Last summer, Maine potato farmers were dealt too much rain, which led to blight and rot.

This year, the farmers are struggling with the opposite extreme — too little rain, which is stunting the potatoes’ growth and will hit farmers’ wallets hard this fall.

Just like the corn crop in the Midwest, Maine potatoes are withering, but they’re underground and out of sight, so how poorly they’re doing is a bit of a guessing game.

The crop, most of which will be harvested this month, is expected to be smaller than normal, both in total yield and the size of the individual potatoes. That means not only fewer potatoes to sell, but also a loss of premiums — bonuses that processors pay for potatoes that are larger or heavier than normal and are particularly attractive to french fry producers.

Those premiums are the key to a profitable season.

“We won’t get any of those bonuses,” lamented Matt Porter, who farms about 700 acres on plots between Washburn and Presque Isle and said he expects that, at best, he will break even this year.

Porter’s not alone in fretting about the crop. Farmers all across Aroostook County, particularly the southern part of the county, are preparing for a poor harvest when they begin pulling potatoes from the ground next month.

June was wetter than normal and so far, the rainfall for the year is actually running a little ahead of average, said meteorologist Rich Norton of the National Weather Service’s Caribou office.

However, just 1.73 inches of rain fell in July, less than half the average, he said. Into the last week of August, only about 2 inches had fallen for the month, and the average for the month is more than 3 inches.

Farmers also note the rain has been particularly spotty, with some areas missing out entirely when showers have moved through. For instance, in July, when Caribou reported 1.73 inches of rain, a University of Maine gauge in Presque Isle — only about 15 miles away — recorded just 0.4 inches.

The timing of the lack of rain was critical, with July and August being the time that most potatoes bulk up.

Russet Burbank potatoes — the variety prized by potato processors — go through about 0.2 inches of water a day, said Steven Johnson, a crop specialist with the University of Maine Cooperative Extension. Without that vital moisture, some plants die; and those that survive produce smaller potatoes.

Bruce Flewelling, of Flewelling Farms in Easton, said he lost about 200 of his 900 acres of potatoes to the drought and unusual warmth.

“They just burned up, dried up,” Flewelling said.

Last year’s rain caused some potatoes to rot in the warehouses, he said. This year’s dry weather means warehouses won’t be as full.

“It’s a one-two punch,” Flewelling said. “It’s just one extreme to another, and boy, that’s rough.”

Flewelling said he has irrigation on about 25 percent of his crop, and that helped save fields he was able to water. He hopes to add irrigation for another 100 acres before next summer, but he doesn’t have access to enough water to go beyond that.

Porter said irrigation is expensive. He said the federal government provides some financial help, but only to upgrade inefficient irrigation systems, not for installing new ones.

To be eligible, he said, laughing, he’d have to put in an inefficient system and then wait five years to apply for aid to pay for a more efficient one. So he relies on Mother Nature, who hasn’t been too reliable lately,

Gregory Porter, a professor in the University of Maine’s department of plant, soil, and environmental sciences and — this being close-knit Aroostook County — Matt Porter’s uncle, said no more than 25 to 30 percent of Maine’s potato fields are irrigated, either because of cost or a lack of access to adequate water.

He said the lack of rain, ironically, will result in pretty good quality potatoes, but that won’t offset the low yields and small sizes.

Size matters to farmers because processors pay more for potatoes that weigh more than 10 ounces, said Dana Wright, executive director of the Presque Isle-based Agricultural Bargaining Council, which represents about 80 growers in negotiating contracts with processors — primarily McCain Foods, which has a large plant in Easton.

The growers also get premiums for potatoes that are free from defects, are particularly dense or are whiter than average, Wright said.

The dry weather “will affect the size profile and, of course, the size profile dictates yield,” Wright said.

He said the 10-ounce threshold will be difficult for many farmers to meet, although he doesn’t want to give up hope that they may be able to get some late-season growth if it rains in September.

Potatoes that don’t earn a premium bring low prices from processors.

Wright said the contracts with processors mean run-of-the-mill potatoes often fetch less than the going price for table potatoes — the kind that find their way to supermarket produce sections.

The current contract with the processors calls for a median price of $10.40 for 100 pounds. That’s for potatoes that are pulled from warehouses in April, the halfway point of the storage season, and meet five-year averages on a range of quality measures. The last two years, that’s been less than the price for table potatoes, he said.

Wright said that in the best of times, potato farming is a gamble that can pay well, but is more often a losing bet.

He said red table potatoes were hot last year, getting $30 for a hundred pounds. This year, when farmers reacted by planting more red potatoes, the price declined to about $3 for 100 pounds.

Many farmers prefer to sell to processors because they have a better handle on what they’ll earn, rather than being subject to the vagaries of the market, Wright said. This year probably will show that even with a guaranteed buyer, the business involves a little luck.

“If you get a perfect storm on that contract, you can make a lot of money,” he said, “and if you don’t get a perfect storm, you either break even or you don’t make any money.”

Matt Porter said he, like many farmers, plans to “push” his Russet Burbanks and harvest later. The potato is normally among the last varieties to be harvested anyway, and the farmers hope another week or two will boost the size.

“We’re going to let our potatoes grow for a little bit and take some risks,” he said.

It’s no sure thing. Early October rain doesn’t evaporate as quickly or as completely as that of July and August, so potatoes harvested after a rainstorm are likely to be wet. Potatoes that are wet when they go into storage can rot.

Also, a hard frost can damage the potatoes, so delaying the harvest increases the possibility of exposing the spuds to freezes.

Johnson said delaying the harvest probably isn’t worth it.

Potatoes just don’t grow much in September, he said, and even some additional rain — and rain that’s not too much and not too close to harvest — isn’t likely to make a difference

“I do not see a rapid recovery,” Johnson said. “They’re not going to really bulk up,” and the downside risks are pretty steep.

Even an inch of rain a week would fall short of the Russet Burbank’s normal needs, he noted.

Matt Porter said he, like most farmers, buys crop insurance, which covers some of the shortfall if the fields fail to produce; but that only averts a disaster. It doesn’t really substitute for a solid crop.

“That’s not the answer for paying all the bills out at the end of the year,” he said.