Blossom end rot: identify, prevent, control

Worst during hot, dry spells and growth spurts in summer, December to February, on the first fruit to set.

Blossom end rot is a calcium and watering disorder, not a pathogen, so it cannot spread from plant to plant. It shows as a sunken brown patch on the bottom of tomatoes, capsicum and zucchini when uneven watering stops the fruit getting enough calcium during a growth spurt.

How to identify it

• A sunken, leathery brown or black patch on the blossom end (bottom) of the fruit
• First fruit of the season often worst affected
• The patch staying dry and firm rather than turning to soft mould
• Appearing after hot spells or irregular watering

How to prevent it

• Water deeply and consistently, since erratic watering is the main cause
• Mulch well to hold even moisture in the root zone
• Avoid heavy nitrogen feeding, which pushes fast growth that outruns calcium supply
• Lime acidic soil before planting so calcium is available to the roots

Organic control, step by step

• Even out your watering routine, as this is the real fix rather than any spray
• Mulch thickly to buffer soil moisture between waterings
• Remove affected fruit so the plant puts its energy into healthy fruit
• Check soil pH and lime if it is too acidic for calcium uptake
• Ease off high-nitrogen fertilisers during fruiting
• Expect later fruit to come good once watering steadies

Plants it attacks