Proceedings 107

Induced resistance to fungal diseases with special reference to yellow rust of wheat

BY R. JOHNSON Plant Breeding Institute, Trumpington, Cambridge, CB2 2LQ

There is ample evidence that plants can respond to infection by parasitic fungi in ways that influence the fate of other more or less closely related parasitic fungi on the same host plant. Resistance induced as a result of the presence of a parasitic fungus in host tissue is also referred to as ‘acquired immunity’ or ‘cross protection’. Among the many papers referring to these topics relatively few have considered potential effects of this type of resistance on the development of diseases in crops and even fewer have reported practical tests in field experiments.

My interest has been primarily in the practical significance of induced resistance, rather than in its mechanism and I have considered four factors which might influence this. These are (1) whether the induced resistance is local or systemic, (2) the timing of its development after the inducing inoculation, (3) its specificity towards other strains of the same pathogen or other pathogens and (4) whether the converse, induced susceptibility, could counteract effects of induced resistance. Induced resistance might have the greatest potential practical significance if it was systemic. effective immediately after the inducing inoculation, against a wide spectrum of races of the same pathogen or against other pathogens and if induced susceptibility was negligible.

L O C A L I S A T I O N O F I N D U C E D R E S I S T A N C E

Elliston, Kuc & Williams (197 1) noted that etiolated hypocotyls of Phaseolus uulgaris L. inoculated with incompatible races of Colletotrichum lindemuthianum (Sacc. & Magn.) were protected at distances up to 5 mm against anthracnose caused by compatible races of this fungus. In contrast, Skipp & Deverall (1973), using the same organisms but without etiolation of the host, found that resistance was only induced at the site of inoculation with the incompatible race, in hypocotyls, leaves and pods. They attributed the difference between these two sets of data to etiolation of the hypocotyls used by Elliston et al. With powdery mildew, Ouchi, Oku & Hibino (1976) noted that resistance to virulent strains of Erysiphe graminis DC ex Merat f. sp. hordei Marchal on barley was detectable after inoculation with non- virulent, inducer strains at a distance not greater than 5 mm from the site of inoculation with the inducer. Kochman & Brown (1975) stated that cross-protection of oat seedlings by inoculation with Puccinia graminis Pers. f. sp. tritici Erikss. & Henn. or P. recondita Rob. & Desm. f. sp. tritici Erikss. & Henn. was only evident when the challenging, virulent, fungi (P . coronata Corda and P. graminis Pers. f. sp. auenae Erikss. & Henn.) attempted to enter the leaf through the stoma already occupied by an appressorium of the inducing fungus. Our results with yellow rust on wheat indicated that the inducer races of P. striiformis West. could induce resistance when applied on the opposite surface of the leaf to the challenging race (Johnson & Allen, 1975; Johnson & Taylor, 1976). The resistance was not evident more than a few millimetres from the chlorotic flecks caused by the inducer races. Similar restriction of the effect of induced resistance has been shown for flax rust (Littlefield, 1969), and for stem rust of wheat (Cheung & Barber, 1972). Most of these cases suggest that induced resistance to fungal diseases does not become systemic in the plant.

In marked contrast is the recent evidence of Kuc, Shockley & Kearney (1975) that systemic resistance to Colletotrichum lagenarium (Pass.) Ell. & Halst. occurred in cucumber plants which had been inoculated with a sub-lethal dose of the same pathogen or in which a single leaf had been infected with a higher dose. This result, which might be described as a true example of acquired immunity in plants, is surprising when compared with results of McLean (1967) who found that inoculation of melons with race I of C. lagenarium gave only local protection in leaves to race I1 and was not effective in inducing resistance in stems. Yarwood (1954) showed a zone of resistance to bean rust, surrounding areas of bean leaves already infected with the same rust.

