Black spot on citrus — symptoms and treatment

Severity: medium

What is Black spot

Citrus black spot is caused by Phyllosticta citricarpa. It causes dark sunken lesions on fruit rind, reducing marketability. This is a disease of appearance rather than of the tree's survival: the flesh inside is usually unaffected, but the rind is disfigured, and for anyone selling fruit that is the whole problem. In bad cases it also causes fruit to drop before it is ready to pick.

Symptoms

The damage shows on the fruit. You see hard, sunken spots on the rind, dark brown to black, often with a slightly depressed centre that you can feel with a fingertip. Alongside these, red-brown lesions with dark margins appear on the rind — the darker rim around a lighter centre is a useful identifying detail. Symptoms typically become obvious as the fruit matures and colours, which is frustratingly late, because infection happened much earlier in the season. Premature fruit drop follows in affected trees.

• Early: small hard, sunken spots beginning to show on the rind
• Early: red-brown lesions with distinctly darker margins
• Advanced: dark brown to black sunken spots, sometimes numerous across the rind
• Advanced: premature fruit drop

Do not confuse it with citrus canker, which produces raised, corky lesions with yellow halos and also marks leaves and stems. Black spot lesions are sunken rather than raised, and the disease concentrates on the fruit.

Causes and conditions

The key to this disease is where the inoculum comes from: fallen leaves on the orchard floor are the primary source. As that leaf litter breaks down it releases spores, which are carried to the developing fruit — spores from the litter travel on air currents, and spores formed on infected fruit and twigs move in splashing rain. Infection needs wetness, so wet periods drive it, and the vulnerable window runs from fruit set onward while the fruit is still developing. There is a long silent gap between infection and visible symptoms, so the spots you see on ripening fruit reflect conditions from much earlier in the season. That lag is exactly why control has to be preventive.

Treatment

Sanitation and leaf litter removal — cultural

Timing: after harvest. Remove fallen leaves (primary inoculum source). Prune for air circulation. Use disease-free nursery stock.

Copper fungicide — chemical

Timing: fruit set through summer. Apply copper-based fungicide from fruit set through summer. Repeat every 3-4 weeks during wet periods. Pre-harvest interval: 21 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Clear fallen leaves from under the tree — this is the primary inoculum source, and removing it attacks the disease at its origin.
• Prune for air circulation so the canopy and fruit dry faster after rain.
• Plant only disease-free nursery stock; do not import the pathogen on new trees.
• Protect fruit through its susceptible period from fruit set rather than reacting to spots at colouring.
• Do not move infected fruit or plant material to clean areas.

Frequently asked questions

Can I eat fruit with black spot? The lesions affect the rind, not the flesh, so blemished fruit is generally fine to eat once you observe the pre-harvest interval of 21 days after any copper application. The loss is cosmetic and commercial — such fruit is hard to sell.

Is it contagious to my other plants? It affects citrus. The relevant risk is to other citrus trees nearby, particularly through leaf litter accumulating under an infected tree, rather than to unrelated plants in the garden.

When should I treat? Fruit is infected long before the spots appear, so treatment runs from fruit set through summer, repeating every 3-4 weeks during wet periods. Spraying when you first see spots on ripening fruit does not undo infections that took hold months earlier.