of the various F6 wheat lines with isolate IPO93001 showed that there was clear transgressive segregation for DS in many of the crosses. Transgressive segregation was observed for increased resistance, increased susceptibility or both. The extent of transgressive segregation varied among the crosses (Table 2). Cross nrs. 17 and 18 had more than 50% of the population segregating towards more resistance than the most resistant parent. On the other hand, the cross nrs. 8, 10, 20, 21, 22, 23, 26, 27, 28, 30, 31 33, 35 and 36 showed over 50% segregation towards more susceptibility than the susceptible parent. There was an increase in resistance in relation to the mid-parent (MP) values in five out of the 36 crosses (Table 3). The greatest increase in resistance obtained was 45% in relation to the mid-parent (MP) value from the cross 244 x 019. However, most crosses showed a decrease in resistance compared to the MP-value. The highest decrease of resistance obtained was 179.8% from the cross 303 x 001. The overall mean MP value was 34.8% while the corresponding F6 mean was 47.7%, a considerable increase in susceptibility. The standard deviations for F6 and the MP value were both 12.3. The correlation coefficient between F6 and MP-values was 0.44, while the correlation coefficient between the MP-values and percentage change of the F6 from the MP-values was 0.73, both being highly significant.
DISCUSSION Genetic studies in inheritance of resistance to STB have generally been conducted on seedlings in the greenhouse and have been oriented towards examining effects of simple Mendelian inheritance. However, in practical plant breeding situations, selection is done in the field for an array of traits, most of which are exposed in later stages of plant development and are quantitatively inherited. Ballantyne (1985) and Camacho-Casas et al. (1995) reported that plant reaction to S. tritici in F3 lines did not suggest simple inheritance. Moreover, there have been reports of resistant cultivars being obtained from parents of susceptible background (Shaner et al., 1975; Shaner and Finney, 1982; Wallwork and Johnson, 1983; Lee and Shaner, 1985; Milus and Line, 1986; Rose-Fricker et al., 1986; Schultz and Line, 1992; Poysa, 1993; Campbell and Wernsman, 1994; Campbell and White, 1995), suggesting that resistance can result from a combination of genes that individually are ineffective (epistatic effects). The data from this study show that transgressive segregation towards more resistance and/or more susceptibility to STB in wheat occurred in most of the crosses. Because so many parents were involved, it suggests that transgressive segregation should be obtained from many other crosses as well and is not just an occasional phenomenon. The frequent occurrence of transgressive segregants indicates the multigenic nature of resistance in the cultivars used. As an illustration, Table 4 gives a half diallel crosses involving five parents, from Table 2, with all F6’s showing transgressive segregation. It can be deduced that 287 and 327 must differ for at least two genes. In this case recombination is possible and some F6 lines are either more susceptible or resistant than the parents. But then 287 differs also from 396, 343 and 127 for at least two genes and that cannot be the same genes, otherwise the transgression among the others cannot be explained. So with so many parents, most showing transgression, the conclusion is that a fair number of genes are involved.
Transgressive segregation for higher resistance to S. tritici was demonstrated in progenies of a number of crosses. These transgressive segregants clearly originated from a combination of genetic components from both parents of each cross where the phenomenon was occurring. The highest increase in resistance was obtained from the cross 244 x 019 (susceptible x resistant). Of the F6 population, 58% were more resistant than 019 (Table 2). This suggests that there are genes in the susceptible parent 244 that contribute to resistance. This is confirmed from other crosses with 244 (crosses 1, 2, 5 and 7) where transgression beyond the resistant parent occurs also. Moreover, it was observed that in most of the crosses involving 244, the F6 means were almost equal to the MP values. Another interesting increase in resistance was observed in the cross 287 x 244 (susceptible x susceptible). Pope (1968) hypothesised that genes controlled functions in a sequence of events leading to resistance. In this case, each gene alone has no effect, but high levels of resistance can be achieved when the necessary combination of genes is produced by crossing. The combination of quantitative resistance and transgressive segregation is indicative of at least a few genes operating in an additive way. More work need to be done to identify the resistance factors in the susceptible cultivars 244 (HAHN‘S’*/PRL‘S’ and 287 (Frontatch). Out of the 36 crosses analysed, 31 crosses had F6 population means higher than the mid-parent value, indicating greater susceptibility than expected from the parental performance. It was observed that the crosses involving the parents 001, 282 and 343 had most of the transgressive segregants towards more susceptibility than the susceptible parent. It could be said that these cultivars had a poor general combining ability. Their resistance being to a fair extent of a non-additive nature (inter-locus interactions). Such cultivars are of little use in breeding for resistance. The high correlation between MP and the loss in resistance is interesting. So, the higher the resistance of the parents, the more the F6 tended to less resistance, an observation of importance for breeders. It is clearly shown that resistance to STB can be obtained by crossing commercial wheat cultivars which are considered fairly to moderately susceptible. This not only avoids introducing undesirable traits which are often associated with resistant parents of exotic or non agronomic types but also increases the probability of obtaining progenies with well adapted agronomic traits suitable for cultivar development. Selection can then be done on resistant transgressive segregants within the F6 population. This reduces the chances of a single gene based resistance, generally vulnerable to adaptation by the pathogen. ACKNOWLEDGEMENTS The trial discussed in this paper was supported by the Kenya Agricultural Research Institute and by the Dutch Ministry for International Cooperation (DGIS). The authors wish to thank the technical and support staff at each participating research institutes for their role in trial execution and data collection. REFERENCES
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