Virginia Tech tree fruit researcher Dr. Greg Peck says growers are under a lot of stress throughout the growing season, stress that begins even before trees start to bloom in spring.

When Virginia growers were asked to list the most critical tree fruit research areas, the number one response was managing crop load in apples. “A lot of people say that it’s the most important management decision they make all year,” noted Peck. “There’s a lot of stress associated with that; it can either make or break the crop for this year and also for next year. If we don’t do a good job thinning, we lose out on size [and] quality and ruin the structure of the tree.” The big issue is return bloom. Earlier thinning means there’s a better chance of good return bloom.

Peck, who has production experience on both coasts, has been involved with the development of a bloom thinning model that’s been in the works for more than 10 years. The models have been developed largely through funding by the Washington Tree Fruit Research Commission.

He says improved models are giving growers an edge when it comes to determining the optimal time for bloom thinning, as well as timing other treatments throughout the growing season.

“We want to reduce the stress of the decision,” said Peck. “The way to do that is to spread out the management decisions.” He listed pruning and training, flower thinning, petal fall thinning, 10-millimeter thinning, 18 to 25-millimeter thinning, hand-thinning 30 days after bloom, and the use of return bloom sprays as opportunities for the grower to manage crop load throughout the year. He also noted that the cultivar, rootstock and training system influence crop load over the life of the tree.

However, many growers, especially in the East, consider flower thinning too risky. There are reasons not to thin blooms, including unpredictable spring weather.

“The biggest con is risk of spring frost,” said Peck. “But what’s a bigger risk – that you have a potential spring frost and knock out blooms, or is it a bigger risk to not have the fruit size to make market grade? There’s a trade-off, and there’s more than one risk to consider.”

Another negative for bloom thinning is that timing is subjective – 20 percent bloom might mean different things to different growers.

The benefits of bloom thinning include larger fruit, greater return fruit the following year, less biennial bearing, and the potential for fewer fungicide applications. According to Peck, lime sulfur applications are available for organic growers, and some trials are examining other products for bloom thinning that also have some fungicidal activity.

He explained that models help growers simplify complex phenomena so information can be used to predict future events. The pollen tube model developed at Virginia Tech helps growers apply thinners more precisely, which further reduces grower stress. The model is based on how long it takes the pollen tube to grow between pollination and the point of fertilization.

“The measurement of pollen tube growth at various temperatures is the basis for the pollen tube model. When temperatures are low, there isn’t much growth in the pollen tube. As temperatures increase, there’s an exponential rate of growth up to a certain point, followed by pollen tube mortality,” Peck explained. “We know that the earlier we thin, the larger the fruit and greater return bloom. We want to make more precise thinning applications, not just subjective based on the bloom we see out there. We also want to reduce the risk of over and underthinning.”

The concept of the pollen model allows growers to more accurately predict when a certain percentage of the flowers have been fertilized prior to applying bloom thinners.

“We’re modeling the amount of time it takes for the pollen tube to grow, from the stigma down to the ovule, where fertilization happens,” said Peck, explaining how the model works. “We put on the thinner before fertilization happens. This helps us better apply chemical thinners.”

The model was developed using superdwarf root-bagged trees on M27 that were forced to bloom in growth chambers. “The whole root system fits in a 5-gallon bucket. Because flowering has a very short window, we used some tricks. We dug trees out in November and brought them into a cold room to give them a winter chill. Then we force them to bloom and use controlled hand-pollination, which allows us to choose which genotype we want to cross,” he explained.

During research trials, blooms are collected at predetermined intervals, and then pistils and ovules are processed and stained to observe pollen tubes in the style. Fluorescence microscopy provides a view of germinating pollen grains and the progression of pollen tubes down the style. This shows tube growth over time at the selected temperature and indicates when fertilization should occur based on style length.

Peck noted that because one of the key components of the model is measuring style length – where it begins and ends – growers must know how to take this measurement. As part of the research project, growers in Washington state have been provided with training on how to obtain these measurements and have been successfully using the pollen tube model to determine optimum timing for bloom thinner applications.

At this point, models have been developed for Golden Delicious, Gala, Fuji, Cripps Pink and Honeycrisp. The model is available through Washington State University’s AgWeatherNet website (; log-in is required).

The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania. Comment or question? Visit and join in the discussions.