Tule fog can damage citrus fruit

Like most Sacramentans, I’ve had my fill of tule fog – and so has my citrus trees.

I discovered, as did other citrus growers, that too much fog can damage lemons, grapefruit, oranges, kumquats and other citrus. And this winter, we’ve definitely had “too much” tule fog.

Tule fog is the regional name for dense ground-hugging radiation fog. One of the most persistent kinds of fog, tule fog is named for the valley’s marsh grasses and can reduce visibility to zero. On calm, clear nights, the ground radiates heat. Water vapor near the surface condenses into fog. Without wind to move it around, this thick fog stays put – often days at a time.

Starting Nov. 21, Sacramento saw a streak of gloomy, foggy days that stretched more than three weeks – just when citrus was ripening. Temperatures during that record fog immersion also stayed chilly, rarely moving out of the 40s.

This week, that tule fog returned, along with the same issues.

Tule fog keeps everything moist, and that’s the problem. That constant exposure to chilly moisture bursts the skin cells on citrus fruit, causing blemishes. The damage can look a lot like frost exposure.

“Extended, heavy fog (such as tule fog) causes significant cosmetic damage, rind blemishes and fruit drop in citrus by keeping fruit constantly moist, leading to cell rupture,” say the UC master gardeners. “This, combined with cold temperatures, results in surface staining (black or brown spots), and increased susceptibility to fungal diseases such as anthracnose, which causes dark staining and rot.”

My grapefruit look like they’ve got the measles (or lots of unwanted freckles) with little brown spots pockmarking their yellow skin. (Fortunately, these blemishes don’t affect their juice.)

Usually, I “store” my Meyer lemons on the tree, letting nature take care of them until they’re needed in the kitchen. But this winter, the lemons growing on the outside of the tree – away from the trunk and more exposed to the fog – look like they’re beat up, with little brown scars and pitted blemishes. Several lemons just fell off.

Thankfully, the lemons growing in the interior of the tree’s canopy got some fog protection and look normal.

Tule fog can lead to citrus fruit drop, say the master gardeners. Citrus growers can lose 15% to 20% of their harvest due to prolonged tule fog exposure.

The solution? Pick citrus (especially if prolonged tule fog is in the forecast), dry the fruit well and store in the refrigerator.

And hope for some sunny, dry, fog-free days.