1University College Dublin, Belfield, Dublin 4, Ireland

Background and objectives
Relatively few studies have been made with any crop on possible interactions between pathogens or between pathogen and non-pathogen; but where such studies have been made, substantial evidence has accumulated to indicate that infection and disease development may be considerably influenced by the presence and effects of other microorganisms. The objective of this study was to examine several factors which might influence the complex balance between Septoria tritici and Stagonospora nodorum in the wheat crop. S. tritici and S. nodorum have a common host, therefore situations may occur in which the two organisms come into direct contact with each other and are subjected to the effects of any metabolic by-products released by either or both organisms.

Materials and methods
S. tritici and S. nodorum were grown together in vitro to test the effect each species has on the growth of the other. Using two isolates of each species, 500 ml of liquid Czapek-Dox V8 (CD V8) agar was seeded with 105 spores with various percentages of each species. The effect of two isolates of the same species on each other was also assessed. After 14 days the mycelium was freeze-dried for 48 h and then weighed. Solid and liquid media containing three levels and three forms of nitrogen were inoculated with mycelial plugs, 6 mm in diameter. Three plugs were placed into each 500-ml flask of liquid culture. After 16 days the mycelium was harvested, freeze-dried and weighed. One plug was placed in the centre of each petri dish of solid medium and the diameter of the colonies measured after 14 days. Wheat plants were maintained in a growth chamber using a 16-h light cycle and 8-h dark cycle for 28 days. The youngest fully expanded leaf was then fixed horizontally onto polyacrylamide plates. These leaves were pretreated with a suspension of the yeast 48 h prior to inoculation with 106 spores/ml of either S. nodorum or S. tritici.

Results and conclusions
S. nodorum was inhibited in liquid culture by the presence of S. tritici, when S. tritici comprised 50 and 75% of the total number of spores present. When two S. nodorum isolates were grown together in equal proportions, the amount of mycelium was less than the expected value. This suggests S. nodorum may produce inhibitory substances in vitro. However, the growth of S. tritici was enhanced by the presence of the second S. tritici isolate. These results indicate that S. tritici may be a better competitor than S. nodorum and that S. tritici has a higher plasticity than S. nodorum. In vitro studies to test the effect of different forms and amounts of N showed that S. nodorum had a threshold value for N of 4 g/l in both solid and liquid media. Most growth of S. nodorum was supported by the medium containing NaNO3 at 4 g/l for the three isolates. Isolates of S. tritici had an optimum growth rate at 4 g/l for the three N forms. However, the isolate preference for the N form appeared to be specific for different isolates, suggesting that there is intra-specific variation for this factor in S. tritici. S. nodorum did not sporulate well on most N forms, except for NaNO3, but vigorous mycelial growth was supported by most of the growth media. The form of available N may play an important role in determining sporulation in S. nodorum. The three S. nodorum isolates responded similarly to different N treatments, whereas the S. tritici isolates were more inconsistent in their response. This suggests there is considerably more intra-specific variation in S. tritici than S. nodorum for this factor.

A significant decrease in symptom expression was observed when leaves were pre-treated with a yeast suspension prior to inoculation with S. nodorum compared to S. nodorum alone. This confirms results of other workers, in which ears of wheat dipped in suspensions of phyllosphere yeasts had less than 50% infection caused by S. nodorum [1]. There was a slight decrease in symptom expression and the number of pycnidia produced by S. tritici when leaves were pre-treated with the yeast. There may be induced resistance to S. nodorum as a result of pre-treatment with the yeast. S. tritici does not appear to be as sensitive to the presence of the yeast as S. nodorum.

1. Fokkema NJ, Den Houter JG, Kosterman YJC, Nelis AL, 1979. Transactions of the British Mycological Society 73, 19-29.