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Apple Scab Review

This season is off to a rapid start with green tissue present and plenty of rainfall, so growers have kick started their scab management programs. David Rosenberger, a pathologist from Cornell University stresses the importance of early season scab prevention since the early infections cause the greatest economic loss. Scab is difficult to control when established early, and fungicide application to sporulating scab lesions may increase the risk of resistance.

Good scab management begins early at green tip and continues to the end the primary infection period (late June). If scab is in the orchard at the end of this period, management must continue until harvest. With scab already in some orchards (lesions appear 9 to 17 days after an infection), here is some useful tips for your scab management program:

• Monitor weather in your orchard (maximum and minimum daily temperature as well as leaf wetness). Regional weather forecasts or agri-phones may not give site-specific weather information.
• Be aware of the level of inoculum in your orchard. If your orchard had apple scab last year, or is near abandoned, or high pressure orchards, consider your orchard to be a high inoculum orchard. Remember that the very wet summer last year throughout the province has created higher scab pressure this year.
• When calculating infection periods, consider the inoculum level in your orchard. Research in New York has shown that 96% of ascospores are released during the day (between 8 a.m. and 7 p.m.). The remaining 4% can still cause significant problems in high inoculum orchards, but are not a large concern in low inoculum orchards.
• In low inoculum orchards, rain that occurs between 8 am and 7 pm are counted from the first hour the rain was recorded until it ends. So if rain begins at 9 am, and continues until midnight, count 15 hrs of leaf wetness. When rain begins at night between 7 pm and 8 am, count the hours of leaf wetness from 8 am until the leaves are dry. So if rain falls from 10 pm until 10 am the next day, start counting at 8 am, for a total of 2 hrs of wetting.
• In high inoculum orchards, calculate the length of the wetting period from the start of the rain until the leaves are dry, regardless of the time of day. Add together wet periods caused by intermittent rain, unless wet periods are separated by 10 hours or more of dry sunny weather. Calculate the average temperature over the hours of wetness, and consult Table 4-4, Publication 310, Apple IPM Manual (pg. 111) to see if an infection occurred.
• Once primary infections have occurred, lesions on the leaves and fruit produce secondary spores or conidia, which spread short distances by splashing rain. Conidia require a slightly longer period of leaf wetness than for primary infection, but unlike ascospores, they do not require rainfall to initiate spore release. Early morning dew is more important in initiating wetting periods. Secondary infections can occur day or night, so calculate the length of the wetting period from the beginning of the wetting period regardless of the time of day. Scab infection periods can continue throughout the summer from lesions with conidia.
• As fruit matures, it becomes more resistant to apple scab. As a result longer wet periods are needed for fruit infection to occur as the season progresses. In Publ. 310, Figure 4-5 (pg 112) shows the relationship between temperature, hours of wetting, and weeks after bloom for secondary scab fruit infections. For example, at an average temperature of 14º C, it takes only 8.5 hours of wetting one week after bloom, while at 10 weeks after bloom, 32.5 hours is needed to produce for 2% fruit infection.

In terms of fungicide residues and rainfall as a general rule of thumb:

• Less then 1 inch of rain since the last spray will not significantly affect residues.
• 1-2 inches of rain will reduce the residue by ½. Reduce the number of days until the next spray by ½.
• Over 2 inches of rain since the last spray will remove most of the spray residue. Re-apply a fungicide as soon as possible.