3.4.52
A DETACHED SEEDLING LEAF TECHNIQUE TO SCREEN FOR RESISTANCE TO MYCOSPHAERELLA GRAMINICOLA (SEPTORIA TRITICI) IN WHEAT VARIETIES

LS ARRAIANO, PA BRADING and JKM BROWN

Cereals Research Department, John lnnes Centre, Colney Lane, Norwich, NR4 7UH, UK

Background and objectives
Septoria leaf blotch, caused by the ascomycete fungus Mycosphaerella graminicola (anamorph Septoria tritici), is a major disease of wheat throughout the world, reducing grain yields by up to 60 %. The most favourable condition for infection are warmth and humidity, while pycnidium germination requires a very wet leaf surface. Normally, wheat varieties are screened for resistance to this disease in field trials of adult plants, or in tests in which seedlings are sprayed with a pycnidiospore suspension. Both methods require a great deal of time and space and are very dependent on environmental variables that may be difficult to control. Detached leaf tests have been used in work onPhaeosphaeria nodorum (Septoria nodorum) and show good correlation with results of field trials [1, 2]. We have therefore investigated the use of a similar technique for M. graminicola, so as to able to screen more material more rapidly than is possible in field trials.

Materials and methods
The test uses 3 cm long segments of fully developed first seedling leaves, which are placed on benzimidazole agar, with the cut ends covered by benzimidazole as well, to prevent early senescence. The detached leaves are inoculated by spraying with a suspension of M. graminicola pycnidiospores, taken from one to two week old cultures, adjusted to 107 spores ml-1. The boxes are sealed after inoculation and high humidity is maintained throughout the period of incubation. The leaves are incubated in the dark at 20 C for 48 h after inoculation, then under fluorescent lights at 15 C for 13 days. Pycnidium development is then stiumiated by exposure to near-UV light. Disease, estimated as percentage leaf area covered by pycnidia, is scored 21 and 28 days after inoculation.

Results and conclusions
Six bread wheat varieties, Apollo, Flame, Longbow, Kavkaz-K4500 (KK), Riband and Veranopolis, and three Dutch M. graminicola isolates, IPO 323, IPO 94269 and IPO 001, were used because they have given contrasting results in field trials and seedling tests. KK was resistant in detached leaf tests, seedling tests and field trials, while Longbow and Riband were susceptible. Flame and Veranopolis showed specific interactions with isolates, similar to those observed in the field and in seedling tests. However, Apollo was much more susceptible in both detached leaf tests and whole seedling tests than in the field.

These results indicate that the detached leaf test has potential value for screening wheat varieties for resistance to M. graminicola, in particular for identifying specific variety-isolate interactions. It could therefore be used in genetic analysis (see accompanying poster by PA Brading et al.) and in wheat breeding programmes. However, that the resistance of Apollo is expressed in field conditions but not in detached leaves or in seedlings suggests that, as with many other diseases of wheat, such as powdery mildew and the rusts, there is an adult plant component to the resistance of certain varieties. Further development is therefore needed to establish the extent to which detached leaf data are applicable to a wide range of wheat varieties and M. graminicoia isolates.

This work was supported by Praxis XXI, the EU Biotech programme and MAFF

References
1.Ubels E, 1979. Netherlands Journal of Plant Pathology, 85: 143-150.
2.Benedikz PW, Mappledoram J, Scott PR, 1981. Transactions of the British Mycological Society, 77: 667- 669.