Pelargonium rust

What you see

Small yellow spots on the upper leaf surface, each matched underneath by a ring of cinnamon-brown powdery pustules arranged in a distinct circle. Leaves yellow and drop; heavy infection defoliates the plant.

Conditions

Warm, humid, still conditions with water sitting on leaves - so it is a greenhouse, conservatory and crowded-patio disease more than an open-garden one.

Treatment

Remove and bin affected leaves at first sight - it spreads fast in a collection. Sulphur checks it. Increase spacing and ventilation immediately.

Prevention

Water at the base in the morning. Space pots so leaves do not touch. Inspect new plants carefully before adding them to a collection - this is how it always arrives.

Months: May, June, July, August, September, October

Plants

Pelargonium

Remedies

Wettable sulphur, Potassium bicarbonate spray

The thing people get wrong

The concentric ring of pustules underneath is diagnostic and worth learning, because this spreads through a pot collection far faster than anything else pelargoniums get. It also almost always arrives on a bought plant - quarantining a new pelargonium for two weeks away from the others is unglamorous and completely effective.