Proceedings of the Federation of British Plant Pathologists 257

Implications of host-pathogen variation for resistance breeding in the grass crop

BY P. W. WILKINS Welsh Plant Breeding Station, Aberystwyth

I N T R O D U C T I O N

There has been less artificial selection for disease resistance in grasses than in most other crops and thus little of the natural variation has been lost. When seeking to alter radically a natural host/pathogen balance in favour of the host by means of plant breeding, it is important to consider the nature of the available resistance, its inheritance, and the likely ability of the pathogen to overcome it when subjected to greatly increased selection pressure. The way in which resistance is best utilized by breeding depends on the genetic structure of the crop. Forage grasses are generally outbreeders and synthetic varieties are restricted populations of practically unique individuals ; hence these varieties may contain large numbers of resistance genes and gene combinations. The resistance of ryegrasses to crown rust (Puccinia coronutu Corda) and to ryegrass mosaic virus (RMV) and its utilization by plant breeding is considered in this context.

CROWN RUST

Host resistance

Ryegrasses vary greatly in their resistance to crown rust and a number of resistant varieties have been produced (Cruickshank, 1957; Hanson, 1965; Gibbs, 1966). During field and glass- house tests of a wide range of Italian and perennial ryegrasses at Aberystwyth, it has become apparent that there is continuous variation within both species, ranging from immunity to a very high degree of susceptibility. Within some populations, most of this range of variation is evident whilst in others it is limited to the higher or lower end of the overall spectrum. The resistance of twelve Italian ryegrass genotypes chosen at random from Lemtal (formerly RvP), a variety which shows a wide range of resistance, has been investigated (Wilkins, 1975). The variation of ten of the twelve genotypes was inherited by a relatively large number of genes and in an entirely additive manner. However, the two most resistant parents possessed resistance genes of major effect, probably single dominant ones. Under field conditions, resistance within a population derived from S.24 perennial ryegrass was also polygenically inherited, but no major genes were detected (M. D. Hayward, personal communication). It thus appears that in some populations resistance is polygenically inherited and that in others it is controlled by major genes as well. The degree of resistance conferred by polygenes can be very high, and in practice it can be difficult sometimes to determine if highly resistant genotypes possess major resistance genes without further experimentation. However, the genetic control of resistance in a popula- tion can be to some extent inferred from the way in which resistance is distributed.

Pathogenic specialization

Gibbs (1966) reported that crown rust on perennial ryegrasses in New Zealand consisted of a mixture of several races. Unfortunately, a full account of this work has not been published. G. C. M. Latch (personal communication) found that some perennial ryegrass plants resistant to local isolates of crown rust were susceptible to isolates from another district of New Zealand.

Eighteen genotypes of one perennial ryegrass population were selected as highly resistant to the stock crown rust cultures obtained from the Aberystwyth area. Seven of these were attacked by the fungus to some degree when grown outdoors, but the other eleven remained resistant. Rust collected from these seven genotypes was considerably more pathogenic than the stock cultures on most genotypes of the original variable population from which the resistant genotypes had been selected. The mean scores (on a 1-5 scale of increasing susceptibility) of clonal plants of the same thirty-eight genotypes infected with the new culture and the stock culture were 3.4

9-2

258 Proceedings of the Federation of British Plant Pathologists and 2.4 respectively. The new cultures were, however, less pathogenic on a population of Italian ryegrass. These differences were all of a quantitative nature.

Because the stock cultures had been maintained on Italian ryegrass for several uredial genera- tions, it was thought that this differential pathogenicity was probably due to partial specialization on the two host species, rather than to specialization within perennial ryegrass. In order to test this hypothesis, five Italian and five perennial ryegrass populations were inoculated with equal weights of freeze-dried uredospores of both cultures and later scored for susceptibility on a 1-10 scale. The experiment was replicated six times, a total of thirty plants per population being inoculated with each culture. Populations varied significantly in their susceptibility (P = O.OOI),

but there was very little difference in the overall pathogenicity of the two cultures on either Italian or perennial ryegrass (Table I). Additionally, there was no significant population/culture interaction.

Table I. Pathogenicity" of two crown rust cultures on Italian and perennial yyegrass

Culture source A ,

Perennial ryegrass Population Italian ryegrass

Optima Lemtal (RvP) Combita Sabalan Bb 1277

Mean

S. 24 S. 23 BarIenna Gremie Melle

Mean L.S.D. (5 %)

7.8 5'5 4'2 4'4 3 '5 5''

8.0 5'9 4'9 4'3 3 '5

5'3 1'7

* Scored on a 1-10 scale.

