Blossom drop

also called Flower drop, Failure to set

Flowers open, yellow at the stalk and fall without setting fruit. Almost always the temperature — too hot, too cold, or too dry at the wrong moment.

Disorder Serious Doesn't spread

What you'll see

Open flowers dropping off cleanly at a joint in the stalk, leaving a small stub.
A yellow band across the flower stalk a few days before it goes.
Trusses of tomato flowers lost while the plant itself is dark green and growing hard.
Beans and peppers flowering heavily and setting almost nothing through a hot fortnight.
Fruit setting normally again as soon as the weather moderates, which is the confirmation.

Why it happens

Pollen is the weak point. Tomato pollen is sterile above about 32 °C by day or 24 °C at night, and will not shed below about 13 °C; peppers and beans have their own narrow windows. Drought, waterlogging, a heavy nitrogen feed and deep shade all interfere with the same step. The plant is not sick — it is refusing to invest in fruit it cannot make.

A heatwave or a cold snap during flowering, above all. Low humidity that dries the pollen, or very high humidity that makes it clump, will both do it, as will a plant fed hard on nitrogen and running to leaf.

Treating it

Wait. Once the weather moderates the next trusses will set, and the plant loses nothing by the pause.
Shade tomatoes and peppers with fleece or shade cloth through the worst afternoons of a heatwave.
Water deeply and consistently and mulch; drought stress on top of heat is what turns a pause into a failure.
Stop high-nitrogen feeding and switch to a potassium-rich tomato feed.
Tap or shake the flower trusses at midday in a greenhouse, which shakes the pollen loose when still air will not.

Keeping it away

Grow heat-set tomato cultivars where summers are reliably hot, and early cultivars where they are cool.
Plant out after the cold nights are past rather than at the earliest possible moment.
Mulch and water steadily so the plant never swings between drought and flood.
Ventilate a greenhouse hard, and damp the floor down rather than the plants.
Feed for fruit, not for leaf, from the first flower onward.

Who gets it

Tomatoes, peppers, aubergines and beans above all, and any fruit tree flowering in a cold snap.