Summary:
Vigour or colour of downy mildew infected turfgrass (Poa annua) did not improve significantly with 2 spring applications of azoxystrobin at 3 application rates. Although the fungicide may influence the disease cycle, no visible improvement of the turf was produced within 3 weeks from its first application during the spring of 1999. The product did not have a negative impact on the growth of the turf and also did not make downy mildew more severe during the trial period at the rates tested.

Background:
Since 1997, turf managers have been observing general yellowing of bentgrass and Poa annua during the spring. The cause of the problem appears to be downy mildew caused by a Sclerophthora sp. (tentative identification), which is closely related to Pythium. Downy mildew was identified in yellow patches 2-10 cm in diameter on a practice green at Mt. Brenton Golf Course. This site was used for the research trial.

At the time of the study, no fungicides were registered to control downy mildew on turfgrass. The fungicide azoxystrobin, which is marketed in the USA as Heritage, will be registered in Canada soon. Since azoxystrobin is labeled for Pythium diseases on turf and downy mildew of grape in the USA, it may have efficacy against downy mildew of turf. This study was developed to determine the value of azoxystrobin for the control of downy mildew and to assess the crop tolerance to the fungicide.

Methods:
Three rates of ICIA5504 50 WG (azoxystrobin) were used as separate treatments and an untreated control was included as a check (Table 1). Application dates were April 16, 1999 and again April 28, 1999. Each treatment and control was replicated four times. Plot size was 150 ft2 and spray volume was 400L/ha. Plots were sprayed when the grass was dry and plots were not mowed for 2 days after spraying.

Grass colour and vigour were assessed by randomly placing 4 open 1 ft2 grids in the centre of each plot. Colour was rated from 1-5 with 5 = healthy, 4 = slightly yellow, 3 = yellow-green, 2 = yellowish, 1 = orange-brown. Vigour was rated from 1 to 5 with 5 = dense and 1 = no plants present. Ratings of 2,3 and 4 will be gradations from poor to healthy. Four readings (one in each grid) were taken in each treatment plot to produce an average rating for each plot. ANOVA (analysis of variance) analyses were performed between treatments and across time to determine if there were any differences between treatments or over time for individual treatments.

Table 1. Treatments used in study

Results:
Differences in the average ratings (Tables 2 and 3) were not statistically significant. ANOVA analyses show that there is no difference in the colour or vigour of the turfgrass regardless of the application rate. It is likely that the slight differences seen in the averages are not statistically different because of the amount of variability between replications. The ANOVA analyses also show that the fungicide did not significantly damage turfgrass at the rates used in the study and it did not make the disease worse.

Table 2 - Average colour of turfgrass (rating 1-5 with 5=healthy)

Table 3 - Average vigour of turfgrass (rating 1-5 with 5=most dense growth)

Conclusions and Observations:
Azoxystrobin did not help turf infected with downy mildew to have better colour or grow more vigorously when applied in the spring. It may be that azoxystrobin inhibits the growth of Sclerophthora sp. and may help to reduce the future level of disease, however its impact in this regard cannot be determined from this study. Cultural controls recommended for downy mildew such as avoiding nitrogen deficiencies and reducing moisture and humidity in the plant canopy continue to be the recommended practices. Based on this limited study, greens appear to have good tolerance for azoxystrobin and the chemical did not make the symptoms worse. An observation was that the treated green seemed to be less disease-prone throughout the summer and into the fall. No Fusarium patch was observed on this green.

Acknowledgements
· Western Canada Turfgrass Association (financial support)
· Mt. Brenton Golf Course
· Zeneca (Bill Moons)
· BC Ministry of Agriculture and Food

1 BC Ministry of Agriculture and Food
2 Mt. Brenton Golf Course

Turf Line News December 1999/January 2000

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