T I M I N G O F D E V E L O P M E N T O F I N D U C E D R E S I S T A N C E

There seems to be considerable variation in the time taken for induced resistance to become effective after an inducing inoculation. Skipp & Deverall (1973) were able to apply the challenge inoculation of a compatible race of C. lindemuthianum 1 day before the incompatible race and still observe some resistance. In many cases, it has been reported that induced resistance is not detectable until some hours

108 Proceedings

or days after the inducing inoculation. The minimum delay for powdery mildew on barley was 6 h (Ouchi et al., 1976), and, for rust diseases on oats, 2-3 days (Kochman & Brown, 1975). In our work with yellow rust on wheat seedlings total spore production was reduced as a result of almost simultaneous application of the inducer and challenger races (Johnson & Taylor, 1976). Once established, induced resistance usually remains effective for a number of days. For example, Littlefield (1969) suggested that induced resistance to Melampsora lini (Ehrenb.) Lev. remained effective in flax for about 7 days. Clearly the rate at which induced resistance develops and its duration once established would affect its influence in the field.

S P E C I F I C I T Y O F I N D U C E D R E S I S T A N C E

Usually, resistance induced by a specific incompatible reaction between host and pathogen is effective against a range of normally compatible combinations of the same host and parasite species. I have not found any evidence to indicate race-specificity of the induced resistance. In addition, resistance can be induced in host tissues by pathogenic organisms which are not normally pathogenic on the host (e.g. Hammerschmidt, Acres & Kuc, 1976). It has frequently been shown that one rust species can induce resistance against another. For example resistance against flax rust was induced in flax by inoculation with wheat rust (Littlefield, 1969) and resistance against leaf rust was induced in wheat by oat crown rust (Johnston & Huffman, 1958). Several other examples of this type are known including those given by Yarwood (1956).

I N D U C E D S U S C E P T I B I L I T Y

Several authors have indicated that prior inoculation with a virulent strain of a pathogen can predispose the tissue of a plant to strains to which it would otherwise be resistant. If this effect was equal and opposite to induced resistance it could negate the effect. Evidence from several sources indicates that where induced susceptibility occurs it develops more slowly than induced resistance. Ouchi et al. (1976) suggested that tissue of barley became susceptible to Sphaerorhecafuliginea (Schl.) Pollaci some 25-28 h after infection of the leaves with E. graminis f. sp. hordei. They did not report whether S. fuliginea was able to produce conidia on barley under these conditions. With P. striiformis we were unable to induce susceptibility to a white isolate of a non-virulent race by prior inoculation with a virulent, yellow, race on Maris Templar wheat seedlings. In another experiment there was no evidence of enhanced infection as a result of inoculation of seedlings with a non-virulent race several days after inoculation with a virulent race. On the other hand Moseman, Scharen & Greeley (1965) reported that small numbers of conidia of a non-virulent strain of E. graminis were produced on Algerian and Goldfoil barleys after prior inoculation with virulent strains. Similarly, wheat attacking strains of E. graminis could sporulate on barley plants previously inoculated with barley-attacking races, and vice versa. Thus incompatible races could survive in relatively hostile hosts with the aid of compatible races.

I N D U C E D R E S I S T A N C E IN F I E L D E X P E R I M E N T S

In a few reports on laboratory experiments, comments have been made about the potential effectiveness of induced resistance in the field, but very rarely have these suggestions been supported by any experi- mental data (Yarwood, 1956; Kochmann & Brown, 1975; Johnson & Allen, 1975; Allen, 1975; Nelson & Tung, 1973). There are, however, reports of experiments which seem to indicate some effects of induced resistance under field conditions. Schnathorst & Mathre (1966) used soil infected with a severe strain (T- 1) of Verticillium albo-altrum Reinke & Berth. (5 x lo3 propagules/g) and supplemented it with a mild strain (SS-4) at lo5 propagules/g. All cotton plants in the soil with T-1 alone suffered severe wilt but 94% of those in the supplemented plot were protected. No protection was given if equal quantities of the mild and virulent strains were used. The authors presumed this effect to be due to cross-protection of the plants by the mild strain against the virulent strain.

Borlaug (1945) described antagonism between races 6 and 11 of Fusarium Iini Bolley which causes flax wilt and though he did not claim this to be due to cross-protection the effects seem to be similar to those described by Schnathorst & Mathre (1966). Borlaug claimed the effect to be ‘striking’ but how true this was depends on the meaning to be attached to his words concerning the preparation of the inoculum for the experiment. He states that he applied ‘equivalent quantities’ of inoculum to each treatment block, and that the composite mixed inoculum was prepared ‘by mixing together equal amounts of inoculum of races 6 and 11’. This could mean that an equivalent amount of mixed inoculum consisted of half as much race

ProceediMgs 109

6 and half as much race 11 as in the respective single race inocula. In this case the interaction between races in the mixed inoculum seems to be slight in two of the flax varieties, Linota and NDR 114, the number of plants infected being almost equal to the mean of those infected by the two races separately. In Redwing the mean for the two races separately was 15.5% and for the mixed inoculum 10%; in Bison the respective values were 2 7 . 5 and 13, both indicating a reduction of disease due to the mixtures of races applied.

