Grey Mold

What’s that grey mold that appears overnight on your strawberries? Where did it come from? And how did it happen so fast? Strawberries can go from nearly perfect to practically inedible in an astoundingly short time. Read on to find out why!

'Botrytis cinerea' on strawberry (Rasbak) CC BY-SA 3.0

Grey mold, also known as botrytis fruit rot and botrytis bunch rot, is a fungal disease caused by Botrytis cinerea. Grey mold also attacks:
​

• strawberries
• grapes
• tomatoes
• bulbs
• snapdragons
• rhubarb
• pomegranates
• flowers
• kiwifruit
• cannabis

​The culprit

Botrytis cinerea is everywhere. It flies in on the wind, is carried on clothing, shoes, tools, and pets; it’s floating around in the air we breath. Yep, it’s like that. So, unless you live in a bubble, your plants and food are already in contact with grey mold. There are actually several different strains of grey mold. They get their Latin name, Botrytis, from the Greek words for ‘grapes like ashes’. You might think that’s because they feed on grapes, but it is actually because the fungi itself grows in clusters. The word ‘ash’ refers to the grey color. ​


​This particular garden problem spends most of its time as a dormant, asexual spore that is relatively indestructible. Add moisture in spring (or with irrigation) and voilà, stuff starts to happen!

    Grey mold - This is the gray fuzz we find on our strawberries, grapes, and other fruit and flowers.

    Noble rot - If dry conditions follow wet weather, the fungi suck moisture out of grapes, leaving behind a bitter aftertaste. 

    Antifungal - If that weren’t bad enough, botrytis also interferes with wine-making! As it grows, it produces an antifungal substance (presumably to kill off the competition) that also kills off the yeast that makes grape juice magically transform into your favorite Chardonnay or Pinot Noir. 

    Bunch rot - As soon as an injury occurs to grape vines or fruit, the botrytis fungi cause bunch rot infection.

Botrytis cinerea on Riesling grapes (Tom Maack) CC BY-SA 3.0

Preventing infection

Most commercial grape growers apply fungicides to prevent infection, since the fungi is present all the time. As with downy mildews, black spot, and powdery mildew, moisture control is the key. If leaves and fruit are left wet, they are more likely to become infected. [Remember those strawberries you washed before putting them in the fridge last night? Yep, that’s what happened!]


Controlling grey mold

Once infection begins, potassium bicarbonate-based fungicides can be applied to reduce the spread of the disease. Dead and diseased plant tissue should be carefully removed. I say ‘carefully’ because each bit can contain millions of fungal spores.


Some people have an allergic reaction to grey mold that causes a rare lung disease known as ‘winegrower’s lung’.


So, the next time you buy strawberries or grapes, wait until you are ready to eat them before rinsing them off. It’s not a guarantee, but it helps! Also, keep them in their container and put them in the crisper drawer. Strawberries are best stored at 32 to 36° F, at 90 to 95% humidity.