Chlorothalonil and Oil for Scab Control -- To Late? Yes.

Figure 1: The effect of dormant treatmentson the incidence of almond scab sporulation.Observations were made on May 3rd, 2011.Different letters indicate significanttreatment differences at p<0.05.

There has been quite a bit of talk about the use of Chlorothalonil and oil applied during dormant for Scab control. Although this treatment has shown to reduce overwintering scab populations and delay scab lesion sporulation (Figure 1), caution needs to be taken when considering application. This product needs to be applied when the tree is completely dormant. Once the tree starts to push and the buds begin to swell/green tissue appears at the tip (Figure 2), avoid using this spray treatment. Chlorothalonil mixed with oil applied to green tissue will kill the green tissue, explaining why this product is labeled for dormant use no later than December.

Figure 2: Delayed dormant stage of analmond bud. Note the bud swell andexposed green tissues.

If a dormant scab treatment is to be applied at this time (delayed dormant/bud swell), consider making an application of copper and oil. This treatment has also been shown to be quite effective (Figure 1), especially when used in combination with in-season scab sprays timed 2-5 weeks post petal fall.