We conducted trials to see whether induced resistance to P. striiformis on wheat could be detected in the field (Johnson & Taylor, 1 9 7 6 ) . In 1 9 7 5 wheat varieties were grown in pairs of plots each 25 cm diameter, containing 25 plants. The trial was of split-plot design and a sub-plot consisted of three pairs of 25 cm diameter plots, the middle pair being the variety to be tested. The adjacent pair on one side was a susceptible variety and was inoculated with a race of P. striiformis virulent upon the variety being tested. The pair on the opposite side was, in one sub-plot, a susceptible variety inoculated with a race non-virulent on the test variety, i.e. producing the ‘inducer’ inoculum, and in the other sub-plot a resistant, uninoculated variety. A 1 m barrier of a resistant variety separated main plots and sub-plots. Plots were inoculated in April and assessed in May and June. There was a small but significant reduction in the amount of rust

Table 1 Percent leaf area of wheat cultivars infected with Puccinia striiformis in plots inoculated with a virulent race alone ( V ) or with both a virulent and a non-virulent race (V + N V ) in 1975.

,-*-, ,-*-, ,.pA-, for V versus V + N V Maris Ternplar Maris Beacon Maris Ranger L.S.D.

Date V V + N V V V + N V V V + N V (I‘ = 0.05)

29May 5.1 4.6 10.9 9.6 7.5 7.8 NS 6 June 24.1 21.6 39.1 35.3 30.0 24.1 2.5

19June 23.1 23. I 37.8 34.1 25.6 22.5 1.9

on the test varieties in plots where non-virulent inoculum was present (Table 1). Unfortunately the observer could not avoid being aware of which treatment was being scored and some bias may have entered the results. We hope to arrange future trials to avoid this possibility.

C O N C L U S I O N S

There is thus evidence that induced resistance can affect the development of disease in the field to a limited extent. It may, therefore, be one component of disease control where genetically diverse host and parasite populations exist as in mixed cropping systems or in multiline varieties (Allen, 1 9 7 5 ; Johnson & Allen, 1 9 7 5 ) . Both the non-specificity of induced resistance and, in some cases, its speed of development after the inducing inoculation would appear to favour its effectiveness in the field. However the resistance does not usually become systemic in the plant and high levels of inducing inoculum are therefore required to give significant resistance.

The acquired immunity described by Kui: et al. ( 1 9 7 5 ) and Yarwood ( 1 9 5 4 ) appears to be more systemic but if it were normally important in the field it should preclude the development of epidemics. It has been noted, however, that the incidence of anthracnose on melons, caused by C. lagenarium, is very dependent upon the environment (McLean, 1 9 6 7 ) . It is possible that the acquired immunity described by KuC et al. ( 1 9 7 5 ) is also dependent upon the environment and would be sporadic in the field.

It seems unlikely, from the results of laboratory experiments, that induced susceptibility could counter- act the effects of induced resistance under field conditions; it could however affect the survival of pathogens and lead to changes in pathogen populations that would otherwise not occur (M. S . Wolfe, personal communication).

R E F E R E N C E S

ALLEN, D. J. ( 1 9 7 5 ) . Induced resistance to bean rust and its possible epidemiological significance. Report of

BORLAUG, N. E. ( 1 945). Variation and variability of Fusarium h i . Minnesota Agricultural Experiment

CHEUNG, D. s . M. & BARBER, H. N. ( 1 9 7 2 ) . Activation of resistance of wheat to stem rust. Transactions of

the Bean Improvement Co-operative 18, 15-16 .

Station Technical Bulletin no. 1 6 8 , 4 0 pp.

the British Mycological Society 58, 333-336.

110 Proceedings

ELLISTON, J. E., KUC, J.

HAMMERSCHMIDT, R., ACRES, s.

JOHNSON, R.

JOHNSON, R.