6.4 6.7 6.4 3'9 I .7 5'0

6.9 6.5 5'5 4'3 3 '4 5'3

Clearly, further work is needed to assess the extent and magnitude of pathogenic specialization on ryegrasses and to relate it to the inheritance of host resistance. Meanwhile, resistance would be more reliably assessed by the use of a wide range of rust isolates, maintained on as many different genotypes as possible.

Prospects for control by breeding

The hazards of relying heavily on major-gene resistance to obligate air-borne pathogens are well known. Fortunately, polygenically inherited resistance to crown rust appears to be wide- spread and genetically straightforward. There i s no reason in theory why major-gene resistance should not be introduced into a population to supplement a high level of polygenic resistance, but in practice it tends to mask the underlying level of resistance and this becomes a serious barrier to further improvement when possessed by more than half the plants in a population. Evidence is also accumulating that even the partial, minor gene type of resistance is subject to some extent to erosion by pathogenic specialization (Leonard, 1969; Zadocks, 1972; Johnson & Taylor, 1972; Caten, 1974). In view of similar specialization of crown rust on ryegrass, it is clearly desirable to maintain the present high level of genetic diversity amongst commercial varieties. In this way it should be possible to effectively control crown rust by the use of resistant varieties.

Proceedings of the Federation of British Plant Pathologists 259

RYEGRASS M O S A I C V I R U S

Host resistance

Ryegrasses are also very variable in their resistance to the effects of systemic RMV infection. This type of resistance is here referred to as tolerance, in order to distinguish it from resistance to infection. Plants infected with the same isolate of the virus show great differences in the in- tensity of mosaic symptoms and the amount of leaf necrosis. These two characters can be scored on a five-point scale and combined to give an estimate of the total leaf symptoms. Earlier work has shown a high degree of correlation (Y = -0.83; P = 0.001) between the mean symptom score of thirty-two Italian ryegrass genotypes and their yields when expressed as a percentage of their healthy counterparts (Wilkins & Catherall, 1974). Subsequently, it was found that the yields of four perennial ryegrass varieties infected with RMV ranked in exact reverse order to the severity of their visual symptoms. Also, highly significant regressions of similar value between individual genotype score and yield were obtained within all four populations (Wilkins, I 974). It is thus reasonable to assume that tolerance can be measured largely by symptom expression.

Table 2. Susceptibility of fifteen experimental ryegrass populations to ryegrass mosaic virus

Species

Italian ryegrass

Italianlperennial ryegrass hybrids

Perennial ryegrass

W.P.B.S.* population Mean score number (1-10)

6.6 5'9 4'4 4'4 3 '9 3 6 4'4 4'4 3'9 3'7 3 '4 3 '3 3'2 2.3 1'3

L.S.D. (5 %) I '0

* Welsh Plant Breeding Station.

The extent of the variation is illustrated by a test of forty-two genotypes of each of the experimental populations shown in Table 2. The hybrids between Italian and perennial ryegrass were nearer to perennial ryegrass in their susceptibility. The perennial ryegrasses varied between a moderate degree of susceptibility and a very high degree of tolerance. Most genotypes of Ba 8941 showed barely perceptible symptoms whilst others had none at all.

The distribution of tolerance in all populations so far investigated has been continuous and thus probably controlled by a relatively large number of genes. The inheritance of this variation within Lemtal Italian ryegrass and S.24 perennial ryegrass has been investigated by Wilkins (1974) by intercrossing twelve randomly selected plants of each variety in a North Carolina Model 2 mating design, similar to that used for crown rust. A substantial proportion of the genetic variation in Lemtal was due to gene interaction, which could have been due to both dominance and epistasis, and a slightly smaller part to additive effects. Consequently, although some improvement in this population could be made by straightforward mass selection, usually used in grass breeding, a large part of the genetic variation would remain unutilized. In contrast, the genetic variation within S.24 perennial ryegrass was entirely additive and considerable improvement should be attainable.

260 Proceedings of the Federation of British Plant Pathologists

Pathogen variation

In order to determine if this tolerance is equally effective with different isolates of the virus, clonal plants of three Italian ryegrass genotypes were inoculated with RMV from twenty-nine localities in the United Kingdom, ranging from Cumberland to Hampshire, and from one locality in Ireland. The mean genotype scores were: A = 3.4, B = 3.5, C = 8.1. Isolates which ex- hibited the greatest deviation from the means were used to infect further clonal material of each genotype, replicated five times. Thus, two isolates from Cardiganshire were chosen as causing the most severe symptoms on all three genotypes, and two, one from Worcester and one from Ireland, as being more severe on genotype B than on A. Additionally, an isolate from Pembroke- shire was included as representative of the average isolate. There were highlysignificant differences both between genotypes and between isolates (P = 0.001). There was also a highly significant genotype/isolate interaction (P = o-oor). As Table 3 shows, most of this interaction was due to some isolates having a much more severe effect on genotype C than others, but being only slightly more severe on genotypes A and B. This confirms previous results (Wilkins & Catherall, 1974) that isolate severity has less influence on tolerant genotypes than on more susceptible ones. There was some suggestion of differences in genotype ranking according to isolate. The Worcester isolate produced slightly more severe symptoms on A than on B, but these differences were not significant and contributed very little to the total genotype/isolate interaction.

