MaxCel® - A New Chemical Thinner for Apples

Valent Biosciences announced in December 2007, that the Pest Management Regulator Authority (PMRA) has approved the registration of a new formulation of 6-BA for thinning apples, called MaxCel®. The company reports that MaxCel® will replace Accel®, first registered in Canada in the spring of 1996, however both will be available for the 2008 growing season. 

The active ingredient in MaxCel® is 6-benzyadenine (6-BA). 6-BA is a cytokinin, a class of growth regulator that promotes cell division. MaxCel® contains 1.9% 6-BA, which is slightly more concentrated than Accel®, and unlike Accel®, it contains no gibberellic acid (GA₄₊₇). Prior research has clearly demonstrated that GA₄₊₇ contained in the Accel® formulation, can in some instance decrease the thinning efficacy of 6-BA at higher concentrations (> ~150 ppm) and may also inhibit flowering the following season. While this was rare and very uncommon because such rates were seldom used, it was of sufficient concern that the company reformulated Accel®. One further advantage the manufacturer states is that the new Maxcel® formulation contains additives to enhance absorption and provide increased product stability and solubility of the active ingredient. Many comparisons of Maxcel® and Accel® are shown in Table 1.

^[1] – Based on survey of pricing provided by supplies as of March 28, 2008 (Many did not have 2008 pricing available at the time of print). Prices may vary by supplier and purchasing volumes.

^[2] – The Canadian distributor has indicated that the cost per litre for MaxCel® will be the same as Accel®.

What is New

Rates and Number of Sprays: MaxCel® is limited to two sprays if used for thinning, and four sprays if used for fruit size enhancement. The total amount of product applied per season cannot exceed 446 grams 6-BA/ha (22.5 L/ha), which is nearly six times more the 6-BA than was permitted under the Accel® label.

Using Maxcel® to Increase Fruit Size: The product label states that Maxcel® can be used to enhance fruit with mild or no thinning. Two to four applications, beginning at petal fall and repeating every 3-10 days, are required to enhance fruit size. Suggested rates are 10-50 mg/L 6-BA (ppm), but the label cautions that some easy to thin cultivars and/or environmental conditions may result in fruit thinning.

Using Maxcel® to Thin: The product label states that MaxCel® can be used at rates of 75 to 200 mg/L 6-BA. Our experience has shown that 6-BA at concentrations ranging from 50-75 mg/L 6-BA is a mild thinner. However, if used alone at rates up to 200 mg/L or combined with Carbaryl for harder to thin cultivars, the treatment becomes much more aggressive.

Suggested Use Pattern: While 6-BA is not a ideal thinning compound for all cultivars, it has exhibited effectiveness for Empire, McIntosh, Idared, and Gala and many other varieties. MaxCel® and Accel® currently have the advantage over other chemical thinners (NAA and Carbaryl) by thinning as well as increasing fruit size. This is achieved by stimulating cell division in the early stages of fruit growth and development. One would need to carefully consider its use on Honeycrisp and other very large fruited cultivars, only for the reason that excessive fruit size can be problematic.

The thinning response to 6-BA is concentration dependant, meaning that increasing the concentration applied generally will result in increased thinning activity. MaxCel® at 100 to 150 ppm will provide a stronger thinning response than what might be expected from Accel®. While most growers considered Accel® to be a mild thinner, the MaxCel® label will permit a range of rate options from mild through aggressive thinning.

If mild thinning is desired, similar to the results obtained with Accel®, then 75 ppm MaxCel® is a good starting point. For moderate thinning with easy to moderately difficult cultivars, 75-100 ppm is acceptable, while 100-150 ppm might be used for more difficult to thin cultivars. MaxCel® can be used by itself or in combination with Sevin where more aggressive thinning is desired. Table 2 contains suggested rates of MaxCel® and Sevin® XLR Plus. It is important to review Publication 360 Fruit Production Recommendations for further information on cultivar sensitivity to fruit thinners and other information regarding the chemical thinning of apples.

[1] there are several factors that influence the chemical thinning outcome. Rates are generally chosen on the degree of cultivar sensitivity to chemical thinners. Consult Publication 360 for further information on cultivars sensitivity to chemical thinners.

[2] 1 ppm is equivalent to 1 mg/L.

[3] Based on April 2008 prices. May vary by supplier. Calculation based on manufacturer suggest retail price of $16.40/L Sevin® XLR Plus and $112/L MaxCel®, and excludes application costs. For comparison purposes, the estimated costs of one 10 ppm spray of NAA (Fruitone-N® is approximately $67.60/ha, based on a suggested retail price of $209.52/kg Fruitone N and application of 1000 Litres of spray/ha.

[4] Mild thinning may occur under some conditions (weak trees, young trees, sensitive cultivars, and environmental conditions that favour the thinning response.

[5] While 6-BA has the potential to increase cell division and enhance fruit size beyond the thinning (crop load) effect alone, this is not observed in all years because the response can be affected by spray concentration, coverage, cultivar, tree health, time of application, tree stress, and environmental conditions during and following spray application.

Since the label is based on a per hectare basis, the maximum concentration applied depends upon the size of the tree and volume of water used to obtain good coverage. Table 3 provides the relationship between water volumes, grams active ingredient (BA) per unit area, and concentration (ppm).

Pre-harvest Interval: Another change on the MaxCel® label is that the pre-harvest interval (PHI) has been increased from 28 to 86 days. This means that MaxCel® may not be a good choice for very early season varieties, but apart from that, this change is minor.

Timing: The window of best response for MaxCel® for thinning is between 5 and 15 mm fruit size. To determine the average fruit size, select five to 10 spurs and measure fruit size of all the fruit on the spurs and then calculate an average fruit size. The label states that for enhancing fruit size, applications spray should begin at petal fall and repeated up to four times at 3-10 day intervals.

MaxCel® should be applied dilute (do not concentrate more than 2X) in 500 - 2000 litres of spray solution per hectare. Uniform and thorough coverage is essential. Research at the Simcoe Horticultural Experiment Station indicates that concentrations below 50 ppm 6-BA are ineffective for thinning and single applications of at least 50 ppm are necessary for improving fruit size. While multiple applications below 50 ppm 6-BA may be effective, no research to corroborate this recommendation has been conducted at Simcoe.

Do not apply MaxCel® in combination with NAA or NAD (either tank mix or separate sprays) during the same growing season to Delicious or Fuji, as this combination may result in the formation of pygmy fruit.

Environmental Conditions: The effectiveness of chemical thinners is influenced by the weather in several ways. MaxCel® is no exception. To optimize plant uptake of the spray solution, apply MaxCel® during periods of slow drying (for example, early morning). Best results are obtained when warm temperatures (greater than 20^oC) occur during and following application.

Conclusion: The registration of MaxCel® is important because it represents an improvement in 6-BA technology. Growers will be able to use highly effective concentrations, either as a stand-alone thinner or in combination with Carbaryl. 

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