Studies on fungal pathogens and control of eyespot disease in winter wheat

Eyespot disease of wheat has been observed and widespread in Hokkaido since 1983. The purpose of the present study was to identify the pathogens, and to develop the measures to control eyespot disease by investigating the ecology, the damage, and the effect of fungicides and cultural control. 1. Identification of the pathogens Isolates of the eyespot pathogen of cereals were divided into two groups based on colony morphology on PDA: fast-growing, even-edged (FE) and slow-growing, feathery or uneven-edged (SF). These two groups of the pathogens could not be clearly distinguished by conidial morphology and the pathogenicity to rye. Genetic relationships between isolates of Oculimacula yallundae and Oculimacula acuformis, from diverse geographical origins including Japan, Europe, North America, South Africa and New Zealand, were determined by DNA relatedness on the basis of nDNA reassociation kinetics and a quantitative analysis of restriction fragment length polymorphisms of mitochondrial DNAs. This study suggested that isolates of O. yallundae and O. acuformis consist of two genetically distinct taxonomic groups, corresponding to separate species. We collected 1017 and 1931 isolates of wheat eye spot pathogen from 555 fields in various regions of Hokkaido in 1989 and 1992, respectively. Of these, 52.3% in 1989 and 43.7% in 1992 were identified as Oculimacula acuformis based on mycelial growth and culture morphology. The others (47.7% in 1989 and 56.7% in 1992) were O. yallundae. The two-year data showed that O. acuformis was widely distributed in Hokkaido, and was the dominant species in northern and eastern regions. 2. Ecology of eyespot disease Eyespot disease of wheat occurred in previously pathogen-free-soil that was even slightly mixed with infested soil. When wheat was grown in a field newly converted from a paddy field located next to a diseased continuous wheat-cropping field, eyespot disease was found during the first wheat growing season, and disease occurrence in the second season was almost equal to that in the diseased continuous wheat-cropping field. This suggests that the eyespot pathogen easily invades and establishes itself in non-infested fields. Influence of climatic factors on eyespot disease of winter wheat was investigated in Hokkaido from 1987 to 1996. The disease severity was associated negatively with the lowest temperature in early May and positively with the number of days with precipitation above 1mm in early November. The development of eyespot disease was examined for two years. Initial occurrence of the lesions was observed in late October, and incidence rate increased sharply after late April. We considered that rainfall in early November promoted primary infection, and the low temperature in early May extended the infection period. 3. Estimating disease losses We estimated yield and quality losses in winter wheat due to eyespot disease based on field experiments over three years in Hokkaido. The yield and quality of wheat decreased when the stems were completely girdled by lesions but no lodging occurred. Eyespot infection did not affect ear number per square m or grain number per ear, but reduced 1,000-grain dry weight under severe infection, resulting in a decrease in the yield. The yield did not decrease significantly when less than 90% of stems were covered with lesions and disease severity index was less than 40. 4. Chemical control The effects of benzimidazole fungicide thiophanate-methyl, propiconazole and prochloraz of DMIs, and anilinopyrimidine fungicide cyprodinil on eyespot disease were examined. Surveys in 1989 and 1992 showed that benzimidazole resistant strains were common and widespread in Hokkaido, and especially in the eastern region, most of the strains were resistant to benzimidazole. Experiments in the fields of varying resistant isolate frequencies suggested that thiophanate-methyl was able to control eyespot only in fields whose frequency of resistant isolates was low. The susceptibility of O. yallundae and O. acuformis to demethylation inhibitor (DMI) fungicides was tested. The two-year results indicated that the susceptibility of O. acuformis to propiconazole was lower than O. yallundae, whereas there was little difference in susceptibility to prochloraz between the two pathogens. Efficacy of propiconazole and prochloraz on wheat eyespot was investigated in the fields where the proportion of O. acuformis was different from 5.0 to 100%. Propiconazole was effective in the fields dominated by O. yallundae, whereas effect was not observed in those dominated by O. acuformis. Prochloraz showed a stable effect in all the fields tested. Cyprodinil could control eyespot except for fields where the resistant strains were isolated. The control effects of thiophanate-methyl and prochloraz were examined in different growth stages of winter wheat for three years. Early May (GS 31) was the most effective, followed by late April (GS30) and mid- May (GS 33). This study suggested that disease severity of eyespot disease could be predicted from climatic factors to determine the necessity of control. 5. Cultural control Crop rotation effects on eyespot disease severity in winter wheat were investigated in Hokkaido. Cultivating non-host crops for one, two and three years reduced disease severity by 22.2%～38.7%, 48.2%～56.3% and 69.0%～73.l %, respectively. This effect was lost after the first cropping of wheat. There was no difference in effects between the tested non-host crop species: potato and adzuki bean. Eyespot disease severity was higher when wheat stem number/square m was greater, which resulted from higher seeding rates and earlier sowing dates. This indicated that disease losses could be reduced by proper sowing and nitrogen application to prevent excessive tillering in wheat. A total of 108 wheat varieties collected from Japan and other countries were examined for resistance to eyespot disease. All of the varieties bred in Japan were susceptible, whereas four foreign varieties, "VPM-1", "Madsen", "Cappelle-Desprez", and "Cerco", showed resistance, and could be used as parents in the development of resistant varieties in Hokkaido. Near-isogenic lines (NILs) was developed by backcrossing between the non-recurrent patent "Madsen" with eyespot resistance and the recurrent parent "Kitahonami", followed by DNA marker-assisted selection. Agricultural characteristics, such as maturity, culm length and yield, of the NILs were similar to those of "Kitahonami".