Life with limited carbaryl use

As of September 20th, 2016 the label for carbaryl (Sevin XLR) changed to meet the PMRA re-evaluation decision. Carbaryl was under evaluation due to the post application risk to growers. Changes to the label are listed in the New Products for Ontario Apple Growers in 2017 article in this newsletter. The most impactful change to the label is that only one application is allowed. This is going to cause great difficulty with thinning programs as many growers use at least two applications. With this label change, other options and thinning programs will need to be considered with less reliance on carbaryl. Perhaps we will have to think beyond chemical fruitlet thinning and look at blossom thinning through caustics like ATS or mechanically.

Alternative fruitlet thinning programs

Thinning with only one application of carbaryl will pose difficulty for growers. Even with full use of carbaryl, fruitlet thinning is very stressful with planning on when to apply, number of applications and how much product to apply based on the weather, fruitlet stage and crop load. If you thin too much, you have a reduced crop. If you thin too little, your crew spends many hours hand thinning, raising your cost of production. Future thinning programs will have to rely less on carbaryl and more on other thinning products available in Ontario:

  • NAA (Fruitone) - The application window for NAA is from petal fall (3-7 mm) to early fruit set (8-10 mm) with NAA being most effective when the king bloom is at 5-10 mm. One application can provide adequate thinning but if needed, make a second application. NAA can be tank mixed with carbaryl on hard-to-thin cultivars but there is risk of pygmy or small fruit on some cultivars like Delicious and Fuji. Do not apply NAA during the same growing season as MaxCel or Cilis Plus as pygmy fruit may result.
  • 6-BA (Maxcel or Cilis Plus) - Apply 6-BA at 5-15mm. Apply up to two applications for fruit thinning and apply up to 4 applications for improving fruit size. When applying 6-BA to improve fruit size, begin applications at petal fall. 6-BA can be tank mixed with carbaryl on hard-to-thin cultivars.

With only application of Carbaryl allowed, you will have to be strategic with when you should apply it; perhaps as a final spray or with 6-BA or NAA to give those two products more strength.

Chemical blossom thinning

Thinning at blossom time is a widely used technique to manage crop load and improve fruit size in Washington State. Chemical blossom thinning would not be the only method to thinning but a tool in the whole program followed by fruitlet thinning and hand thinning. The advantage to thinning a portion of blossoms off is that earlier on, more carbohydrates will be allocated to the remaining developing fruitlets going through cell division. Caustics are used to thin blossom; ammonium thiosulfate is quite commonly used as well as lime sulphur. The goal of blossom thinning is to apply the product once the king bloom has been fertilized but before the side blooms have been fertilized. Blossom thinning in Ontario and other areas in eastern North America isn't widely adopted because we have a higher risk of frost, as is evident in recent history; having as many blossoms as possible through the bloom period acts as insurance. With limited use of carbaryl, though, it may be a method to consider.

Mechanical blossom thinning

In trials in New York State by Mario Miranda Sazo from Cornell Cooperative Extension and in Ontario by Dr. John Cline and Kendra McClure from the University of Guelph, mechanical blossom thinning (MBT) was evaluated and shown to effectively reduce fruit set. In both of these trials, a Darwin string thinner was used and was also on display at the most recent International Fruit Tree Association (IFTA) Study Tour (Figure 1). It was recommended by Miranda Sazo to use the string thinner at 180 and 200 RPM, not at 240, to reduce damage to spur leaves, reproductive and vegetative structures. Also, allowing the strings to reach into the canopy so that the strings are hitting the trunk and branches, will slow down the momentum of the strings and cause less damage. MaxCel was applied after MBT when fruitlets were 8-10 mm, to improve fruit size in Gala.

A main concern to MBT is the spread of fire blight throughout the orchard. At the IFTA Study Tour in New York, Miranda Sazo found that the incidence or severity of fireblight did not increase with MBT.

Figure 1. A Darwin string thinner used to mechanically thin blossoms

Figure 1. A Darwin string thinner used to mechanically thin blossoms

Thinning presentations at the Ontario Fruit and Vegetable Convention

At the Ontario Fruit and Vegetable Convention, the Apple Program will have Zac Farmer and Charles Stevens, two growers from Durham Region and Sean Bartlett from N.M. Bartlett Inc. speak on their experience and viewpoints on how to manage thinning programs with new carbaryl label changes. Following the panel and lunch, Dr. John Cline will also be speaking on thinning apples without carbaryl.

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