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Apple Pest Update

Crop Stages
The cool, wet spring has made for what feels like a long start to the season. Many are reporting to be a few days to a week behind last year, which was also a late season in terms of growth. In all regions, growth stages are quite spread out depending on variety and proximity to water:
  • In Essex, Chatham-Kent, Lambton, Middlesex, Elgin, Norfolk, Brant and Niagara, early varieties are at pink to bloom. However, tight cluster can still be found in late varieties.
  • In Grey, Durham, Northumberland and Quinte, most varieties are tight cluster to early pink.
  • Further along the St. Lawrence and into the Ottawa Valley, apples are at tight cluster.
A big issue this spring has been the above average rainfall. Snow melt was long and the soil didn’t have much drying time before the snow turned to rain. Orchards across the province have stayed water-logged. Growers are trying to avoid ruts and getting equipment stuck; some by using partial tanks when spraying to lighten the load.
With the wet weather this spring, some areas have seen as many as 6 potential scab events since Easter weekend including events that have lasted 3 or more days. Most areas have reached the period of rapid ascospore maturation, at about 30-50% of ascospores matured. The forecast this weeks looks to have dryer weather. However, that means there will be a significant build-up of mature ascospores ready to be released during the next rainfall.
The length of leaf wetness required for infection will also be greatly reduced with higher temperatures. Refer to the revised Mills table in Table 1 below for more information. Make sure to keep covered every 5-7 days during this time, especially ahead of rain and be sure to tank mix single-site fungicides with protectant Group M fungicides for resistance management.
Scab lesions have not been found yet. However, the cooler weather has likely slowed down the formation of conidia on lesions (secondary scab) so symptoms may not be present for up to 17 days following infection. These can develop quickly  should weather warm up (Figure 1 & 2). With the frequent rains and reports from growers that were having difficulty keeping good coverage, I imagine we will start hearing of lesions being observed in susceptible varieties shortly.
Table 1. Revised Mills table relating leaf wetness duration and temperature to predict infection time and symptom development of apple scab.
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