Citrus greasy spot

What you see

Yellow blotches on the upper leaf that develop into raised, brown-black, distinctly greasy-looking swellings on the underside. Heavy infection causes serious leaf drop in winter, leaving a thin, unproductive tree.

Conditions

Warm, humid nights and long leaf wetness through late summer and autumn. The fungus develops on fallen leaves and releases spores upward onto the canopy.

Treatment

Rake up and remove fallen leaves from beneath the tree - this is the main source. A copper or horticultural oil application in early summer protects the new flush.

Prevention

Keep the ground under the tree clear of leaf litter. Prune for an open canopy. Keep the tree vigorous with balanced feeding.

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

Plants

Orange, Lemon, Tangerine, Lime, Grapefruit

Remedies

Bordeaux mixture (copper), Horticultural oil (white oil)

The thing people get wrong

The tree defoliating in midwinter is what people notice, and by then the infection happened five months earlier on the summer flush. Greasy spot is the reason many home citrus trees look permanently thin. The whole cycle depends on the tree's own fallen leaves rotting underneath it - so a rake in autumn is genuinely the most powerful treatment available.