The epidemiology of dicarboximide resistant strains of Monilinia fructicola (Wint.) Honey in stone fruit orchards

Abstract

Dicarboximide resistant strains of Moniliniajructicola (Wint.) Honey were detected in New Zealand stone fruit orchards in 1985. In contrast to benzimidazole resistance in M. jructicola, the occurrence of dicarboximide resistance had not led to disease control failures at that time.

A mass screening system was developed to detect low frequencies of resistant strains in orchards. Resistant strains were detected on agar discs amended with 5 mg a.i./l iprodione and 250 mg/l streptomycin sulphate. In 1987 the proportion of resistant strains in eight blocks ranged from 0.2 to 5%. The frequency of dicarboximide applications influenced the proportion of resistant strains from 1987-1989. However, there was no positive correlation between brown rot incidence and the proportion of resistant strains at the frequencies tested

The characteristics of resistant and sensitive strains were investigated. EC50 values ranged from 3-217 mg a.i/l iprodione for resistant strains and 0.3 to 0.7 mg for sensitive strains. Strains resistant to iprodione were cross-resistant to vinclozolin, but not to carbendazim, triforine or bitertanol. Dicarboximide resistance was phenotypically stable for the majority of resistant strains. However, the degree of resistance of four of 11 resistant strains decreased significantly after nine generations on unamended nectarine fruit.

Three resistant strains and two sensitive strains were selected for more detailed analysis of biological characteristics on agar and host tissues. A detached peach shoot system was developed to study the virulence and fitness of strains on flowers (cv. Redhaven) in controlled environments. Resistant strains were pathogenic on flowers (cv. Redhaven) and fruit (cv. Fantasia) amended with dicarboximide. Resistant strains were as virulent and fit as sensitive strains on flowers, but significantly less fit on fruit, in the absence of dicarboximide. One resistant strain was at least as virulent and fit as sensitive strains in both flower and fruit tests. There was a poor correlation between the fitness of resistant strains on flowers and on fruit. Resistant strains were significantly less competitive than sensitive strains on flowers and fruit. Virulence and fitness on flowers in the absence of competition was not a reliable indicator of competitive ability.

Resistant strains were aggregated in all blocks, indicated by three spatial indices. This was confirmed by spatial autocorrelation analysis. The greatest aggregation occurred in an orchard where the mean density of resistant strains was the lowest. Spatial autocorrelation analysis indicated that resistant strains were aggregated at a maximum of one to three tree sites, which suggested limited spread from resistant foci. Aggregates occurred within and between orchard rows which indicated a degree of directional bias in some blocks. Spatial autocorrelation also indicated that higher frequencies of resistant strains were associated with more foci rather than focus expansion. In contrast to the spatial pattern of resistant strains, brown rot incidence was less aggregated and disease foci were larger. Spatial autocorrelations indicated that within row spread of disease was more important than across rows when conditions were favourable for brown rot development at one orchard.

The incidence of resistant strains was not significantly correlated between successive seasons in seven of eight blocks. Absence of significant temporal correlation indicates that resistant strains did not persist at sample sites. There was some evidence to support the hypothesis that resistant strains may not overwinter effectively. There were no significant spatiotemporal correlations for resistant strains, which suggested that the selection of dicarboximide resistant strains occurred during the current growing season.

Modified spray programmes are now recommended in New Zealand to manage dicarboximide resistance and maintain disease control. These recommendations are based on these studies of the effect of frequency of dicarboximide applications on resistant populations, reduced fitness and competitive ability of resistant strains on fruit in the absence of dicarboximide and the limited spread of resistant strains within and between growing seasons.