Activation of resistance of wheat to stem rust

Notes and Brief Articles

333

REFERENCES

C. (1937). Some hyphomycetes that prey on free-living terricolous nematodes. Mycologia 29, 447-552. DRECHSLER, C. (1940) . Three new hyphomycetes preying on free-living terricolous nematodes. Mycologia 32,448-47°.

DRECHSLER,

ACTIVATION OF RESISTANCE OF WHEAT TO STEM RUST D. S. M. CHE UNG AND H. N. BARBER

School

ofBotany, University ofNew South Wales, Kensington, Australia

There have been several reports on induced resistance of plants to pathogenic fungi. The relevant literature has been reviewed by Littlefield (1969) who also confirmed that resistance of flax to virulent races of Melampsora lini can be induced by prior inoculation of the plants with one of several avirulent races of the rust fungus. The present paper reports a similar mechanism operating in wheat inoculated with races of stem rust. Two varieties of wheat (Triticum aestivum L. var. Reliance and var, W 2691) and two races of stem rust (Puccinia graminis f sp. tritici Erikss. & Henn., race 21-2,3,7 and race 126-6,7) were selected for study. Reliance is immune to 21-2, 3, 7 but susceptible to 126-6, 7, while W2691 is susceptible to both races (see infection types illustrated in PI. 32 A, B). The seedlings were planted in vermiculite, irrigated with Hoagland's solution I (H oagland & Arn on, 1950) and kept in a growth cabinet (700 fc, photoperiod, 18 h ; temperature, 20 °C; dark temperature 7° ± 1°). The procedures of inoculation with stem rust and other experimental details are summa rized in Table 1. The typical rust infections of th e leaves are illustrated in PI. 32. From previous contact with th e avirulent race 21-2, 3, 7 for 3 or 6 days, the primary leavesof Reliance acquired a measure of resistanc e to th e virulent race 126-6, 7. This was estimated as a reduction of about 80 % of the number of pustules jcm- leaf area (T ext-fig. 32). At the time of inoculation with 21-2 , 3, 7, some of the second leaves had partly emerged and the area exposed to the avirulent rust race had also acquired a similar degree of resistance (PI. 32 E). Johnston & Huffman (1958) recorded a similar ph enomenon for leaf rust of wheat and suggested that the induced resistance may result from killing or plugging of many stomata by the avirulent fungus. We observed that both races grew well in th e doubly infected W 2691 which had significantly higher infection densities than plants inocul ated with only one race (Text-fig. I). The diffusion of enzymes from germinating urediniospores has already been recorded (Cheung & Barb er, 1971) and these enzymes, when produced by the avirulent spores, ma y activate the synth esis of a resistance factor which is also effective against virulent races of th e pathogen. Trans. Br, mycol. Soc. 58 (2), (1972). Printed in Great Britain

Transactions British Mycological Society

334 Table

1.

Summary

of inoculation procedures and other experimental details

Seedlings at one-leaf stage (Reliance and W26 gl , in a growth ca binet)

I

Control (sprayed with M obisol roo)

I

Inoculated with ra ce 21 - 2 , 3, 7 (ur ediniospores suspended in 1\Iobisol 100) (Incubated at 18° in a dark moist chamber for 20 h and then retu rned to the growt h cabinet) STEP I

I

(noCon~ s;~~~)

m

~,ml r'r (o~~;~ race

3 D. " after 3 step I

9

9

2 1-2, 3, 7 from step I

STEP 2

Inoculat ed with race 126-6, 7

10 days after step 2

I

M ost reliance leaves inoculated with both races died soon after step 2

Nu mber of pustulcsjcm" of leaf area estima ted

Histological examinations showed that the avirulent race 21- 2, 3, 7 penetrated young Reliance leaves 1-2 days after inoculation, and a few necrotic cells developed near the infection sites. One to three small haustoria were found in about two of 200 sites studied. Further development of th e fungus was not observed during the longer incubation periods of 4,6,8, 12 and 14 days. However, 20 days after inoculation, growth was resumed in about 60 of 300 sites examined. Small fleckssoon appeared and then pustules. This ren ewed growth coincided with the yellowing of the primary leaves associated with senescence. Obviously, necrosis of host cells did not kill nor permanently inhibit the growth of the pathogen. This agrees with the observations of Brown, Shipton & White (1966). It appears that, in response to infection by an avirulent race, some host cells synthesize a fungistatic compound(s). The metabolic changes associated with senescence result in the compound(s) being not produced or produced in low and ineffective concentrations. Trans. Br. mycol. Soc.

