The first-ever fungus infestation of Hawaii’s $6.8 million sweet basil crop discovered late last week has started affecting some businesses while farmers scramble to save their fields.

Most of Hawaii’s sweet basil crop is grown across Oahu and farmers are hastily pruning back their plants and applying fungicide to combat "basil downy mildew" after the pathogen Peronospora belbahrii was confirmed Friday on multiple farms in Waianae, according to the University of Hawaii’s College of Tropical Agriculture and Human Resources.

It has since been spotted on farms in Ewa and Waimanalo, said Jari Sugano, a UH extension agent who works with commercial farmers in the field.

Ba-Le Sandwiches & Bakery already has seen a price increase in its sweet basil purchases and will bear the extra costs for now, said operations manager Trung Lam.

"We definitely do use a lot of basil — we have basil in drinks and a lot of food items, as well," Lam said. "We have to absorb it (price increase) for a little while. We can’t just raise prices tomorrow."

Foodland Super Markets has turned to sweet basil imports, rather than locally grown sweet basil, spokeswoman Sheryl Toda said.

She was told the shortage of local sweet basil is due to the recent rain and "we expect to receive our normal delivery next week."

The recent cool weather and heavy rain were a factor because they created a friendly environment for Peronospora belbahrii to latch onto the sweet basil leaves, said Janice Uchida, a plant pathologist at UH’s College of Tropical Agriculture and Human Resources.

As the infestation drags on, the hardest-hit customers will likely be large hotels and restaurants that often buy sweet basil in 10-pound bulk packages, rather than much smaller packages sold individually at grocery stores, according to Armstrong Produce Ltd., Hawaii’s largest produce distributor.

Basil downy mildew creates black, gray, clear or brownish spores on the underside of sweet basil leaves, which then turn yellow, black or brown before dying, according to the College of Tropical Agriculture and Human Resources.

It has affected crops throughout Europe, Israel, New Zealand, Argentina, parts of Africa, the U.S. mainland and Canada.

The majority of sweet basil is grown in Waianae, as well as "in little pockets here and there" across Oahu, Sugano said.

Several farmers declined to be interviewed or did not respond to requests for interviews.

Once the fungus is discovered, Sugano said it spreads quickly across fields and farmers are now scrambling to prune their plants and apply fungicide, hoping the fungus will disappear in the next few weeks, Sugano said.

"They’re going to be shut down for a while until they evaluate whether it’s working or not," she said.

The infestation comes at the peak season for Hawaii growers, who usually send the bulk of their shipments this time of year to colder climates in Canada and the mainland, Sugano said.