Apple Disease Management: Part One – Early Season (Green Tip to Tight Cluster)

With the very spring-like temperatures, apple buds are moving along quickly. Most areas of the province are seeing green tip in earlier varieties, while some regions are already at ½” green. If protectant fungicides haven’t gone on yet, now would be an ideal time, especially in anticipation of the forecasted wet weather over the long weekend.

The best disease management is early disease management

Where there is green tissue, apple scab and powdery mildew spores can find it. Risk of infection by these diseases steadily increases as temperatures rise and growth continues. If you were dealing with scab or mildew last year, overwintering spores are present in the orchard now in leaf litter (apple scab, Figure 1) or dormant buds (powdery mildew, Figure 2). The best defense to prevent scab and mildew this season is getting protectant fungicide sprays on as soon as possible to keep those early spore release periods from taking hold.

Overwintering apple scab in leaf litter.

Figure 2. Healthy (left) and powdery mildew infected (right) apple buds.

Despite a dry 2016, disease control is still important

While it is possible the dry season last year reduced apple scab inoculum, it’s very unlikely levels went down to zero. Why take the chance of fighting infection the rest of the year? If an early season infection period is predicted, putting on a protectant fungicide will never be a wasted spray.

This is particularly important in orchards with historically high scab pressure. The level of infection risk for this season is dependent on the overwintering inoculum from last year. If you were chasing scab in your orchard and did not do any sanitation practices (urea spray, leaf mulching) in the fall or this spring, the inoculum levels are likely still high enough that mature ascospores could be released with early wetting events.

Besides, although apple scab favours wet weather, powdery mildew thrives in dry and humid conditions. A dry season last year followed by the mild winter could have us seeing rather high overwintering mildew inoculum.

As well, though apple scab may have been low last year, fire blight was a significant issue across the province. It’s reasonable to say many orchards will have overwintering fire blight cankers oozing this spring (Figure 3), particularly as temperatures warm above 10-15⁰C with humidity or moisture. Dormant copper such as Copper Spray, Copper 53W and Cueva can safely be applied up to ½” green for management of fire blight bacteria. Using 1-2% dormant oil with Copper Spray or Copper 53 W will act as a sticker, as well as provide efficacy on scale, European red mite and suppression of powdery mildew. Cueva is formulated with a fatty acid so there is little benefit to adding oil as a sticker. Dormant copper may also have some efficacy on apple scab if any spores are being released at the time.

If using dormant oil, do not apply captan or sulphur products within 14 days of application. Oil can enhance penetration of these products into sensitive tissue, resulting in phytotoxicity issues.

Tips for an early season disease management program

• Dormant copper can be applied up to ½” green. However, ideal timing is silver tip to ¼” green. This type of early season application should not have any compatibility issues with oil.
• Rainfall is needed for the release of apple scab ascospores. The length of a wetting period required for infection varies with temperature as outlined in the Revised Mills Table. Typically during temperatures above 12⁰C, less than 8 hours of leaf wetness is required for infection.
• Green tip to tight cluster is a period of extensive new growth. Keep covered. Re-apply fungicides every 5-7 days during periods conducive to disease development or following heavy (greater than 1”) rain.
  • A protectant program consists of contact fungicides and does not provide effective post-infection or anti-sporulant activity. That means if sprays are applied in less than ideal conditions, ie., windy, alternate rows, or washed off in rain, the risk of scab infection is increased.
  • Captan (Maestro, Supra Captan), mancozeb (Dithane, Manzate, Penncozeb) and metiram (Polyram) have good redistribution properties. Apply these immediately prior to an infection event. Rain will help wash these fungicides further into the canopy.
  • Consider “captozeb” – ½ rate captan plus ½ rate mancozeb. This combination provides the good retention (rainfastness) of mancozeb with better redistribution and efficacy of captan. It will not be as harsh to foliage as captan alone.
• In prolonged wetting events, it is better to re-apply fungicides during a break in the rain to provide temporary protection, than to not spray at all. Apply again once conditions are finally dry to replace residues washed off by rain.
  • A combination of dodine (Syllit) and mancozeb might also be a good early season option until tight cluster. Dodine is a strong oil-compatible scab fungicide with good retention, redistribution and anti-sporulant activity. This may be useful during times when coverage is a concern with protectant fungicides alone. However, dodine-resistant scab populations are present in Ontario so this product should only be used in a tank mix with a protectant and no later than tight cluster.
  • If there is concern about coverage during a potential infection period, using a product with good kick-back, such as Scala or Luna Tranquility may provide post-infection activity up to 48-72 hours in cooler temperatures, depending on the product, from the start of the infection period.
• Protectant fungicides do not control powdery mildew. Include 3-5 kg/ha sulphur (Microthiol Disperss, Microscopic Sulphur, Kumulus) with the protectant sprays until tight cluster when more effective mildew products will be used. Keep in mind mildew does better in dry conditons with high relative humidity. Protectant sprays may be needed during dry periods when there is little risk from scab.