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Ontario Apple Update: April 19

April 30, 2024  By ONFruit

Growth Stages

With the cooler temperatures, growth has been moving quite slowly this week. Regionally, growth stages are fairly spread out depending on cultivar and proximity to water:

  • In Essex, Chatham-Kent, Lambton, Middlesex, Elgin, Norfolk, Brant and Niagara, most cultivars are at tight cluster. However, earliest regions are seeing some signs of pink.
  • In Grey, most cultivars are green tip to half-inch green.
  • In Durham, Northumberland and Quinte, most cultivars are at half-inch green with some in very early tight cluster.
  • Further along the St. Lawrence and into the Ottawa Valley, apples are at green tip to half-inch green.

Weather Forecast

Certain areas are expecting cool weather overnight this weekend and into next week. For information about critical temperatures, assessments and mitigation, please refer to the article below:

Disease Development

Rainfastness of Fungicides

With the on-going rainy weather, I thought it would be important to touch on rainfastness of fungicides. Timing fungicides can be a challenge. Infection for most diseases requires rainfall. Fungicides are used mostly protectively so work best if applied before rain. But how well do fungicides stand up to rains that occur after application?


A general rule of thumb often used is that 1 inch (2.5 cm) of rain removes approximately 50% of protectant fungicide residue and over 2 inches (5 cm) of rain will remove most of the residue. However, many newer formulations or with the addition of spreader-stickers, some products may be more resistant to wash-off. However, avoid putting on fungicides within several hours before a rainstorm as much can be lost to wash-off regardless of formulation. As well, there are exceptions to the general rule in regard to truly systemic fungicides such as Aliette and Phostrol.

The following are some points to consider to improve fungicide efficacy during wet weather:

  • During rainy periods, systemic fungicides tend to perform better than protectant (or contact) fungicides since they are less prone to wash-off.
  • Applying a higher labelled rate can extend the residual period.
  • Apply protectant fungicides such as captan (Supra Captan, Maestro), mancozeb (Manzate, Dithane, Penncozeb) and folpet (Folpan) during sunny, dry conditions to allow for quick drying on the leaves. These types of fungicides are better absorbed and become rainfast over several days after application.
  • Apply systemic fungicides such as sterol inhibitors (Cevya, Nova, Fullback, Inspire Super), SDHI (Excalia, Fontelis, Sercadis, Kenja, Aprovia Top, Luna Tranquility) and strobilurins (Flint, Pristine, Merivon) under humid, cloudy conditions. The leaf cuticle will be swollen, allowing quicker absorption. In dry, hot conditions, the cuticle can become flattened and less permeable, so product can breakdown in sunlight, heat or microbial activity or be washed off by rain.


Most areas have now reached (or are very near) the accumulation of 125 degree days since green tip where we start to see rapid maturation of ascospores. Major apple scab infection events can occur during rainy periods when conditions are conducive to infection. Average temperatures in the 7-day forecast look to be around 6 to 10C, which only requires 11-18 hours of leaf wetness for a primary infection to occur. For more information, check out Relationship of Temperature and Moisture to Apple Scab Infection.

As I mentioned last week, even with the freezing temperatures, it is possible for ascospores to continue to develop.

Keep good fungicide coverage throughout this infection period. If there are any concerns about residue wash-off, you may want to consider going in after the rains have stopped with a post-infection, or kickback product. Refer to Characteristics of Apple Scab Fungicides for more information.

By tight cluster, apples are typically entering into a time of critical infection period for scab and powdery mildew with higher daily temperatures, large amounts of lush growth and rapid maturation of spores. Management programs at this point should begin incorporating systemic fungicides (Groups 3, 7 and 11). However, unlike protectant fungicides like Group M, which have multi-site activity and low resistance potential, systemic fungicides are typically single-site and are at high risk of resistance development. For resistance management, where possible, half to full rate protectant fungicides should still be included with all applications and rotate between chemical groups.

Chemical groupings of common single site fungicides registered for apples.

Insects of Interest

Spring-Feeding Caterpillar

For the most part, insect activity has been low. However, this week there have been a few signs of early spring-feeding caterpillar damage, in particular leafrollers. Look for signs of leaf feeding, rolled terminal leaves and webbing.

Small obliquebanded leafroller on apple leaf (Photo: Amanda Dooney)

When scouting between tight cluster to petal fall, check 5 terminal shoots and 5 fruit buds in 10 trees (50 terminals and 50 fruit buds in total). Control is typically warranted when 12-15 terminals and buds are infested. Click here for registered prebloom control products for spring-feeding caterpillar.

Apple Leafcurling Midge

Another insect pest we commonly start to see an increase in activity at tight cluster to pink is apple leafcurling midge. Despite the recent rains, leafcurling midge flight has started to kick off. There are still no signs of egg laying but I imagine this will begin in the coming weeks. I will continue to provide updates on egg laying as well as regional degree day predictions.

Adult leafcurling midge on developing cluster.


Trying to find a window for dormant oil targeting scale timing was difficult in some areas this year. Another prebloom timing to consider is a tight cluster to pink application of Sivanto Prime or Closer to target the maturing scale. The high rate of Closer at this timing will also have efficacy on leafcurling midge, woolly apple aphid, tarnished plant bug and mullein bug.


If this spring continues to be on the cool, wet side, be prepared for rosy apple aphid populations to build. Egg hatch is likely on-going currently but infestations really start to be observed into tight cluster. Pay particular attention to fruit clusters of susceptible cultivars like Cortland, Idared, Golden Delicious and Ambrosia.

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