Table 3. The effect of selected ryegrass mosaic virus isolates of diverse origin on three Italian ryegrass genotypes

Mean RMV score Virus source Host genotype (1-10)

Cardiganshire I

Cardiganshire z

Worcester

Eire

Pembrokeshire

A B C A B C A B C A B C A B C

L.S.D. (5 %)

That the overall severity of isolates can be altered by selection has been shown by Wilkins & Catherall (1974). In view of the variability in host tolerance demonstrated here, it is possible that this flexibility has some function and might tend to counteract changes in the host/pathogen balance.

Prospects for control by breeding

In assessing the possibility that the widespread use of tolerant varieties might lead to patho- genic specialization or change in overall virulence, it is important to determine whether or not tolerance is associated with reduced multiplication and spread of the virus. The threat posed by changes in overall virulence may not be great, as some populations are highly tolerant of even the most severe isolates.

Although the production and use of tolerant varieties would considerably reduce losses due to RMV, the exact degree of control that can be achieved is not yet known. To determine this,

Proceedings of the Federation of British Plant Pathologists 261 it would be necessary to examine the tolerance of populations that show few symptoms on infection under sward conditions.

I thank Miss D. 0. Hughes for technical assistance, and Dr A. J. H. Carr for discussion of the manuscript.

REFERENCES

CATEN, C. E. (1974). Intra-racial variation in Phytophthora infestans and adaptation to field

CRUICKSHANK, I. A. M. (1957). Crown rust of ryegrass. New Zealand Journal of Science and

GIBBS, J. G. (1966). Field resistance in Lolium sp. to leaf rust (Puccinia coronata). Nature, London 209, 420.

HANSON, A. A. (1965). Grass varieties in the United States. U S D A Agricultural Handbook No. 170, 102 pp.

JOHNSON, R. & TAYLOR, A. L. (1972). Isolates of Puccinia striifomzis collected in England from the wheat varieties Maris Beacon and Joss Cambier. Nature, London 238, 105-106.

LEONARD, K. J. (1969). Selection in heterogeneous populations of Puccinia graminis f.sp. avenae. Phytopathology 59, 1851-1857.

WILKINS, P. W. (1974). Tolerance to ryegrass mosaic virus and its inheritance. Annals of Applied Biology 78, iQ7-192.

WILKINS, P. W. (1975). Inheritance of resistance to Puccinia coronata and Rhynchosporium orthosporum in Italian ryegrass. Euphytica 24, 191-196.

WILKXNS, P. W. & CATHERALL, P. L. (1974). The effect of some isolates of ryegrass mosaic virus on different genotypes of Lolium multiflorum. Annals of Applied Biology 76, 209-216.

ZADOCKS, J. C. (1972). Modern concepts of disease resistance in cereals. Proceedings of the 6th Congress of Eucarpia (1971), pp. 89-98.

resistance for potato blight. Annals of Applied Biology 77, 259-270.

TechnologY 38, 539-545.

The transmission and effect on yield of ryegrass mosaic virus in a filtered air environment

By R. W. GIBSON AND R. T. PLUMB

To be published later in Annals of Applied Biology.

Epidemiology and effects of grass viruses in relation to their insect vectors

BY J. A’BROOK

Welsh Plant Breeding Station, Aberystwyth

C O C K S F O O T MOTTLE VIRUS

Benigno & A‘Brook (1972~) have shown that the Chrysomelid beetle (Oulema melanopa (L.)) is an inefficient vector of cocksfoot mottle virus (CFMV), but nevertheless, is probably respon- sible for the introduction of the virus into cocksfoot swards. Spread of the virus within swards is mainly mechanical by stock and tillage implements (Serjeant, 1965 ; A’Brook, 1968). Partly due to CFMV, acreages of cocksfoot are declining. Benigno & A’Brook (197zb) who examined a large number of Dactylis species and cultivars found no sources of resistance but did not test for tolerance since CFMV kills Dactylis glomerata L. in the field.

A’Brook & Benigno (1972) have also shown that 0. melanopa transmits only CFMV or phleum mottle virus (PMV) after acquisition of both, and that the mortality of adult beetles