Noon, Mon, April 14: Early-season apple cultivars at the Hudson Valley Lab reached the green tip bud stage (50% of fruiting buds showing green tip) on Sunday, April 13. Temperatures over the past three days were 5 to 8 degrees higher than last Friday’s forecast had suggested, and the predicted high for today is 80 F. Forecasted temperatures for the rainy period that will be arriving tomorrow (Tuesday) have also been increased slightly, with all indicators suggesting that we will sustain a moderate scab infection period with perhaps 20 hr of leaf wetting and a mean temperature of 53 to 56 degrees.
Apple leaves collected this morning from an abandoned orchard just north of the Hudson Valley Lab provided a mean discharge of 208 spores in our shooting tower, well above the 50-60 spores that we have traditionally considered an economic threshold for our spore tower discharge tests. Thus, our spore assessments suggest that apple scab ascospores will be discharged during the coming rain, and that orchards with carry-over inoculum will be at risk of infection if they are not covered with fungicide before the rain begins. The apple scab model in the NEWA system (http://newa.cornell.edu/index.php?page=apple-diseases) predicts that 3% of the total season’s ascospores will be released on Tuesday. Note that the NEWA model is predicting actual spore discharge, not percentage of mature spores, which would be much higher.
Of particular concern is the fact that daytime temperatures for Tuesday are predicted to be in the low 60’s. Research conducted in various locations has shown that spore release during the first three to five hours of a wetting period is roughly twice as great when temperatures are above 50 F as compared to temperatures in the low 40’s. The total number of spores released at lower temperatures may eventually be the same as at warmer temperatures, but the more rapid release at warmer temperatures means that a lot of spores have more time to be blown around, find green tissue, and initiate infections before the temperatures drop and/or the wetting period ends.
Orchards that are not sprayed with fungicides before Tuesday’s rain may benefit from post-infection fungicides applied on Wednesday or Thursday. Vangard, Scala, and Syllit are the preferred post-infection fungicides for this stage of bud development, and all three of them will all provide 48 to 72 hours of post-infection activity counting from the start of the wetting period. Post-infection activity may be limited to 48 hours if temperatures remain warm, but 72 hr of post-infection activity is possible with lower temperatures, such as those predicted for Tuesday evening, Wednesday, and Thursday.
The disadvantage of post-infection spraying is that windy weather often follows spring rains, thereby making it difficult to achieve good spray coverage. Also, one cannot depend on rains to redistribute fungicides applied in post-infection mode. Finally, whereas Syllit (dodine) at 1.5 pt/A applied in combination with mancozeb will provide good protection when applied ahead of rains, the original research on dodine conducted in the 1960’s showed that the equivalent of 3 pt of Syllit/A is required for efficient post-infection activity. With Vangard and Syllit, however, research trials at the Hudson Valley Lab showed the same level of post-infection activity against scab when we used the lower label rates as when we used the higher label rates for those two products. Thus, whereas Syllit must be used at higher rates when applied in post-infection timings, there is no need to use higher rates of Vangard or Scala. All three of these products should always be applied in combinations with mancozeb in early-season sprays.