58 (2),

(t972). Printed in Great Britain

Notes and Brief Articles

335

400

Experiment 1

Experiment 2

200

o Ll L2 L3 ReI.

Ll L2 L3 W2691

LlL2L3 ReI.

Ll L2 L3 W2691

Text-fig. I. Relative infection (average number ofpustules/cm'leafarea of six leaves from each sample) of wheat varieties Reliance (Rel.) and W 2691 inoculated with one or two races of stem rust. LI = primary leaves inoculated with race 21-2, 3, 7; L2 = primary leaves inoculated with race 126-6, 7; L3 = primary leaves inoculated first with race 21-2, 3, 7 and 3 days later with race 126-6, 7. Significant differences were observed between the infection densities of samples L2 and LS of Reliance and between those of Lr , L2 and L3 OfW2691 (analysis of variance, P = 0'05%).

The results obtained in this investigation suggest that the types of resistance of wheat to stem rust (and there are probably more than one), include in this particular host-pathogen combination, an inducible, active process. This process is of a local nature (PI. 32E) and maintained by the continuing presence of the viable, avirulent rust organism. When activated, the process can operate against at least one other race of the pathogen which is virulent to the host under ordinary conditions. We gratefully acknowledge the support of the Australian Research Grant Committee. Our thanks are due also to Drs H. J. Willets and R. S. Vickery, Associate Professor C. J. Driscoll and Mr C. E. May for valuable suggestions, Dr N. H. Luig for supplying seeds and rust cultures, and Mr H. Cook for technical assistance. Trans. Br. mycol. Soc. 58 (2), (1972). Printed in Great Britain

Transactions British Mycological Society REFERENCES

CHEUNG, D. S. M. & BARBER, H. N. (1971). Uredospore wall proteins of wheat stem rust: localization and enzymatic activities. Archio fiir Mikrobiologie 77, 139-146. HOAGLAND, D. R. & ARNON, D. 1. (1950). The water-culture method of growing plants without soil. California Agricultural Experiment Station Circular 347 (Revised, 1950). JOHNSTON, C. O. & HUFFMAN, M. D. (1958). Evidence oflocal antagonism between two cereal rust fungi. Phytopathology 48, 69-70. LITTLEFIELD, L.J. (1969). Flax rust resistance induced by prior inoculation with an avirulent race of Melampsora lini. Phytopathology 59, 1323-1328.

EXPLANATION OF PLATE

32

Response of primary leaves of wheat var, Reliance and var. W 2691 to infection by stem rust race 21-2,3,7 (21) and race 126-6, 7 (126); 21 + 126 signifies that leaves were first inoculated with !1.7-2, 3, 7 and then 3 days later with 126-6, 7. Note only a few pustules in sample C of Reliance and the absence of pustules in the apex of E of Reliance. Samples D and E are the upper parts of the second leaves of two seedlings.

FUSARIUM WILT OF VOANDZEIA SUBTERRANEA IN TANZANIA D. L. EBBELS* AND R. V. BILLINGTONt

Ministry of Agriculture, Food andCo-operatives Research and Training Institute, Ukiriguru, P.O. Box 1433, Mtoanra, Tanzania Voandzeia subterranea (L.) Thou. (the Bambarra groundnut) is a common food crop in north-western Tanzania. It frequently suffers from a wilt disease which is often fatal. First symptoms appear about 7-8 weeks after sowing in the field. Growth slowsor ceases so that the affected plants appear stunted, and the vascular system quickly becomes stained reddish brown to brownish purple. Interveinal chlorosis of the leaflets is followed by a more general yellowing, only the larger veins remaining green. The leaves then wilt, necrosis progresses from the margins inwards, and death of the plant often ensues. Fusarium oxysporum Schl. was isolated from wilted Bambarra groundnuts at Ukiriguru on 16 May 1969, and pathogenicity of the isolate (UKP 218) was confirmed in pot tests. Further isolations have been made since then, and the identity of the fungus has been confirmed by Dr C. Booth at the Commonwealth Mycological Institute (IMI 151395). This is believed to be the first published record of this disease from Tanzania, although F. osysporum has been reported from V. subterranea in Kenya (Nattrass, 1961). Isolates UKP 218 and UKP 322 of Fusarium from the Voandzeia-wilt were used in a series of pot experiments to inoculate the crop plants listed in Table I. Seedlings were inoculated at 7- 10 days after sowing (according to the species) by dipping the roots in an aqueous suspension made from a

* t

Cotton Research Corporation. British Graduate Volunteer.

Trans. Br. mycol. Soc. 58 (2), (1972). Printed in Great Britain