Sweet as Pie

Developing sweeter solutions to blackberry disorders

By: Kathleen Forman

Sweet as Pie

A bowl of blackberries awaits preparation (Stock photo)

From pies and cobblers, to jellies and jam, blackberries are synonymous with the South, though frequently grown in the Pacific Northwest. The small edible fruit is not exactly a berry, in the traditional sense of the word. It contains drupelets, small round pockets of flavor that are clumped together to form the berry. As consumer taste grows for blackberries across the U.S., the South has stepped up to meet consumer demand. However, the best management practices for growing blackberries and ensuring a crop free of disorders are not so black and white.

Dr. Eric Stafne, extension and MAFES research professor in plant and soil sciences, is investigating common disorders in blackberries to help producers and consumers enjoy a more consistently high-quality crop. With a research focus on small fruit crops, Stafne recognized white drupelet disorder as an emerging issue in blackberries and began research designed to help producers manage it in their crops.

"Previously, there weren't a lot of blackberry issues in south Mississippi, south Georgia, or even Florida, since this crop was not typically grown in those locales," said Stafne. "However, when you start growing crops in areas where they haven't been previously grown, different problems can arise. This is the case with blackberry. When I came to Mississippi, I saw white drupelet disorder as an issue, especially in the "Sweetie Pie", a new blackberry cultivar, and knew this could be significant for a Southern grower."

White drupelet disorder is an abiotic affliction for blackberry crops, causing individual drupelets on the plant to have a white color instead of the traditional dark black coloring. This disorder does not harm the plant, but does change the physical appearance of the fruit, which can discourage wholesalers and confuse consumers. Consumers unaware of white drupelet disorder may assume the blackberry is diseased and avoid purchasing affected berries, which could result in a substantial economic loss for growers. While there is an unnoticeable taste difference in blackberries with few affected drupelets, an increased bitterness occurs when the majority of the berry is affected, making them more unattractive to buyers.

Stafne, with the help of doctoral student Haley Williams, is investigating the causes of white drupelet disorder. There is little research into why this condition occurs but many scientists, including Stafne, suspect that high temperatures and light intensity are factors. Extreme heat stress, fluctuating humidity, and heavy rainfalls are other suspected causes that possibly increase white drupelet disorder. Why some varieties are more commonly affected than others is unknown, but Stafne believes that genetics paired with environmental factors may play a part.

"The complex genetic-environment interaction is not easy to predict or eliminate and it is not well understood," Stafne said. "Several cultivars are known to exhibit white drupelets, with some varieties more sensitive than others, and it can vary from year to year."

Since the causes of white drupelet disorder are not fully known, Stafne chose to test the effects of nitrogen on the "Sweetie Pie" variety to determine if the addition of the crop booster will minimize the number of affected drupelets. Stafne and his team conducted three treatments with varying degrees of nitrogen to the blackberry crops located at the MAFES South Mississippi Branch Experiment Station in Poplarville, Mississippi. Nitrogen is essential to crop growth, particularly for its necessary role in the process of photosynthesis, and although blackberries require less nitrogen than other fruit crops, adding additional nitrogen to the plant could strengthen its production. This chemical element encourages more vegetative growth, assisting the plant in producing more leaves for shading in heat stress environments.

"The nitrogen worked as an indirect effect, relieving some of the environmental stress of the high temperatures of the South," said Stafne. "Overall, the nitrogen is keeping the plant healthier, less stressed, and more functionally active during time periods that prove additionally stressful."

In this experiment, nitrogen application began at bud break. The control group had no further nitrogen supplements after the initial application. The first and second variable groups had two and five additional nitrogen applications, respectively. The nitrogen was applied at one time for the first variable group, and applied one week apart for five consecutive weeks for the second variable group. With the singular application of the nitrogen treatment, the occurrence of white drupelet disorder decreased from 13 percent to ten percent of blackberries affected. Five applications of nitrogen lowered the disorder's presence to nine percent. Overall, the percentage decrease in white drupelet disorder for the two research groups was 23 and 30 percent, respectively.

The economic and environmental cost-benefit analyses are still being conducted, but Stafne is optimistic that this management strategy will benefit producers. He hopes to continue this research by applying the same test to other blackberry varieties and further investigate other management practices such as applying mulch to keep the soil cooler and reduce drastic temperature changes. Creating an effective management strategy for producers will help consumers around the country, and specifically the South, have easier access to blackberries.

"My personal view of research is that in a study or series of studies, we solve one piece of the puzzle," said Stafne. "In this study, we were able to add one more piece into this puzzle with hopes that one day we'll see the whole picture. I enjoy adding each piece and being able to see the direction I need to go in the future to complete the whole image."

In addition to Stafne, this research team included Jenny Ryals, MSU research associate, and Barbara Smith, USDA researcher. This research was funded through the USDA Agricultural Research Service and the Mississippi Agricultural and Forestry Experiment Station.

When I came to Mississippi, I saw white drupelet disorder as an issue, especially in the "Sweetie Pie", a new blackberry cultivar, and knew this could be significant for a Southern grower.

Dr. Eric Stafne

Behind the Science

Eric Stafne

Eric Stafne

Extension/Research Professor

Education: B.S., Forestry, Michigan State University; M.S., Horticulture, Ph.D., Plant Science, University of Arkansas

Years At MSU: 11

Focus: Production of small fruit crops

Passion At Work: Fruit is a healthy food source that people need and desire to have as part of their diet.