Department of Genetic Resources I, National Institute of Agrobiological Resources, Ministry of Agriculture, Forestry and Fisheries, 2-1-2 Kannondai, Tsukuba, Ibaraki 305, Japan

Background and objectives
Because of excess production of rice, huge areas of paddies (rice fields) in Japan are requested to be converted for other crops, e.g. wheat, barley, soybean and vegetables. Fusarium diseases on wheat and barley, such as head scab, still remain as important problems to be solved. The objective of this study is to analyse the Fusarium flora on wheat plants and to estimate the influence of field conditions on the potential infection of causal agents of such diseases.

Materials and methods
Healthy wheat plants were collected on two separate occasions in the city of Tsukuba, Ibaraki, Japan, from a wheat field converted from a paddy (CP) in 1990, and from a normal upland field (NF) in 1995. Both the fields are nowadays typical for wheat cultivation in Japan. Collection of wheat plants (cv. Norin 61), which were sown in the previous autumns in both CP and NF, started from the winters following their growth stages until harvest. The collected plants were cut into different parts as study samples, i.e. leaf blades and stems of 5 cm radius above the ground, stems under the ground, remnants of seeds attached, and roots of 5 cm radius. From spring to summer, growing heads (ears) of the wheat were cut from the plants to obtain spikelets and seeds. At intervals, 5-cm culms just above ground level and 5-cm leaf blades of a middle position were also obtained. The study samples were washed in detergent, placed on CMA and incubated. Species of Fusarium were isolated and identified following Gerlach & Nirenberg [1]. Percentage frequencies of occurrence for each species were calculated by counting numbers of samples from which the species was detected.

Results and conclusions
Occurrence of Fusarium spp. from wheat plants cultivated under two different field conditions, CP of 1990 and NF of 1995, differed greatly. In CP of 1990, Fusarium flora on the winter to early spring wheat consisted of species such as F. graminearum and F. equiseti, occurring mainly on underground parts of the plants. But F. oxysporum occurred just once and F. solani was not detected. In NF of 1995, on the contrary, F. oxysporum and F. solani were the main Fusarium spp. from all parts of the wheat plant in winter and early spring, but no occurrence was observed for F. graminearum and F. equiseti.

Another remarkable contrast between in CP and in NF was found in the flora of growing wheat heads. In CP of 1990, only two species of Fusarium, i.e. F. graminearum and F. proliferatum, were recovered from the spikelets and the ripe seeds, but 12 species of Fusarium, including both these species, from NF of 1995. Succession of Fusarium species on the 5-cm above-ground parts of wheat showed similar features. Differences in field conditions, e.g. soil type, remnants of rice stubble, drainage ability of the converted paddy, were suspected to be possible causes of the difference in flora.

1. Gerlach W, Nirenberg H, 1982. The Genus Fusarium - a Pictorial Atlas. Paul Parey, Berlin.