Though proving uncommon in mature orchards, the green spot disorder that can develop in WA 38 fruit seems to be influenced by tree vigor, crop load, climate conditions and nutrient levels in the fruit, similar to bitter pit in one of its parents, Honeycrisp, according to Washington State University’s Bernardita Sallato.

Growing a new apple cultivar poses growing pains, and for WA 38, one of those challenges has been the green spot disorder.

Scientists trying to understand the cosmetic disorder still don’t have it entirely solved but said the 2021 season offered both good news and insight into how growers can reduce the problem going forward.

First the good news: In about two dozen commercial orchards surveyed by Washington State University, there were much lower rates of green spot in 2021 than in 2020. Physiologist Lee Kalcsits reported the incidence dropped from 12 precent to 3.7 percent — a finding that likely reflects both the trees maturing and the differing environmental conditions between the growing seasons. The same study also found the rate of bitter pit incidence in Honeycrisp in 2021 was half that of 2020, Kalcsits said when presenting the findings to the Washington Tree Fruit Research Commission in January.

Speaking of bitter pit, the research project found many similarities between green spot development in WA 38 and bitter pit development in Honeycrisp, one of the new cultivar’s parents. Risk factors for both include high vigor, low crop load and high levels of potassium and nitrogen relative to calcium in the fruit tissue, according to WSU Extension specialist Bernardita Sallato, who started studying green spot with funding from Washington state’s Specialty Crop Block Grant program in 2018 and now collaborates with Kalcsits to survey commercial orchards.

“In my perspective, the green spot is very similar to bitter pit. The symptom development is just different because it’s a different cultivar,” Sallato said. The disorder is characterized by development of dark green circles in the peel and necrotic, corky tissue underneath, which renders the fruit unmarketable. A more mild form of the symptoms, which Sallato calls “greening,” is limited to peel discoloration.

Learn full figure pic on goodfruits.com

Like bitter pit, green spot appears to be a complex disorder influenced by everything from rootstock to crop load to climate. One key difference: Green spot doesn’t continue to develop in storage.

The Research Commission has funded several research efforts looking into green spot from different angles, because the industry needs it solved from all angles, said Ines Hanrahan, the commission director. While Kalcsits and Sallato focused on the role of nutrient management, another project found that fruit microclimate, specifically higher humidity, may reduce the development of the disorder.

“It’s not all that disconnected, and scientists are figuring out how it all connects,” Hanrahan said. After all, vigor and water stress and nutrient uptake are all related physiologically, she said.

Both projects found high-vigor rootstocks, such as G.41, are associated with higher green spot incidence. In her prior work, published last year, Sallato also found lower calcium levels and higher nitrogen in apple tissue from trees on G.41 than in apples grown on M.9 rootstock, which also had higher crop load.

Sallato urged growers who are struggling with green spot to focus on vigor.

“I do hope that the message — when we say, ‘This is similar to bitter pit’ — the growers won’t think that calcium applications will solve the problem,” Sallato said. “It’s more about the management of vigor and the balance of the tree. They are managing bitter pit by managing vigor and crop load and water in the orchard.”

Nitrogen, which drives vigorous growth, appears to be a key nutrient indicator. The more out of balance the ratio of nitrogen to calcium, the higher the severity of green spot, she said, similar to other calcium-related disorders.

“Any apple grower that grows Honeys knows that, too, especially if you are pushing trees hard to get to the top of the wire, you will sacrifice some fruit quality due to that nitrogen and calcium imbalance,” Sallato said. “The more balance, the more crop load, the more dwarfing rootstock, the less green spot development. It’s been very consistent.”

WA 38 seems to be easier to bring into balance than Honeycrisp, growers said. At Monument Hill Orchards in Quincy, which hosted a WSU field day last fall, orchard manager Jorge Acevedo said that in the fourth leaf, he found virtually no green spot. The fruit gets better with age, he said.

Others in the industry agree.

“I don’t think it’s going to be nearly as sensitive as Honeycrisp. Our experience is that it seems to grow out of that,” said Garrett Henry, area manager for Douglas Ag Services.

“We’re really getting to the stage where people are backing off a bit on inputs for tree growth; trees are filling space now and we can focus on those other aspects of consistent cropping and fruit quality that can come from a more balanced tree,” said Dave Gleason, horticulturist and variety developer for Domex Superfresh Growers.

Allan Bros. engineer Matt Miles, a research commission board member, said it’s important for growers to hear about the research they are funding, but “we may grow out of the problem before we fix it,” he said. “My perspective is that it’s just a little wonky in its younger years and matures into a great piece of fruit.”

People who continue to plant WA 38 will still need to deal with the balance problem in young trees, Gleason added, and the research findings should help them do that.

Moreover, it is possible for mature orchards to develop green spot if the trees are out of balance, Sallato said. At WSU’s Roza orchard, she found lots of green spot development in a mature block after very aggressive pruning pushed up the vigor, she said.

Sallato is also looking at how plant growth regulators that control vigor could play a role. She and Kalcsits plan to collect more data this season, before finalizing their recommendations around nutrient management and rootstock selection. Until then, Kalcsits told the commission, “Be careful with nitrogen applications and make sure you are applying the right amount for the right reasons.”

Feature image courtesy of Elmi Lotze via scibraai.co.za