WILLIAMS, E. B. (1971). Induced resistance to bean anthracnose at a distance from the site of the inducing interaction. Phytopathology 61, 1 1 10-1 1 12.

KUC, J. (1976). Protection of cucumber against Colletotrichum lagenarium and Cladosporium cucumerinum. Phytopathology 66,790-793.

ALLEN, D. J. (1975). Induced resistance to rust diseases and its possible role in the resistance of multiline varieties. Annals of Applied Biology 80, 359-364.

TAYLOR, A. J. (1976). Effects of resistance induced by non-virulent races of Puccinia striiformis. Proceedings of the Fourth European and Mediterranean Cereal Rusts Conference 1976, InterlakenlSwitzerland, 49-5 1 .

JOHNSTON, c. 0. & HUFFMAN, M. D. (1968). Evidence of local antagonism between two cereal rust fungi. Phytopathology 48,69-70.

KOCHMAN, J. K. & BROWN, J. F. (1975). Studies on the mechanism of cross-protection in cereal rusts. Physiological Plant Pathology 6, 19-2 7.

KUC, J., SHOCKLEY, G. & KEARNEY, K. (1975). Protection of cucumber against Colletotrichum lagenarium by Colletotrichum lagenarium. Physiological Plant Pathology 7, 195- 199.

LITTLEFIELD, L. J. (1969). Flax rust resistance induced by prior inoculation with an avirulent race of Melampsora lini. Phytopathology 59, 1323-1328.

MCLEAN, D. M. (1967). Interaction of race I and race I1 of Colletotrichum orbiculare on watermelon. Plant Disease Reporter 51,885-887.

MOSEMAN, J. G., sCHAREN, A. L. & GREELEY, L. w . (1965). Propagation of Ervsiphe graminis f. sp. tritici on barley and Erysiphe graminis f. sp. hordei on wheat. Phytopathology 55,92-96.

NELSON, R. R. & TUNG, G. (1973). Cross protection by race 0 against race T of Helminthosporium maydis. Plant Disease Reporter 51,971-973.

OUCHI, s., OKU, H. & HIBINO, C. (1976). Localization of induced resistance and susceptibility in barley leaves inoculated with the powdery mildew fungus. Phytopathology 6 6 , 9 0 1-905.

SCHNATHORST, W. C. & MATHRE, D. E. (1966). Cross-protection in cotton with strains of Verticilhm albo- atrum. Phytopathology 56, 1204- 1209.

SKIPP, R. A. & DEVERALL, B. J. (1973). Studies on cross-protection in the anthracnose disease of bean. Physiological Plant Pathology 3,229-3 13.

YARWOOD, c . E. (1954). Mechanism of acquired immunity to a plant rust. National Acadetny of Science Proceedings 40,374-377.

YARWOOD, c . E. (1956). Cross protection with two rust fungi. Phytopathology 46, 540-544.

The use of avirulent virus strains to protect plants against the effects of virulent strains

by J. T. FLETCHER Agricultural Development and Advisory Service, Kenton Bar, Newcastle upon Tyne, NE1 2 YA

McKinney (1929), working with tobacco mosaic virus (TMV) was the first to report the ability of one strain of a virus to protect against infection and invasion by a second related strain. Later, Salaman (1933) found that tobacco plants invaded by a mild strain of potato virus X were protected against other strains of this virus and he suggested the possible economic significance of this type of interaction. Such inter- action between strains of viruses has generally been referred to as cross-protection, antagonism or interferance and is now known to commonly occur between related strains of the same virus. For some years, virologists used the cross-protection technique to test relationships between virus isolates but more recently it has been demonstrated that this technique is not reliable as some strains, shown to have close relationships by other techniques, do not protect against each other. Also some unrelated viruses can show quite strong degrees of interference. Since Salaman (1933) suggested that cross-protection might have economic significance, there have been a number of attempts to use avirulent virus strains to improve yield. None have been adopted on a wide scale with the exception of an avirulent mutant strain of TMV which has been used extensively in tomato crops in Europe and other parts of the world.

There is still little precise information on the mechanism of cross-protection, a subject not within the scope of this paper, but one which has been reviewed by several authors (Bennett, 1953; Matthews, 1970; de Zoeten & Fulton, 1975; Gibbs & Harrison, 1976; and Zaitlin, 1976).